kernfs: fix dentry unexpected skip

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Hatayama, Daisuke
Sent: Monday, May 28, 2018 9:54 PM
To: '[email protected]' <[email protected]>; '[email protected]' <[email protected]>
Cc: Okajima, Toshiyuki <[email protected]>; [email protected]; '[email protected]' <[email protected]>
Subject: [RESEND PATCH v2] kernfs: fix dentry unexpected skip

kernfs_dir_next_pos() overlooks the situation that the dentry corresponding to a given pos object has already been inactive. Hence, when kernfs_dir_pos() returns the dentry with a hash value larger than the original one, kernfs_dir_next_pos() returns the dentry next to the one returned by kernfs_dir_pos(). As a result, the dentry returned by kernfs_dir_pos() is skipped.

To fix this issue, try to find a next node only when the returned object is less than or equal to the original one.

Note that this implementation focuses on getting guarantee that the existing nodes are never skipped, not interested in the other nodes that are added or removed during the process.

We found this issue during a stress test that repeatedly reads /sys/module directory to get a list of the currently loaded kernel modules while repeatedly loading and unloading kernel modules simultaneously.

v2: Fix the case where nodes with the same hash but with the name
larger than the original node could still be skipped. Use kernfs_sd_compare() to compare kernfs_node objects. Imporove patch description.

Signed-off-by HATAYAMA Daisuke <[email protected]>

Suggested-by Toshiyuki Okajima <[email protected]>

Cc Eric W. Biederman <[email protected]>

---
 fs/kernfs/dir.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

fs/kernfs/dir.c +3 -1

index 89d1dc1..3aeeb7a 100644
@@ -1621,8 +1621,10 @@ static int kernfs_dir_fop_release(struct inode *inode,
 struct file *filp)
 static struct kernfs_node *kernfs_dir_next_pos(const void *ns,
 	struct kernfs_node *parent, ino_t ino, struct kernfs_node *pos)
 {
+	struct kernfs_node *orig = pos;
+
 	pos = kernfs_dir_pos(ns, parent, ino, pos);
-	if (pos) {
+	if (pos && kernfs_sd_compare(pos, orig) <= 0) {
 		do {
 			struct rb_node *node = rb_next(&pos->rb);

if (!node)
--
1.7.1

-----Original Message-----
From: Tejun Heo [mailto:[email protected]] On Behalf Of '[email protected]'
Sent: Wednesday, May 30, 2018 1:26 AM
To: Hatayama, Daisuke <[email protected]>
Cc: '[email protected]' <[email protected]>; Okajima, Toshiyuki <[email protected]>; [email protected]; '[email protected]' <[email protected]>
Subject: Re: [RESEND PATCH v2] kernfs: fix dentry unexpected skip

Hello,

fs/kernfs/dir.c static int kernfs_dir_fop_release(struct inode *inode,

Quoted 5 lines · Daisuke

fs/kernfs/dir.c +0 -0

index 89d1dc1..3aeeb7a 100644
@@ -1621,8 +1621,10 @@ static int kernfs_dir_fop_release(struct inode *inode,

struct file *filp)

fs/kernfs/dir.c static int kernfs_dir_fop_release(struct inode *inode, · line 1627

Quoted 8 lines

fs/kernfs/dir.c +3 -1

 static struct kernfs_node *kernfs_dir_next_pos(const void *ns,
 	struct kernfs_node *parent, ino_t ino, struct kernfs_node *pos)
 {
+	struct kernfs_node *orig = pos;
+
 	pos = kernfs_dir_pos(ns, parent, ino, pos);
-	if (pos) {
+	if (pos && kernfs_sd_compare(pos, orig) <= 0) {

Hmm... the code seems a bit unintuitive to me and I wonder whether it's because there are two identical skipping loops in kernfs_dir_pos() and kernfs_dir_next_pos() and we're now trying to selectively disable one of them. Wouldn't it make more sense to get rid of it from kernfs_dir_pos() and skip explicitly only when necessary?

kernfs_dir_pos() checks if a kernfs_node object given as one of its arguments is still active and if so returns it, or returns a kernfs_node object that is most equal (possibly smaller and larger) to the given object.

kernfs_dir_next_pos() returns a kernfs_node object that is next to the object given by kernfs_dir_pos().

Two functions does different things and both need to skip inactive nodes. I don't think it natural to remove the skip only from kernfs_dir_pos().

OTOH, throughout getdents(), there is no case that the kernfs_node object given to kernfs_dir_pos() is used afterwards in the processing. So, is it enough to provide kernfs_dir_next_pos() only? Then, the skip code is now not duplicated.

The patch below is my thought. How do you think?

But note that this patch has some bug so that system boot get hang without detecting root filesystem disk :) I'm debugging this now.

-----Original Message-----
From: Tejun Heo [mailto:[email protected]] On Behalf Of '[email protected]'
Sent: Saturday, June 2, 2018 2:07 AM
To: Hatayama, Daisuke <[email protected]>
Cc: '[email protected]' <[email protected]>; Okajima, Toshiyuki <[email protected]>; [email protected]; '[email protected]' <[email protected]>
Subject: Re: [RESEND PATCH v2] kernfs: fix dentry unexpected skip

Hello,

Quoted 4 lines · Daisuke

kernfs_dir_pos() checks if a kernfs_node object given as one of its arguments is still active and if so returns it, or returns a kernfs_node object that is most equal (possibly smaller and larger) to the given object.

Sometimes they're duplicate operations tho, which is exactly the bug the posted patch is trying to fix. What I'm suggesting is instead of leaving both instances and skipping one conditionally, put them in one place and trigger only when necessary. The sequence of operations would be exactly the same. The only difference is how the code is organized.

Quoted 12 lines

kernfs_dir_next_pos() returns a kernfs_node object that is next to the object given by kernfs_dir_pos().

Two functions does different things and both need to skip inactive nodes. I don't think it natural to remove the skip only from kernfs_dir_pos().

OTOH, throughout getdents(), there is no case that the kernfs_node object given to kernfs_dir_pos() is used afterwards in the processing. So, is it enough to provide kernfs_dir_next_pos() only? Then, the skip code is now not duplicated.

Quoted 4 lines

The patch below is my thought. How do you think?

But note that this patch has some bug so that system boot get hang without detecting root filesystem disk :) I'm debugging this now.

I haven't looked into the code that closely but given that we had cases where both skippings were fine and not, the condition is likely gonna be a bit tricker?

Thanks.

--
tejun

kernfs_dir_next_pos() overlooks the situation that the dentry corresponding to a given pos object has already been inactive. Hence, when kernfs_dir_pos() returns the dentry with a hash value larger than the original one, kernfs_dir_next_pos() returns the dentry next to the one returned by kernfs_dir_pos(). As a result, the dentry returned by kernfs_dir_pos() is skipped.

To fix this issue, try to find a next node only when the returned object is less than or equal to the original one.

Note that this implementation focuses on getting guarantee that the existing nodes are never skipped, not interested in the other nodes that are added or removed during the process.

We found this issue during a stress test that repeatedly reads /sys/module directory to get a list of the currently loaded kernel modules while repeatedly loading and unloading kernel modules simultaneously.

v2: Fix the case where nodes with the same hash but with the name
larger than the original node could still be skipped. Use kernfs_sd_compare() to compare kernfs_node objects. Imporove patch description.

-----Original Message-----
From: Eric W. Biederman [mailto:[email protected]]
Sent: Monday, June 4, 2018 3:51 AM
To: Hatayama, Daisuke <[email protected]>
Cc: '[email protected]' <[email protected]>; '[email protected]' <[email protected]>; Okajima, Toshiyuki <[email protected]>; [email protected]; '[email protected]' <[email protected]>
Subject: [CFT][PATCH] kernfs: Correct kernfs directory seeks.

