uprobe: add support for overlayfs

uprobes cannot successfully attach to binaries located in a directory mounted with overlayfs.

To verify, create directories for mounting overlayfs (upper,lower,work,merge), move some binary into merge/ and use readelf to obtain some known instruction of the binary. I used /bin/true and the entry instruction(0x13b0):

	$ mount -t overlay overlay -o lowerdir=lower,upperdir=upper,workdir=work merge
	$ cd /sys/kernel/debug/tracing
	$ echo 'p:true_entry PATH_TO_MERGE/merge/true:0x13b0' > uprobe_events
	$ echo 1 > events/uprobes/true_entry/enable

This returns 'bash: echo: write error: Input/output error' and dmesg tells us 'event trace: Could not enable event true_entry'

This change makes create_trace_uprobe() look for the real inode of a dentry. In the case of normal filesystems, this simplifies to just returning the inode. In the case of overlayfs(and similar fs) we will obtain the underlying dentry and corresponding inode, upon which uprobes can successfully register.

Running the example above with the patch applied, we can see that the uprobe is enabled and will output to trace as expected.

Reviewed-by Josef Bacik <[email protected]>

Signed-off-by Howard McLauchlan <[email protected]>

---
 kernel/trace/trace_uprobe.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

kernel/trace/trace_uprobe.c +1 -1

index 2014f4351ae0..17c65fa4136d 100644
@@ -446,7 +446,7 @@ static int create_trace_uprobe(int argc, char **argv)
 	if (ret)
 		goto fail_address_parse;
 
-	inode = igrab(d_inode(path.dentry));
+	inode = igrab(d_real_inode(path.dentry));
 	path_put(&path);
 
 	if (!inode || !S_ISREG(inode->i_mode)) {

Quoted 13 lines · On Tue, 10 Apr 2018 16:10:30 -0700 Howard McLauchlan <[email protected]> wrote:

uprobes cannot successfully attach to binaries located in a directory mounted with overlayfs.

To verify, create directories for mounting overlayfs (upper,lower,work,merge), move some binary into merge/ and use readelf to obtain some known instruction of the binary. I used /bin/true and the entry instruction(0x13b0):

	$ mount -t overlay overlay -o lowerdir=lower,upperdir=upper,workdir=work merge
	$ cd /sys/kernel/debug/tracing
	$ echo 'p:true_entry PATH_TO_MERGE/merge/true:0x13b0' > uprobe_events
	$ echo 1 > events/uprobes/true_entry/enable

Quoted 2 lines

This returns 'bash: echo: write error: Input/output error' and dmesg tells us 'event trace: Could not enable event true_entry'

I tried the above and verified that it is a problem and your patch solves it.

Thanks, I'll apply it and start my tests on it.

-- Steve

Quoted 29 lines

This change makes create_trace_uprobe() look for the real inode of a dentry. In the case of normal filesystems, this simplifies to just returning the inode. In the case of overlayfs(and similar fs) we will obtain the underlying dentry and corresponding inode, upon which uprobes can successfully register.

Running the example above with the patch applied, we can see that the uprobe is enabled and will output to trace as expected.

Reviewed-by Josef Bacik <[email protected]>

Signed-off-by Howard McLauchlan <[email protected]>

---
 kernel/trace/trace_uprobe.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

kernel/trace/trace_uprobe.c +1 -1

index 2014f4351ae0..17c65fa4136d 100644
@@ -446,7 +446,7 @@ static int create_trace_uprobe(int argc, char **argv)
 	if (ret)
 		goto fail_address_parse;
 
-	inode = igrab(d_inode(path.dentry));
+	inode = igrab(d_real_inode(path.dentry));
 	path_put(&path);
 
 	if (!inode || !S_ISREG(inode->i_mode)) {

Quoted 8 lines · On Tue, 10 Apr 2018 16:10:30 -0700 Howard McLauchlan <[email protected]> wrote:

uprobes cannot successfully attach to binaries located in a directory mounted with overlayfs.

To verify, create directories for mounting overlayfs (upper,lower,work,merge), move some binary into merge/ and use readelf to obtain some known instruction of the binary. I used /bin/true and the entry instruction(0x13b0):

Quoted 7 lines

	$ mount -t overlay overlay -o lowerdir=lower,upperdir=upper,workdir=work merge
	$ cd /sys/kernel/debug/tracing
	$ echo 'p:true_entry PATH_TO_MERGE/merge/true:0x13b0' > uprobe_events
	$ echo 1 > events/uprobes/true_entry/enable

This returns 'bash: echo: write error: Input/output error' and dmesg tells us 'event trace: Could not enable event true_entry'

I tried the above and verified that it is a problem and your patch solves it.

Thanks, I'll apply it and start my tests on it.

-- Steve

Quoted 10 lines

This change makes create_trace_uprobe() look for the real inode of a dentry. In the case of normal filesystems, this simplifies to just returning the inode. In the case of overlayfs(and similar fs) we will obtain the underlying dentry and corresponding inode, upon which uprobes can successfully register.

Running the example above with the patch applied, we can see that the uprobe is enabled and will output to trace as expected.

Quoted 19 lines

Reviewed-by Josef Bacik <[email protected]>

Signed-off-by Howard McLauchlan <[email protected]>

---
 kernel/trace/trace_uprobe.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

kernel/trace/trace_uprobe.c +1 -1

index 2014f4351ae0..17c65fa4136d 100644
@@ -446,7 +446,7 @@ static int create_trace_uprobe(int argc, char **argv)
 	if (ret)
 		goto fail_address_parse;
 
-	inode = igrab(d_inode(path.dentry));
+	inode = igrab(d_real_inode(path.dentry));
 	path_put(&path);
 
 	if (!inode || !S_ISREG(inode->i_mode)) {