fs, proc: don't guard /proc/<pid>/task/<tid>/children on CONFIG_CHECKPOINT_RESTORE

From: Alban Crequy <alban-973cpzSjLbNWk0Htik3J/[email protected]>

commit 818411616baf ("fs, proc: introduce /proc/<pid>/task/<tid>/children entry") introduced the children entry for checkpoint restore and the file is only available on kernels configured with CONFIG_EXPERT and CONFIG_CHECKPOINT_RESTORE.

This is available in most distributions (Fedora, Debian, Ubuntu, CoreOS) because they usually enable CONFIG_EXPERT and CONFIG_CHECKPOINT_RESTORE. But Arch does not enable CONFIG_EXPERT or CONFIG_CHECKPOINT_RESTORE.

However, the children proc file is useful outside of checkpoint restore. I would like to use it in rkt. The rkt process exec() another program it does not control, and that other program will fork()+exec() a child process. I would like to find the pid of the child process from an external tool without iterating in /proc over all processes to find which one has a parent pid equal to rkt.

Since the children proc file is useful outside of checkpoint-restore, I am removing the guard on CONFIG_CHECKPOINT_RESTORE.

---
 fs/proc/array.c | 2 --
 fs/proc/base.c  | 2 --
 2 files changed, 4 deletions(-)

index fd02a9e..6edec57 100644
@@ -569,7 +569,6 @@ int proc_pid_statm(struct seq_file *m, struct pid_namespace *ns,
 	return 0;
 }
 
-#ifdef CONFIG_CHECKPOINT_RESTORE
 static struct pid *
 get_children_pid(struct inode *inode, struct pid *pid_prev, loff_t pos)
 {

@@ -692,4 +691,3 @@ const struct file_operations proc_tid_children_operations = {
 	.llseek  = seq_lseek,
 	.release = children_seq_release,
 };
-#endif /* CONFIG_CHECKPOINT_RESTORE */

index 093ca14..b743007 100644
@@ -2922,9 +2922,7 @@ static const struct pid_entry tid_base_stuff[] = {
 	ONE("stat",      S_IRUGO, proc_tid_stat),
 	ONE("statm",     S_IRUGO, proc_pid_statm),
 	REG("maps",      S_IRUGO, proc_tid_maps_operations),
-#ifdef CONFIG_CHECKPOINT_RESTORE
 	REG("children",  S_IRUGO, proc_tid_children_operations),
-#endif
 #ifdef CONFIG_NUMA
 	REG("numa_maps", S_IRUGO, proc_tid_numa_maps_operations),
 #endif

2.1.4

This will add a lump of code to kernels which don't need it.

It's a bit of a pain, but I suppose we should still keep the presence of get_children_pid() configurable. That would be by adding a new CONFIG_PROC_CHILDREN (or similar) and making CONFIG_CHECKPOINT_RESTORE select that.

commit 818411616baf ("fs, proc: introduce /proc/<pid>/task/<tid>/children entry") introduced the children entry for checkpoint restore and the file is only available on kernels configured with CONFIG_EXPERT and CONFIG_CHECKPOINT_RESTORE.

This is available in most distributions (Fedora, Debian, Ubuntu, CoreOS) because they usually enable CONFIG_EXPERT and CONFIG_CHECKPOINT_RESTORE. But Arch does not enable CONFIG_EXPERT or CONFIG_CHECKPOINT_RESTORE.

However, the children proc file is useful outside of checkpoint restore. I would like to use it in rkt. The rkt process exec() another program it does not control, and that other program will fork()+exec() a child process. I would like to find the pid of the child process from an external tool without iterating in /proc over all processes to find which one has a parent pid equal to rkt.

This commit introduces CONFIG_PROC_CHILDREN and makes CONFIG_CHECKPOINT_RESTORE depend on it. This allows enabling /proc/<pid>/task/<tid>/children without needing to enable CONFIG_CHECKPOINT_RESTORE and CONFIG_EXPERT.

Alban tested that /proc/<pid>/task/<tid>/children is present when the kernel is configured with CONFIG_PROC_CHILDREN=y but without CONFIG_CHECKPOINT_RESTORE

---
 fs/proc/Kconfig | 4 ++++
 fs/proc/array.c | 4 ++--
 fs/proc/base.c  | 2 +-
 init/Kconfig    | 1 +
 4 files changed, 8 insertions(+), 3 deletions(-)

index 2183fcf..d751fcb 100644
@@ -71,3 +71,7 @@ config PROC_PAGE_MONITOR
 	  /proc/pid/smaps, /proc/pid/clear_refs, /proc/pid/pagemap,
 	  /proc/kpagecount, and /proc/kpageflags. Disabling these
           interfaces will reduce the size of the kernel by approximately 4kb.
+
+config PROC_CHILDREN
+	bool "Include /proc/<pid>/task/<tid>/children file"
+	default n

index fd02a9e..ddc4b2c 100644
@@ -569,7 +569,7 @@ int proc_pid_statm(struct seq_file *m, struct pid_namespace *ns,
 	return 0;
 }
 
-#ifdef CONFIG_CHECKPOINT_RESTORE
+#ifdef CONFIG_PROC_CHILDREN
 static struct pid *
 get_children_pid(struct inode *inode, struct pid *pid_prev, loff_t pos)
 {

@@ -692,4 +692,4 @@ const struct file_operations proc_tid_children_operations = {
 	.llseek  = seq_lseek,
 	.release = children_seq_release,
 };
-#endif /* CONFIG_CHECKPOINT_RESTORE */
+#endif /* CONFIG_PROC_CHILDREN */

index 093ca14..5477a4a 100644
@@ -2922,7 +2922,7 @@ static const struct pid_entry tid_base_stuff[] = {
 	ONE("stat",      S_IRUGO, proc_tid_stat),
 	ONE("statm",     S_IRUGO, proc_pid_statm),
 	REG("maps",      S_IRUGO, proc_tid_maps_operations),
-#ifdef CONFIG_CHECKPOINT_RESTORE
+#ifdef CONFIG_PROC_CHILDREN
 	REG("children",  S_IRUGO, proc_tid_children_operations),
 #endif
 #ifdef CONFIG_NUMA

index dc24dec..daa7941 100644
@@ -1145,6 +1145,7 @@ endif # CGROUPS
 
 config CHECKPOINT_RESTORE
 	bool "Checkpoint/restore support" if EXPERT
+	depends on PROC_CHILDREN
 	default n
 	help
 	  Enables additional kernel features in a sake of checkpoint/restore.