Over the years two bugs have crept into the code that handled the last returned kernfs directory being deleted, and handled general seeking in kernfs directories. The result is that reading the /sys/module directory while moduled are loading and unloading will sometimes skip over a module that was present for the entire duration of the directory read.

The first bug is that kernfs_dir_next_pos was advancing the position in the directory even when kernfs_dir_pos had found the starting kernfs directory entry was not usable. In this case kernfs_dir_pos is guaranteed to return the directory entry past the starting directory entry, making it so that advancing the directory position is wrong.

The second bug is that kernfs_dir_pos when the saved kernfs_node did not validate, was not always returning a directory after the the target position, but was sometimes returning the directory entry just before the target position.

To correct these issues and to make the code easily readable and comprehensible several cleanups are made.

- A function kernfs_dir_next is factored out to make it straight-forward to find the next node in a kernfs directory.

- A function kernfs_dir_skip_pos is factored out to skip over directory entries that should not be shown to userspace. This function is called from kernfs_fop_readdir directly removing the complication of calling it from the other directory advancement functions.

- The kernfs_put of the saved directory entry is moved from kernfs_dir_pos to kernfs_fop_readdir. Keeping it with the kernfs_get when it is saved and ensuring the directory position advancment functions can reference the saved node without complications.

- The function kernfs_dir_next_pos is modified to only advance the directory position in the common case when kernfs_dir_pos validates and returns the starting kernfs node. For all other cases kernfs_dir_pos is guaranteed to return a directory position in advance of the starting directory position.

- kernfs_dir_pos is given a significant make over. It's essense remains the same but some siginficant changes were made.

+ The offset is validated to be a valid hash value at the very beginning. The validation is updated to handle the fact that neither 0 nor 1 are valid hash values.

+ An extra test is added at the end of the rbtree lookup to ensure the returned position is at or beyond the target position.

+ kernfs_name_compare is used during the rbtree lookup instead of just
comparing
the hash. This allows the the passed namespace parameter to be used, and if it is available from the saved entry the old saved name as well.

+ The validation of the saved kernfs node now checks for two cases. If the saved node is returnable.
If the name of the saved node is usable during lookup.

In net this should result in a easier to understand, and more correct version of directory traversal for kernfs directories.

Reported-by "Hatayama, Daisuke" <[email protected]>

Fixes a406f75840e1 ("sysfs: use rb-tree for inode number lookup")

Fixes 1e5289c97bba ("sysfs: Cache the last sysfs_dirent to improve readdir

scalability v2")

Signed-off-by "Eric W. Biederman" <[email protected]>

---

Can you test this and please verify it fixes your issue?

 fs/kernfs/dir.c | 109
++++++++++++++++++++++++++++++++++----------------------
 1 file changed, 67 insertions(+), 42 deletions(-)

fs/kernfs/dir.c +12 -19

index 89d1dc19340b..8148b5fec48d 100644
@@ -1584,53 +1584,75 @@ static int kernfs_dir_fop_release(struct inode *inode,
 struct file *filp)
 	return 0;
 }
 
+static struct kernfs_node *kernfs_dir_next(struct kernfs_node *pos)
+{
+	struct rb_node *node = rb_next(&pos->rb);
+	return node ? rb_to_kn(node) : NULL;
+}
+
 static struct kernfs_node *kernfs_dir_pos(const void *ns,
-	struct kernfs_node *parent, loff_t hash, struct kernfs_node *pos)
+	struct kernfs_node *parent, loff_t off, struct kernfs_node *saved)
 {
-	if (pos) {
-		int valid = kernfs_active(pos) &&
-			pos->parent == parent && hash == pos->hash;
-		kernfs_put(pos);
-		if (!valid)
-			pos = NULL;
-	}
-	if (!pos && (hash > 1) && (hash < INT_MAX)) {
-		struct rb_node *node = parent->dir.children.rb_node;
-		while (node) {
-			pos = rb_to_kn(node);
-
-			if (hash < pos->hash)
-				node = node->rb_left;
-			else if (hash > pos->hash)
-				node = node->rb_right;
-			else
-				break;
+	struct kernfs_node *pos;
+	struct rb_node *node;
+	unsigned int hash;
+	const char *name = "";
+

fs/kernfs/dir.c +16 -5

+		} else {
+			if (kernfs_active(saved))
+				return saved;
+			name = saved->name;
 		}
 	}
-	/* Skip over entries which are dying/dead or in the wrong namespace
 */
-	while (pos && (!kernfs_active(pos) || pos->ns != ns)) {
-		struct rb_node *node = rb_next(&pos->rb);
-		if (!node)
-			pos = NULL;
+
+	/* Find the closest pos to the hash we are looking for */
+	pos = NULL;
+	node = parent->dir.children.rb_node;
+	while (node) {
+		int result;
+
+		pos = rb_to_kn(node);
+		result = kernfs_name_compare(hash, name, ns, pos);
+		if (result < 0)
+			node = node->rb_left;
+		else if (result > 0)

fs/kernfs/dir.c +13 -11

 	return pos;
 }
 
 static struct kernfs_node *kernfs_dir_next_pos(const void *ns,
-	struct kernfs_node *parent, ino_t ino, struct kernfs_node *pos)
+	struct kernfs_node *parent, loff_t off, struct kernfs_node *start)
 {
-	pos = kernfs_dir_pos(ns, parent, ino, pos);
-	if (pos) {
-		do {
-			struct rb_node *node = rb_next(&pos->rb);
-			if (!node)
-				pos = NULL;
-			else
-				pos = rb_to_kn(node);
-		} while (pos && (!kernfs_active(pos) || pos->ns != ns));
-	}
+	struct kernfs_node *pos = kernfs_dir_pos(ns, parent, off, start);
+	if (likely(pos == start))
+		pos = kernfs_dir_next(pos);
+	return pos;
+}
+
+static struct kernfs_node *kernfs_dir_skip_pos(const void *ns,
+					       struct kernfs_node *pos)
+{
+	/* Skip entries we don't want to return to userspace */
+	while (pos && !(kernfs_active(pos) && (pos->ns == ns)))
+		pos = kernfs_dir_next(pos);
 	return pos;

}

fs/kernfs/dir.c +1 -1

@@ -1638,7 +1660,7 @@ static int kernfs_fop_readdir(struct file *file, struct
 dir_context *ctx)
 {
 	struct dentry *dentry = file->f_path.dentry;
 	struct kernfs_node *parent = kernfs_dentry_node(dentry);
-	struct kernfs_node *pos = file->private_data;
+	struct kernfs_node *pos, *saved = file->private_data;
 	const void *ns = NULL;
 

fs/kernfs/dir.c +8 -5

 	if (!dir_emit_dots(file, ctx))

@@ -1648,23 +1670,26 @@ static int kernfs_fop_readdir(struct file *file,
 struct dir_context *ctx)
 	if (kernfs_ns_enabled(parent))
 		ns = kernfs_info(dentry->d_sb)->ns;
 
-	for (pos = kernfs_dir_pos(ns, parent, ctx->pos, pos);
-	     pos;
+	for (pos = kernfs_dir_pos(ns, parent, ctx->pos, saved);
+	     (pos = kernfs_dir_skip_pos(ns, pos));
 	     pos = kernfs_dir_next_pos(ns, parent, ctx->pos, pos)) {
 		const char *name = pos->name;
 		unsigned int type = dt_type(pos);
 		int len = strlen(name);
 		ino_t ino = pos->id.ino;
 
-		ctx->pos = pos->hash;
-		file->private_data = pos;
-		kernfs_get(pos);
+		kernfs_put(saved);
+		saved = pos;
+		ctx->pos = saved->hash;
+		file->private_data = saved;
+		kernfs_get(saved);
 
 		mutex_unlock(&kernfs_mutex);
 		if (!dir_emit(ctx, name, len, ino, type))
 			return 0;
 		mutex_lock(&kernfs_mutex);
 	}
+	kernfs_put(saved);
 	mutex_unlock(&kernfs_mutex);
 	file->private_data = NULL;

ctx->pos = INT_MAX;
--
2.14.1

Hello,

Thanks for this patch, Eric.

Quoted 72 lines

-----Original Message-----
From: Eric W. Biederman [mailto:[email protected]]
Sent: Monday, June 4, 2018 3:51 AM
To: Hatayama, Daisuke <[email protected]>
Cc: '[email protected]' <[email protected]>; '[email protected]' <[email protected]>; Okajima, Toshiyuki <[email protected]>; [email protected]; '[email protected]' <[email protected]>
Subject: [CFT][PATCH] kernfs: Correct kernfs directory seeks.

Over the years two bugs have crept into the code that handled the last returned kernfs directory being deleted, and handled general seeking in kernfs directories. The result is that reading the /sys/module directory while moduled are loading and unloading will sometimes skip over a module that was present for the entire duration of the directory read.

The first bug is that kernfs_dir_next_pos was advancing the position in the directory even when kernfs_dir_pos had found the starting kernfs directory entry was not usable. In this case kernfs_dir_pos is guaranteed to return the directory entry past the starting directory entry, making it so that advancing the directory position is wrong.

The second bug is that kernfs_dir_pos when the saved kernfs_node did not validate, was not always returning a directory after the the target position, but was sometimes returning the directory entry just before the target position.

To correct these issues and to make the code easily readable and comprehensible several cleanups are made.

- A function kernfs_dir_next is factored out to make it straight-forward to find the next node in a kernfs directory.

- A function kernfs_dir_skip_pos is factored out to skip over directory entries that should not be shown to userspace. This function is called from kernfs_fop_readdir directly removing the complication of calling it from the other directory advancement functions.

- The kernfs_put of the saved directory entry is moved from kernfs_dir_pos to kernfs_fop_readdir. Keeping it with the kernfs_get when it is saved and ensuring the directory position advancment functions can reference the saved node without complications.

- The function kernfs_dir_next_pos is modified to only advance the directory position in the common case when kernfs_dir_pos validates and returns the starting kernfs node. For all other cases kernfs_dir_pos is guaranteed to return a directory position in advance of the starting directory position.

- kernfs_dir_pos is given a significant make over. It's essense remains the same but some siginficant changes were made.

+ The offset is validated to be a valid hash value at the very beginning. The validation is updated to handle the fact that neither 0 nor 1 are valid hash values.

+ An extra test is added at the end of the rbtree lookup to ensure the returned position is at or beyond the target position.

+ kernfs_name_compare is used during the rbtree lookup instead of just
comparing
the hash. This allows the the passed namespace parameter to be used, and if it is available from the saved entry the old saved name as well.

+ The validation of the saved kernfs node now checks for two cases. If the saved node is returnable.
If the name of the saved node is usable during lookup.

In net this should result in a easier to understand, and more correct version of directory traversal for kernfs directories.

Quoted 9 lines

Reported-by "Hatayama, Daisuke" <[email protected]>

Fixes a406f75840e1 ("sysfs: use rb-tree for inode number lookup")

Fixes 1e5289c97bba ("sysfs: Cache the last sysfs_dirent to improve readdir

scalability v2")

Signed-off-by "Eric W. Biederman" <[email protected]>

---

Can you test this and please verify it fixes your issue?

I tried this patch on top of v4.17 but the system fails to boot without detecting root disks by dracut like this:

    [  196.121294] dracut-initqueue[324]: Warning: dracut-initqueue timeout - starting timeout scripts
    [  196.672175] dracut-initqueue[324]: Warning: dracut-initqueue timeout - starting timeout scripts
    [  197.219519] dracut-initqueue[324]: Warning: dracut-initqueue timeout - starting timeout scripts
    [  197.768997] dracut-initqueue[324]: Warning: dracut-initqueue timeout - starting timeout scripts
    [  198.312498] dracut-initqueue[324]: Warning: dracut-initqueue timeout - starting timeout scripts
    [  198.856841] dracut-initqueue[324]: Warning: dracut-initqueue timeout - starting timeout scripts
    [  199.403190] dracut-initqueue[324]: Warning: dracut-initqueue timeout - starting timeout scripts
    [  199.945999] dracut-initqueue[324]: Warning: dracut-initqueue timeout - starting timeout scripts
    [  200.490281] dracut-initqueue[324]: Warning: dracut-initqueue timeout - starting timeout scripts
    [  201.071177] dracut-initqueue[324]: Warning: dracut-initqueue timeout - starting timeout scripts
    [  201.074958] dracut-initqueue[324]: Warning: Could not boot.
	     Starting Setup Virtual Console...
    [  OK  ] Started Setup Virtual Console.
    [  201.245921] audit: type=1130 audit(1528132266.260:12): pid=1 uid=0 auid=4294967295 ses=4294967295 subj=kernel msg='unit=systemd-vconsole-setup comm="systemd" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=success'
	     [  201.256378] audit: type=1131 audit(1528132266.260:13): pid=1 uid=0 auid=4294967295 ses=4294967295 subj=kernel msg='unit=systemd-vconsole-setup comm="systemd" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=success'
    Starting Dracut Emergency Shell...
    Warning: /dev/fedora/root does not exist
    Warning: /dev/fedora/swap does not exist
    Warning: /dev/mapper/fedora-root does not exist

Generating "/run/initramfs/rdsosreport.txt"

Entering emergency mode. Exit the shell to continue.
Type "journalctl" to view system logs.
You might want to save "/run/initramfs/rdsosreport.txt" to a USB stick or /boot after mounting them and attach it to a bug report.

dracut:/# ls /sys/module
8139cp dm_bufio kernel psmouse ttm
8139too dm_mirror keyboard pstore uhci_hcd
8250 dm_mod kgdboc qemu_fw_cfg usbcore
acpi dm_snapshot kgdbts qxl usbhid
acpiphp drm libahci random uv_nmi
ahci drm_kms_helper libata rcupdate virtio
ata_generic dynamic_debug md_mod rcutree virtio_console
ata_piix edac_core mii rng_core virtio_pci
battery ehci_hcd module scsi_mod virtio_ring
block fb mousedev serio_raw vt
button firmware_class pata_acpi sg watchdog
configfs fscrypto pci_hotplug slab_common workqueue
cpufreq hid pcie_aspm spurious xhci_hcd
cpuidle hid_magicmouse pciehp sr_mod xz_dec
crc32c_intel hid_ntrig pcmcia srcutree zswap
cryptomgr i8042 pcmcia_core suspend
debug_core intel_idle printk sysrq
devres ipv6 processor tcp_cubic
dracut:/# lsmod
Module Size Used by
qxl 77824 1
drm_kms_helper 196608 1
ttm 126976 1
drm 454656 4 qxl,ttm
virtio_console 32768 0
ata_generic 16384 0
crc32c_intel 24576 0
8139too 40960 0
virtio_pci 28672 0
8139cp 32768 0
virtio_ring 28672 2 virtio_pci
serio_raw 16384 0
pata_acpi 16384 0
virtio 16384 2 virtio_pci
mii 16384 2 8139too
qemu_fw_cfg 16384 0
dracut:/#

OTOH, there's no issue on the pure v4.17 kernel.

As above, ls /sys/module looks apparently good. But I guess any part of behavior of getdentries() on sysfs must have changed, affecting the disk detection...