2.4.1

--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html

From the linked thread, it seems that it was just lost... The discussion ended up about task/<tid> and /proc/<pid>... That patch should have been applied.

BTW this patch adds the CONFIG guard that was requested by Andrew. There is a v3 coming to make it "select" as suggested.

Thanks!

Djalal Harouni
http://opendz.org
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html

commit 818411616baf ("fs, proc: introduce /proc/<pid>/task/<tid>/children entry") introduced the children entry for checkpoint restore and the file is only available on kernels configured with CONFIG_EXPERT and CONFIG_CHECKPOINT_RESTORE.

This is available in most distributions (Fedora, Debian, Ubuntu, CoreOS) because they usually enable CONFIG_EXPERT and CONFIG_CHECKPOINT_RESTORE. But Arch does not enable CONFIG_EXPERT or CONFIG_CHECKPOINT_RESTORE.

However, the children proc file is useful outside of checkpoint restore. I would like to use it in rkt. The rkt process exec() another program it does not control, and that other program will fork()+exec() a child process. I would like to find the pid of the child process from an external tool without iterating in /proc over all processes to find which one has a parent pid equal to rkt.

This commit introduces CONFIG_PROC_CHILDREN and makes CONFIG_CHECKPOINT_RESTORE select it. This allows enabling /proc/<pid>/task/<tid>/children without needing to enable CONFIG_CHECKPOINT_RESTORE and CONFIG_EXPERT.

Alban tested that /proc/<pid>/task/<tid>/children is present when the kernel is configured with CONFIG_PROC_CHILDREN=y but without CONFIG_CHECKPOINT_RESTORE

v3: change depend -> select
v2: introduce CONFIG_PROC_CHILDREN http://marc.info/?l=linux-fsdevel&m=143229890217280&w=2
v1: http://marc.info/?l=linux-api&m=143220431121869&w=2

---
 fs/proc/Kconfig | 4 ++++
 fs/proc/array.c | 4 ++--
 fs/proc/base.c  | 2 +-
 init/Kconfig    | 1 +
 4 files changed, 8 insertions(+), 3 deletions(-)

index 2183fcf..d751fcb 100644
@@ -71,3 +71,7 @@ config PROC_PAGE_MONITOR
 	  /proc/pid/smaps, /proc/pid/clear_refs, /proc/pid/pagemap,
 	  /proc/kpagecount, and /proc/kpageflags. Disabling these
           interfaces will reduce the size of the kernel by approximately 4kb.
+
+config PROC_CHILDREN
+	bool "Include /proc/<pid>/task/<tid>/children file"
+	default n

index fd02a9e..ddc4b2c 100644
@@ -569,7 +569,7 @@ int proc_pid_statm(struct seq_file *m, struct pid_namespace *ns,
 	return 0;
 }
 
-#ifdef CONFIG_CHECKPOINT_RESTORE
+#ifdef CONFIG_PROC_CHILDREN
 static struct pid *
 get_children_pid(struct inode *inode, struct pid *pid_prev, loff_t pos)
 {

@@ -692,4 +692,4 @@ const struct file_operations proc_tid_children_operations = {
 	.llseek  = seq_lseek,
 	.release = children_seq_release,
 };
-#endif /* CONFIG_CHECKPOINT_RESTORE */
+#endif /* CONFIG_PROC_CHILDREN */

index 093ca14..5477a4a 100644
@@ -2922,7 +2922,7 @@ static const struct pid_entry tid_base_stuff[] = {
 	ONE("stat",      S_IRUGO, proc_tid_stat),
 	ONE("statm",     S_IRUGO, proc_pid_statm),
 	REG("maps",      S_IRUGO, proc_tid_maps_operations),
-#ifdef CONFIG_CHECKPOINT_RESTORE
+#ifdef CONFIG_PROC_CHILDREN
 	REG("children",  S_IRUGO, proc_tid_children_operations),
 #endif
 #ifdef CONFIG_NUMA

index dc24dec..fa09e5e 100644
@@ -1145,6 +1145,7 @@ endif # CGROUPS
 
 config CHECKPOINT_RESTORE
 	bool "Checkpoint/restore support" if EXPERT
+	select PROC_CHILDREN
 	default n
 	help
 	  Enables additional kernel features in a sake of checkpoint/restore.

2.4.1

--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html

On Tue, May 26, 2015 at 05:12:42PM +0200, Iago López Galeiras wrote:
...

Looks ok to me, thanks.

I sent an essentially identical patch a couple years ago, and it got some interesting comments:

http://lkml.kernel.org/g/0e00e9073855c02a382d49ba1ede9c4fda3451b7.1372189875.git.luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org

See also:

http://lkml.kernel.org/g/5f9a6b3ab75b12f2c5ba61ea1f6f3b08e9952b55.1372280661.git.luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org

--Andy

Oh, it is 2k13, I thought it's merged already ;)

Thanks for the pointer, I didn't see it.

Your paragraph seems good to have in the documentation but I am also not qualified to comment on the behavior.