Quoted 50 lines

 fs/kernfs/dir.c | 109
++++++++++++++++++++++++++++++++++----------------------
 1 file changed, 67 insertions(+), 42 deletions(-)

fs/kernfs/dir.c +16 -19

index 89d1dc19340b..8148b5fec48d 100644
@@ -1584,53 +1584,75 @@ static int kernfs_dir_fop_release(struct inode *inode,
 struct file *filp)
 	return 0;
 }
 
+static struct kernfs_node *kernfs_dir_next(struct kernfs_node *pos)
+{
+	struct rb_node *node = rb_next(&pos->rb);
+	return node ? rb_to_kn(node) : NULL;
+}
+
 static struct kernfs_node *kernfs_dir_pos(const void *ns,
-	struct kernfs_node *parent, loff_t hash, struct kernfs_node *pos)
+	struct kernfs_node *parent, loff_t off, struct kernfs_node *saved)
 {
-	if (pos) {
-		int valid = kernfs_active(pos) &&
-			pos->parent == parent && hash == pos->hash;
-		kernfs_put(pos);
-		if (!valid)
-			pos = NULL;
-	}
-	if (!pos && (hash > 1) && (hash < INT_MAX)) {
-		struct rb_node *node = parent->dir.children.rb_node;
-		while (node) {
-			pos = rb_to_kn(node);
-
-			if (hash < pos->hash)
-				node = node->rb_left;
-			else if (hash > pos->hash)
-				node = node->rb_right;
-			else
-				break;
+	struct kernfs_node *pos;
+	struct rb_node *node;
+	unsigned int hash;
+	const char *name = "";
+
+	/* Is off a valid name hash? */
+	if ((off < 2) || (off >= INT_MAX))
+		return NULL;
+	hash = off;

fs/kernfs/dir.c static int kernfs_dir_fop_release(struct inode *inode, · line 1611

Quoted 6 lines

fs/kernfs/dir.c +6 -0

+
+	/* Is the saved position usable? */
+	if (saved) {
+		/* Proper parent and hash? */
+		if ((parent != saved->parent) || (saved->hash != hash)) {
+			saved = NULL;

name is uninitialized in this path.

Quoted 24 lines

fs/kernfs/dir.c +16 -5

+		} else {
+			if (kernfs_active(saved))
+				return saved;
+			name = saved->name;
 		}
 	}
-	/* Skip over entries which are dying/dead or in the wrong namespace
 */
-	while (pos && (!kernfs_active(pos) || pos->ns != ns)) {
-		struct rb_node *node = rb_next(&pos->rb);
-		if (!node)
-			pos = NULL;
+
+	/* Find the closest pos to the hash we are looking for */
+	pos = NULL;
+	node = parent->dir.children.rb_node;
+	while (node) {
+		int result;
+
+		pos = rb_to_kn(node);
+		result = kernfs_name_compare(hash, name, ns, pos);
+		if (result < 0)
+			node = node->rb_left;
+		else if (result > 0)

fs/kernfs/dir.c static int kernfs_dir_fop_release(struct inode *inode, · line 1639

Quoted 10 lines

fs/kernfs/dir.c +7 -1

+			node = node->rb_right;
 		else
-			pos = rb_to_kn(node);
+			break;
 	}
+
+	/* Ensure pos is at or beyond the target position */
+	if (pos && (kernfs_name_compare(hash, name, ns, pos) < 0))
+		pos = kernfs_dir_next(pos);
+

Why not trying to find the final target object here?

Looking at the code, I think the operation needed here is to find the smallest active object in the same namespace from the objects larger than the saved object given as argument. The saved object appears to be used as cache. I think dividing the process into kernfs_dir_pos() is not necessary.

I mean like this:

# git diff

fs/kernfs/dir.c +7 -15

index 8148b5f..eeffc8c 100644
@@ -1605,13 +1605,13 @@ static int kernfs_dir_fop_release(struct inode *inode, struct file *filp)
 
        /* Is the saved position usable? */
        if (saved) {
+               name = saved->name;
                /* Proper parent and hash? */
                if ((parent != saved->parent) || (saved->hash != hash)) {
                        saved = NULL;
-               } else {
-                       if (kernfs_active(saved))
-                               return saved;
-                       name = saved->name;
+               } else if (kernfs_active(saved)) {
+                       pos = saved;
+                       goto skip;
                }
        }
 

@@ -1631,22 +1631,14 @@ static int kernfs_dir_fop_release(struct inode *inode, struct file *filp)
                        break;
        }
 
+skip:
        /* Ensure pos is at or beyond the target position */
-       if (pos && (kernfs_name_compare(hash, name, ns, pos) < 0))
+       if (pos && (kernfs_name_compare(hash, name, ns, pos) <= 0))
                pos = kernfs_dir_next(pos);
 
        return pos;
 }
 
-static struct kernfs_node *kernfs_dir_next_pos(const void *ns,
-       struct kernfs_node *parent, loff_t off, struct kernfs_node *start)
-{
-       struct kernfs_node *pos = kernfs_dir_pos(ns, parent, off, start);
-       if (likely(pos == start))
-               pos = kernfs_dir_next(pos);
-       return pos;
-}
-
 static struct kernfs_node *kernfs_dir_skip_pos(const void *ns,
                                               struct kernfs_node *pos)
 {

@@ -1672,7 +1664,7 @@ static int kernfs_fop_readdir(struct file *file, struct dir_context *ctx)
 
        for (pos = kernfs_dir_pos(ns, parent, ctx->pos, saved);
             (pos = kernfs_dir_skip_pos(ns, pos));
-            pos = kernfs_dir_next_pos(ns, parent, ctx->pos, pos)) {
+            pos = kernfs_dir_pos(ns, parent, ctx->pos, pos)) {
                const char *name = pos->name;
                unsigned int type = dt_type(pos);
                int len = strlen(name);

Quoted 80 lines

fs/kernfs/dir.c +13 -11

 	return pos;
 }
 
 static struct kernfs_node *kernfs_dir_next_pos(const void *ns,
-	struct kernfs_node *parent, ino_t ino, struct kernfs_node *pos)
+	struct kernfs_node *parent, loff_t off, struct kernfs_node *start)
 {
-	pos = kernfs_dir_pos(ns, parent, ino, pos);
-	if (pos) {
-		do {
-			struct rb_node *node = rb_next(&pos->rb);
-			if (!node)
-				pos = NULL;
-			else
-				pos = rb_to_kn(node);
-		} while (pos && (!kernfs_active(pos) || pos->ns != ns));
-	}
+	struct kernfs_node *pos = kernfs_dir_pos(ns, parent, off, start);
+	if (likely(pos == start))
+		pos = kernfs_dir_next(pos);
+	return pos;
+}
+
+static struct kernfs_node *kernfs_dir_skip_pos(const void *ns,
+					       struct kernfs_node *pos)
+{
+	/* Skip entries we don't want to return to userspace */
+	while (pos && !(kernfs_active(pos) && (pos->ns == ns)))
+		pos = kernfs_dir_next(pos);
 	return pos;

}

fs/kernfs/dir.c +1 -1

@@ -1638,7 +1660,7 @@ static int kernfs_fop_readdir(struct file *file, struct
 dir_context *ctx)
 {
 	struct dentry *dentry = file->f_path.dentry;
 	struct kernfs_node *parent = kernfs_dentry_node(dentry);
-	struct kernfs_node *pos = file->private_data;
+	struct kernfs_node *pos, *saved = file->private_data;
 	const void *ns = NULL;
 

fs/kernfs/dir.c +8 -5

 	if (!dir_emit_dots(file, ctx))

@@ -1648,23 +1670,26 @@ static int kernfs_fop_readdir(struct file *file,
 struct dir_context *ctx)
 	if (kernfs_ns_enabled(parent))
 		ns = kernfs_info(dentry->d_sb)->ns;
 
-	for (pos = kernfs_dir_pos(ns, parent, ctx->pos, pos);
-	     pos;
+	for (pos = kernfs_dir_pos(ns, parent, ctx->pos, saved);
+	     (pos = kernfs_dir_skip_pos(ns, pos));
 	     pos = kernfs_dir_next_pos(ns, parent, ctx->pos, pos)) {
 		const char *name = pos->name;
 		unsigned int type = dt_type(pos);
 		int len = strlen(name);
 		ino_t ino = pos->id.ino;
 
-		ctx->pos = pos->hash;
-		file->private_data = pos;
-		kernfs_get(pos);
+		kernfs_put(saved);
+		saved = pos;
+		ctx->pos = saved->hash;
+		file->private_data = saved;
+		kernfs_get(saved);
 
 		mutex_unlock(&kernfs_mutex);
 		if (!dir_emit(ctx, name, len, ino, type))
 			return 0;
 		mutex_lock(&kernfs_mutex);
 	}
+	kernfs_put(saved);
 	mutex_unlock(&kernfs_mutex);
 	file->private_data = NULL;

ctx->pos = INT_MAX;
--
2.14.1

Quoted 4 lines · Daisuke

Quoted 1 line

Can you test this and please verify it fixes your issue?

I tried this patch on top of v4.17 but the system fails to boot without detecting root disks by dracut like this:

Quoted 5 lines

OTOH, there's no issue on the pure v4.17 kernel.

As above, ls /sys/module looks apparently good. But I guess any part of behavior of getdentries() on sysfs must have changed, affecting the disk detection...

I haven't been able to reproduce this yet. My test system boots fine. Which fedora are you testing on?

Quoted 50 lines

Quoted 50 lines

 fs/kernfs/dir.c | 109
++++++++++++++++++++++++++++++++++----------------------
 1 file changed, 67 insertions(+), 42 deletions(-)

fs/kernfs/dir.c +16 -19

index 89d1dc19340b..8148b5fec48d 100644
@@ -1584,53 +1584,75 @@ static int kernfs_dir_fop_release(struct inode *inode,
 struct file *filp)
 	return 0;
 }
 
+static struct kernfs_node *kernfs_dir_next(struct kernfs_node *pos)
+{
+	struct rb_node *node = rb_next(&pos->rb);
+	return node ? rb_to_kn(node) : NULL;
+}
+
 static struct kernfs_node *kernfs_dir_pos(const void *ns,
-	struct kernfs_node *parent, loff_t hash, struct kernfs_node *pos)
+	struct kernfs_node *parent, loff_t off, struct kernfs_node *saved)
 {
-	if (pos) {
-		int valid = kernfs_active(pos) &&
-			pos->parent == parent && hash == pos->hash;
-		kernfs_put(pos);
-		if (!valid)
-			pos = NULL;
-	}
-	if (!pos && (hash > 1) && (hash < INT_MAX)) {
-		struct rb_node *node = parent->dir.children.rb_node;
-		while (node) {
-			pos = rb_to_kn(node);
-
-			if (hash < pos->hash)
-				node = node->rb_left;
-			else if (hash > pos->hash)
-				node = node->rb_right;
-			else
-				break;
+	struct kernfs_node *pos;
+	struct rb_node *node;
+	unsigned int hash;
+	const char *name = "";
+
+	/* Is off a valid name hash? */
+	if ((off < 2) || (off >= INT_MAX))
+		return NULL;
+	hash = off;

Quoted 8 lines

fs/kernfs/dir.c static int kernfs_dir_fop_release(struct inode *inode, · line 1611

Quoted 6 lines

fs/kernfs/dir.c +6 -0

+
+	/* Is the saved position usable? */
+	if (saved) {
+		/* Proper parent and hash? */
+		if ((parent != saved->parent) || (saved->hash != hash)) {
+			saved = NULL;

name is uninitialized in this path.

It is. name is initialized to "" see above.

Quoted 24 lines

Quoted 24 lines

fs/kernfs/dir.c +16 -5

+		} else {
+			if (kernfs_active(saved))
+				return saved;
+			name = saved->name;
 		}
 	}
-	/* Skip over entries which are dying/dead or in the wrong namespace
 */
-	while (pos && (!kernfs_active(pos) || pos->ns != ns)) {
-		struct rb_node *node = rb_next(&pos->rb);
-		if (!node)
-			pos = NULL;
+
+	/* Find the closest pos to the hash we are looking for */
+	pos = NULL;
+	node = parent->dir.children.rb_node;
+	while (node) {
+		int result;
+
+		pos = rb_to_kn(node);
+		result = kernfs_name_compare(hash, name, ns, pos);
+		if (result < 0)
+			node = node->rb_left;
+		else if (result > 0)

Quoted 8 lines

fs/kernfs/dir.c static int kernfs_dir_fop_release(struct inode *inode, · line 1637

Quoted 8 lines

fs/kernfs/dir.c +5 -1

+			node = node->rb_right;
 		else
-			pos = rb_to_kn(node);
+			break;
 	}
+
+	/* Ensure pos is at or beyond the target position */
+	if (pos && (kernfs_name_compare(hash, name, ns, pos) < 0))

Quoted 2 lines

fs/kernfs/dir.c static int kernfs_dir_fop_release(struct inode *inode, · line 1639

Quoted 2 lines

fs/kernfs/dir.c +2 -0

+		pos = kernfs_dir_next(pos);
+

-----Original Message-----
From: Eric W. Biederman [mailto:[email protected]]
Sent: Tuesday, June 5, 2018 11:03 AM
To: Hatayama, Daisuke <[email protected]>
Cc: '[email protected]' <[email protected]>; '[email protected]' <[email protected]>; Okajima, Toshiyuki <[email protected]>; [email protected]; '[email protected]' <[email protected]>
Subject: Re: [CFT][PATCH] kernfs: Correct kernfs directory seeks.

Quoted 6 lines · [email protected] (Eric W. Biederman) writes:

Quoted 4 lines · Daisuke

Quoted 1 line

Can you test this and please verify it fixes your issue?

I tried this patch on top of v4.17 but the system fails to boot without detecting root disks by dracut like this:

[snip]

Quoted 6 lines

Quoted 5 lines

OTOH, there's no issue on the pure v4.17 kernel.

As above, ls /sys/module looks apparently good. But I guess any part of behavior of getdentries() on sysfs must have changed, affecting the disk detection...

Quoted 2 lines

I haven't been able to reproduce this yet. My test system boots fine. Which fedora are you testing on?

I reproduced something similar and fedora 28. So I think I have found and fixed the issue. I believe I simply reversed the test at the end of kernfs_dir_pos. AKA "<" instead of ">".

I am going to see if I can test my changes more throughly on this side and then repost.

Quoted 9 lines

Quoted 9 lines

fs/kernfs/dir.c static int kernfs_dir_fop_release(struct inode

Quoted 9 lines

 fs/kernfs/dir.c | 109
++++++++++++++++++++++++++++++++++----------------------
 1 file changed, 67 insertions(+), 42 deletions(-)

fs/kernfs/dir.c +0 -0

index 89d1dc19340b..8148b5fec48d 100644
@@ -1584,53 +1584,75 @@ static int kernfs_dir_fop_release(struct inode

*inode,

Quoted 76 lines

Quoted 41 lines

Quoted 41 lines

fs/kernfs/dir.c +16 -19

 struct file *filp)
 	return 0;
 }
 
+static struct kernfs_node *kernfs_dir_next(struct kernfs_node *pos)
+{
+	struct rb_node *node = rb_next(&pos->rb);
+	return node ? rb_to_kn(node) : NULL;
+}
+
 static struct kernfs_node *kernfs_dir_pos(const void *ns,
-	struct kernfs_node *parent, loff_t hash, struct kernfs_node *pos)
+	struct kernfs_node *parent, loff_t off, struct kernfs_node *saved)
 {
-	if (pos) {
-		int valid = kernfs_active(pos) &&
-			pos->parent == parent && hash == pos->hash;
-		kernfs_put(pos);
-		if (!valid)
-			pos = NULL;
-	}
-	if (!pos && (hash > 1) && (hash < INT_MAX)) {
-		struct rb_node *node = parent->dir.children.rb_node;
-		while (node) {
-			pos = rb_to_kn(node);
-
-			if (hash < pos->hash)
-				node = node->rb_left;
-			else if (hash > pos->hash)
-				node = node->rb_right;
-			else
-				break;
+	struct kernfs_node *pos;
+	struct rb_node *node;
+	unsigned int hash;
+	const char *name = "";
+
+	/* Is off a valid name hash? */
+	if ((off < 2) || (off >= INT_MAX))
+		return NULL;
+	hash = off;

Quoted 8 lines

fs/kernfs/dir.c static int kernfs_dir_fop_release(struct inode · line 1611

Quoted 6 lines

fs/kernfs/dir.c +6 -0

+
+	/* Is the saved position usable? */
+	if (saved) {
+		/* Proper parent and hash? */
+		if ((parent != saved->parent) || (saved->hash != hash)) {
+			saved = NULL;

name is uninitialized in this path.

It is. name is initialized to "" see above.

Quoted 24 lines

Quoted 24 lines

fs/kernfs/dir.c +16 -5

+		} else {
+			if (kernfs_active(saved))
+				return saved;
+			name = saved->name;
 		}
 	}
-	/* Skip over entries which are dying/dead or in the wrong namespace
 */
-	while (pos && (!kernfs_active(pos) || pos->ns != ns)) {
-		struct rb_node *node = rb_next(&pos->rb);
-		if (!node)
-			pos = NULL;
+
+	/* Find the closest pos to the hash we are looking for */
+	pos = NULL;
+	node = parent->dir.children.rb_node;
+	while (node) {
+		int result;
+
+		pos = rb_to_kn(node);
+		result = kernfs_name_compare(hash, name, ns, pos);
+		if (result < 0)
+			node = node->rb_left;
+		else if (result > 0)

Quoted 8 lines

Quoted 8 lines

fs/kernfs/dir.c static int kernfs_dir_fop_release(struct inode · line 1637

Quoted 8 lines

fs/kernfs/dir.c +5 -1

+			node = node->rb_right;
 		else
-			pos = rb_to_kn(node);
+			break;
 	}
+
+	/* Ensure pos is at or beyond the target position */
+	if (pos && (kernfs_name_compare(hash, name, ns, pos) < 0))

^^^^^^^^^^^^^^^^
should be > 0

Quoted 2 lines

Quoted 2 lines

fs/kernfs/dir.c static int kernfs_dir_fop_release(struct inode · line 1639

Quoted 2 lines

fs/kernfs/dir.c +2 -0

+		pos = kernfs_dir_next(pos);
+

Eric

-----Original Message-----
From: Eric W. Biederman [mailto:[email protected]]
Sent: Monday, June 4, 2018 11:45 PM
To: Hatayama, Daisuke <[email protected]>
Cc: '[email protected]' <[email protected]>; '[email protected]' <[email protected]>; Okajima, Toshiyuki <[email protected]>; [email protected]; '[email protected]' <[email protected]>
Subject: Re: [CFT][PATCH] kernfs: Correct kernfs directory seeks.

Quoted 52 lines · Daisuke

Hello,

Thanks for this patch, Eric.

Quoted 48 lines

-----Original Message-----
From: Eric W. Biederman [mailto:[email protected]]
Sent: Monday, June 4, 2018 3:51 AM
To: Hatayama, Daisuke <[email protected]>
Cc: '[email protected]' <[email protected]>; '[email protected]' <[email protected]>; Okajima, Toshiyuki <[email protected]>; [email protected]; '[email protected]' <[email protected]>
Subject: [CFT][PATCH] kernfs: Correct kernfs directory seeks.

Over the years two bugs have crept into the code that handled the last returned kernfs directory being deleted, and handled general seeking in kernfs directories. The result is that reading the /sys/module directory while moduled are loading and unloading will sometimes skip over a module that was present for the entire duration of the directory read.

The first bug is that kernfs_dir_next_pos was advancing the position in the directory even when kernfs_dir_pos had found the starting kernfs directory entry was not usable. In this case kernfs_dir_pos is guaranteed to return the directory entry past the starting directory entry, making it so that advancing the directory position is wrong.

The second bug is that kernfs_dir_pos when the saved kernfs_node did not validate, was not always returning a directory after the the target position, but was sometimes returning the directory entry just before the target position.

To correct these issues and to make the code easily readable and comprehensible several cleanups are made.

- A function kernfs_dir_next is factored out to make it straight-forward to find the next node in a kernfs directory.

- A function kernfs_dir_skip_pos is factored out to skip over directory entries that should not be shown to userspace. This function is called from kernfs_fop_readdir directly removing the complication of calling it from the other directory advancement functions.

- The kernfs_put of the saved directory entry is moved from kernfs_dir_pos to kernfs_fop_readdir. Keeping it with the kernfs_get when it is saved and ensuring the directory position advancment functions can reference the saved node without complications.

- The function kernfs_dir_next_pos is modified to only advance the directory position in the common case when kernfs_dir_pos validates and returns the starting kernfs node. For all other cases kernfs_dir_pos is

guaranteed

Quoted 1 line

Quoted 1 line

to return a directory position in advance of the starting directory

position.

Quoted 2 lines

Quoted 2 lines

- kernfs_dir_pos is given a significant make over. It's essense remains

the

Quoted 6 lines

Quoted 6 lines

same but some siginficant changes were made.

+ The offset is validated to be a valid hash value at the very beginning. The validation is updated to handle the fact that neither 0 nor 1 are valid hash values.

Quoted 6 lines

Quoted 6 lines

+ An extra test is added at the end of the rbtree lookup to ensure the returned position is at or beyond the target position.

+ kernfs_name_compare is used during the rbtree lookup instead of just
comparing
the hash. This allows the the passed namespace parameter to be used,

and

Quoted 16 lines

Quoted 16 lines

if it is available from the saved entry the old saved name as well.

+ The validation of the saved kernfs node now checks for two cases. If the saved node is returnable.
If the name of the saved node is usable during lookup.

In net this should result in a easier to understand, and more correct version of directory traversal for kernfs directories.

Reported-by "Hatayama, Daisuke" <[email protected]>

Fixes a406f75840e1 ("sysfs: use rb-tree for inode number lookup")

Fixes 1e5289c97bba ("sysfs: Cache the last sysfs_dirent to improve readdir

scalability v2")

Signed-off-by "Eric W. Biederman" <[email protected]>

---

Quoted 7 lines

Quoted 2 lines

Can you test this and please verify it fixes your issue?

I tried this patch on top of v4.17 but the system fails to boot without detecting root disks by dracut like this:

    [  196.121294] dracut-initqueue[324]: Warning: dracut-initqueue

timeout - starting timeout scripts

Quoted 1 line

    [  196.672175] dracut-initqueue[324]: Warning: dracut-initqueue

timeout - starting timeout scripts

Quoted 1 line

    [  197.219519] dracut-initqueue[324]: Warning: dracut-initqueue

timeout - starting timeout scripts

Quoted 1 line

    [  197.768997] dracut-initqueue[324]: Warning: dracut-initqueue

timeout - starting timeout scripts

Quoted 1 line

    [  198.312498] dracut-initqueue[324]: Warning: dracut-initqueue

timeout - starting timeout scripts

Quoted 1 line

    [  198.856841] dracut-initqueue[324]: Warning: dracut-initqueue

timeout - starting timeout scripts

Quoted 1 line

    [  199.403190] dracut-initqueue[324]: Warning: dracut-initqueue

timeout - starting timeout scripts

Quoted 1 line

    [  199.945999] dracut-initqueue[324]: Warning: dracut-initqueue

timeout - starting timeout scripts

Quoted 1 line

    [  200.490281] dracut-initqueue[324]: Warning: dracut-initqueue

timeout - starting timeout scripts

Quoted 1 line

    [  201.071177] dracut-initqueue[324]: Warning: dracut-initqueue

timeout - starting timeout scripts

Quoted 4 lines

    [  201.074958] dracut-initqueue[324]: Warning: Could not boot.
	     Starting Setup Virtual Console...
    [  OK  ] Started Setup Virtual Console.
    [  201.245921] audit: type=1130 audit(1528132266.260:12): pid=1 uid=0

auid=4294967295 ses=4294967295 subj=kernel msg='unit=systemd-vconsole-setup comm="systemd" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=success'

Quoted 1 line

	     [  201.256378] audit: type=1131 audit(1528132266.260:13):

pid=1 uid=0 auid=4294967295 ses=4294967295 subj=kernel msg='unit=systemd-vconsole-setup comm="systemd" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=success'

Quoted 11 lines

Starting Dracut Emergency Shell...
Warning: /dev/fedora/root does not exist
Warning: /dev/fedora/swap does not exist
Warning: /dev/mapper/fedora-root does not exist

Generating "/run/initramfs/rdsosreport.txt"

Entering emergency mode. Exit the shell to continue.
Type "journalctl" to view system logs.
You might want to save "/run/initramfs/rdsosreport.txt" to a USB stick

or /boot

Quoted 11 lines

after mounting them and attach it to a bug report.

dracut:/# ls /sys/module
8139cp dm_bufio kernel psmouse ttm
8139too dm_mirror keyboard pstore uhci_hcd
8250 dm_mod kgdboc qemu_fw_cfg usbcore
acpi dm_snapshot kgdbts qxl usbhid
acpiphp drm libahci random uv_nmi
ahci drm_kms_helper libata rcupdate virtio
ata_generic dynamic_debug md_mod rcutree

virtio_console

Quoted 31 lines

    ata_piix      edac_core       mii          rng_core     virtio_pci
    battery       ehci_hcd        module       scsi_mod     virtio_ring
    block         fb              mousedev     serio_raw    vt
    button        firmware_class  pata_acpi    sg           watchdog
    configfs      fscrypto        pci_hotplug  slab_common  workqueue
    cpufreq       hid             pcie_aspm    spurious     xhci_hcd
    cpuidle       hid_magicmouse  pciehp       sr_mod       xz_dec
    crc32c_intel  hid_ntrig       pcmcia       srcutree     zswap
    cryptomgr     i8042           pcmcia_core  suspend
    debug_core    intel_idle      printk       sysrq
    devres        ipv6            processor    tcp_cubic
    dracut:/# lsmod
    Module                  Size  Used by
    qxl                    77824  1
    drm_kms_helper        196608  1
    ttm                   126976  1
    drm                   454656  4 qxl,ttm
    virtio_console         32768  0
    ata_generic            16384  0
    crc32c_intel           24576  0
    8139too                40960  0
    virtio_pci             28672  0
    8139cp                 32768  0
    virtio_ring            28672  2 virtio_pci
    serio_raw              16384  0
    pata_acpi              16384  0
    virtio                 16384  2 virtio_pci
    mii                    16384  2 8139too
    qemu_fw_cfg            16384  0
    dracut:/#

Quoted 5 lines

OTOH, there's no issue on the pure v4.17 kernel.

As above, ls /sys/module looks apparently good. But I guess any part of behavior of getdentries() on sysfs must have changed, affecting the disk detection...

I haven't been able to reproduce this yet. My test system boots fine. Which fedora are you testing on?

Quoted 9 lines

fs/kernfs/dir.c static int kernfs_dir_fop_release(struct inode

Quoted 9 lines

 fs/kernfs/dir.c | 109
++++++++++++++++++++++++++++++++++----------------------
 1 file changed, 67 insertions(+), 42 deletions(-)

fs/kernfs/dir.c +0 -0

index 89d1dc19340b..8148b5fec48d 100644
@@ -1584,53 +1584,75 @@ static int kernfs_dir_fop_release(struct inode

*inode,

Quoted 41 lines

Quoted 41 lines

fs/kernfs/dir.c +16 -19

 struct file *filp)
 	return 0;
 }
 
+static struct kernfs_node *kernfs_dir_next(struct kernfs_node *pos)
+{
+	struct rb_node *node = rb_next(&pos->rb);
+	return node ? rb_to_kn(node) : NULL;
+}
+
 static struct kernfs_node *kernfs_dir_pos(const void *ns,
-	struct kernfs_node *parent, loff_t hash, struct kernfs_node *pos)
+	struct kernfs_node *parent, loff_t off, struct kernfs_node *saved)
 {
-	if (pos) {
-		int valid = kernfs_active(pos) &&
-			pos->parent == parent && hash == pos->hash;
-		kernfs_put(pos);
-		if (!valid)
-			pos = NULL;
-	}
-	if (!pos && (hash > 1) && (hash < INT_MAX)) {
-		struct rb_node *node = parent->dir.children.rb_node;
-		while (node) {
-			pos = rb_to_kn(node);
-
-			if (hash < pos->hash)
-				node = node->rb_left;
-			else if (hash > pos->hash)
-				node = node->rb_right;
-			else
-				break;
+	struct kernfs_node *pos;
+	struct rb_node *node;
+	unsigned int hash;
+	const char *name = "";
+
+	/* Is off a valid name hash? */
+	if ((off < 2) || (off >= INT_MAX))
+		return NULL;
+	hash = off;

Quoted 8 lines

fs/kernfs/dir.c static int kernfs_dir_fop_release(struct inode · line 1611

Quoted 6 lines

fs/kernfs/dir.c +6 -0

+
+	/* Is the saved position usable? */
+	if (saved) {
+		/* Proper parent and hash? */
+		if ((parent != saved->parent) || (saved->hash != hash)) {
+			saved = NULL;

name is uninitialized in this path.

It is. name is initialized to "" see above.

Quoted 37 lines

Quoted 24 lines

fs/kernfs/dir.c +16 -5

+		} else {
+			if (kernfs_active(saved))
+				return saved;
+			name = saved->name;
 		}
 	}
-	/* Skip over entries which are dying/dead or in the wrong namespace
 */
-	while (pos && (!kernfs_active(pos) || pos->ns != ns)) {
-		struct rb_node *node = rb_next(&pos->rb);
-		if (!node)
-			pos = NULL;
+
+	/* Find the closest pos to the hash we are looking for */
+	pos = NULL;
+	node = parent->dir.children.rb_node;
+	while (node) {
+		int result;
+
+		pos = rb_to_kn(node);
+		result = kernfs_name_compare(hash, name, ns, pos);
+		if (result < 0)
+			node = node->rb_left;
+		else if (result > 0)

fs/kernfs/dir.c static int kernfs_dir_fop_release(struct inode · line 1639

Quoted 10 lines

fs/kernfs/dir.c +7 -1

+			node = node->rb_right;
 		else
-			pos = rb_to_kn(node);
+			break;
 	}
+
+	/* Ensure pos is at or beyond the target position */
+	if (pos && (kernfs_name_compare(hash, name, ns, pos) < 0))
+		pos = kernfs_dir_next(pos);
+

Why not trying to find the final target object here?

Quoted 5 lines

Looking at the code, I think the operation needed here is to find the smallest active object in the same namespace from the objects larger than the saved object given as argument. The saved object appears to be used as cache. I think dividing the process into kernfs_dir_pos() is not necessary.

What you are suggesting below is wrong when restarting readdir.

Ordinarily saved is the next entry we are going to return from readdir. When dir_emit does not succeed we stop returnning entries and when we restart readdir we need to attempt dir_emit on saved again.

Always advancing past saved will cause us to skip saved in the event a single readdir is not enough.

The restart (kernfs_dir_pos) must return saved or if saved is now gone, the first entry past saved.

Saved dying in the middle is what I believe caused the original issue.

Making all of this clear is part of the reason I moved the kernfs_dir_skip_pos logic into it's own function.

Eric

fs/kernfs/dir.c static int kernfs_dir_fop_release(struct inode

Quoted 10 lines

I mean like this:

# git diff

fs/kernfs/dir.c +0 -0

index 8148b5f..eeffc8c 100644
@@ -1605,13 +1605,13 @@ static int kernfs_dir_fop_release(struct inode

*inode, struct file *filp)

fs/kernfs/dir.c static int kernfs_dir_fop_release(struct inode

Quoted 18 lines

fs/kernfs/dir.c +4 -4

        /* Is the saved position usable? */
        if (saved) {
+               name = saved->name;
                /* Proper parent and hash? */
                if ((parent != saved->parent) || (saved->hash != hash)) {
                        saved = NULL;
-               } else {
-                       if (kernfs_active(saved))
-                               return saved;
-                       name = saved->name;
+               } else if (kernfs_active(saved)) {
+                       pos = saved;
+                       goto skip;
                }
        }
 

@@ -1631,22 +1631,14 @@ static int kernfs_dir_fop_release(struct inode

*inode, struct file *filp)

fs/kernfs/dir.c static int kernfs_fop_readdir(struct file *file, struct

Quoted 25 lines

fs/kernfs/dir.c +2 -10

                        break;
        }
 
+skip:
        /* Ensure pos is at or beyond the target position */
-       if (pos && (kernfs_name_compare(hash, name, ns, pos) < 0))
+       if (pos && (kernfs_name_compare(hash, name, ns, pos) <= 0))
                pos = kernfs_dir_next(pos);
 
        return pos;
 }
 
-static struct kernfs_node *kernfs_dir_next_pos(const void *ns,
-       struct kernfs_node *parent, loff_t off, struct kernfs_node *start)
-{
-       struct kernfs_node *pos = kernfs_dir_pos(ns, parent, off, start);
-       if (likely(pos == start))
-               pos = kernfs_dir_next(pos);
-       return pos;
-}
-
 static struct kernfs_node *kernfs_dir_skip_pos(const void *ns,
                                               struct kernfs_node *pos)
 {

@@ -1672,7 +1664,7 @@ static int kernfs_fop_readdir(struct file *file, struct

dir_context *ctx)

Quoted 41 lines

fs/kernfs/dir.c +1 -1

        for (pos = kernfs_dir_pos(ns, parent, ctx->pos, saved);
             (pos = kernfs_dir_skip_pos(ns, pos));
-            pos = kernfs_dir_next_pos(ns, parent, ctx->pos, pos)) {
+            pos = kernfs_dir_pos(ns, parent, ctx->pos, pos)) {
                const char *name = pos->name;
                unsigned int type = dt_type(pos);
                int len = strlen(name);
 

Quoted 32 lines

fs/kernfs/dir.c +13 -11

 	return pos;
 }
 
 static struct kernfs_node *kernfs_dir_next_pos(const void *ns,
-	struct kernfs_node *parent, ino_t ino, struct kernfs_node *pos)
+	struct kernfs_node *parent, loff_t off, struct kernfs_node *start)
 {
-	pos = kernfs_dir_pos(ns, parent, ino, pos);
-	if (pos) {
-		do {
-			struct rb_node *node = rb_next(&pos->rb);
-			if (!node)
-				pos = NULL;
-			else
-				pos = rb_to_kn(node);
-		} while (pos && (!kernfs_active(pos) || pos->ns != ns));
-	}
+	struct kernfs_node *pos = kernfs_dir_pos(ns, parent, off, start);
+	if (likely(pos == start))
+		pos = kernfs_dir_next(pos);
+	return pos;
+}
+
+static struct kernfs_node *kernfs_dir_skip_pos(const void *ns,
+					       struct kernfs_node *pos)
+{
+	/* Skip entries we don't want to return to userspace */
+	while (pos && !(kernfs_active(pos) && (pos->ns == ns)))
+		pos = kernfs_dir_next(pos);
 	return pos;

}

Quoted 1 line

fs/kernfs/dir.c static int kernfs_fop_readdir(struct file *file,

Quoted 1 line

fs/kernfs/dir.c +0 -0

@@ -1638,7 +1660,7 @@ static int kernfs_fop_readdir(struct file *file,

struct

Quoted 46 lines

Quoted 46 lines

fs/kernfs/dir.c +1 -1

 dir_context *ctx)
 {
 	struct dentry *dentry = file->f_path.dentry;
 	struct kernfs_node *parent = kernfs_dentry_node(dentry);
-	struct kernfs_node *pos = file->private_data;
+	struct kernfs_node *pos, *saved = file->private_data;
 	const void *ns = NULL;
 

fs/kernfs/dir.c +8 -5

 	if (!dir_emit_dots(file, ctx))

@@ -1648,23 +1670,26 @@ static int kernfs_fop_readdir(struct file *file,
 struct dir_context *ctx)
 	if (kernfs_ns_enabled(parent))
 		ns = kernfs_info(dentry->d_sb)->ns;
 
-	for (pos = kernfs_dir_pos(ns, parent, ctx->pos, pos);
-	     pos;
+	for (pos = kernfs_dir_pos(ns, parent, ctx->pos, saved);
+	     (pos = kernfs_dir_skip_pos(ns, pos));
 	     pos = kernfs_dir_next_pos(ns, parent, ctx->pos, pos)) {
 		const char *name = pos->name;
 		unsigned int type = dt_type(pos);
 		int len = strlen(name);
 		ino_t ino = pos->id.ino;
 
-		ctx->pos = pos->hash;
-		file->private_data = pos;
-		kernfs_get(pos);
+		kernfs_put(saved);
+		saved = pos;
+		ctx->pos = saved->hash;
+		file->private_data = saved;
+		kernfs_get(saved);
 
 		mutex_unlock(&kernfs_mutex);
 		if (!dir_emit(ctx, name, len, ino, type))
 			return 0;
 		mutex_lock(&kernfs_mutex);
 	}
+	kernfs_put(saved);
 	mutex_unlock(&kernfs_mutex);
 	file->private_data = NULL;

ctx->pos = INT_MAX;
--
2.14.1

Quoted 9 lines

Quoted 8 lines

Review

Quoted 6 lines

(quoted diff) +6 -0

+
+	/* Is the saved position usable? */
+	if (saved) {
+		/* Proper parent and hash? */
+		if ((parent != saved->parent) || (saved->hash != hash)) {
+			saved = NULL;

name is uninitialized in this path.

Quoted 2 lines

It is. name is initialized to "" see above.

Or when either of the conditions is true, it has resulted in some inconsistent state, right? So, why not terminating this session of readdir() immediately by
returning NULL just as when off is turned out to be invalid?

What I have above is not the clearest, and in fact the logic could be better.

The fundamental challenge is because hash collisions are possible a file offset does not hold complete position information in a directory.

So the kernfs node that is to be read/displayed next is saved in the struct file. The it is tested if the saved kernfs node is usable for finding the location in the directory. Several things may have gone wrong.

- Someone may have called seekdir.
- The saved kernfs node may have been renamed.
- The saved kernfs node may have been moved to a different directory in kernfs.
- the saved kernfs node may have been deleted.

If any of those are true the code needs to do the rbtree lookup.

Hello,

Quoted 17 lines · Eric W. Biederman

What I have above is not the clearest, and in fact the logic could be better.

The fundamental challenge is because hash collisions are possible a file offset does not hold complete position information in a directory.

So the kernfs node that is to be read/displayed next is saved in the struct file. The it is tested if the saved kernfs node is usable for finding the location in the directory. Several things may have gone wrong.

- Someone may have called seekdir.
- The saved kernfs node may have been renamed.
- The saved kernfs node may have been moved to a different directory in kernfs.
- the saved kernfs node may have been deleted.

Quoted 1 line

If any of those are true the code needs to do the rbtree lookup.

So, given that the whole thing is protected by a mutex which protects modifications, it could be an option to simply keep track of who's iterating what and shift them on removals. IOW, always keep cursor pointing to the next thing to visit and if that gets removed shift the cursor to the next one.

Sigh, I have found another issue with kernfs_fop_readdir.

We are not currently protecting file->private_data with the kernfs_mutex or any other kind of serialization. Which means if two processes are calling readdir on the same file descriptor we might get unpredictable behavior.

It doesn't look too bad and easy enough to fix, but definitely something to be watchful of.