[PATCH] staging/android: sync_debug: unproxify debugfs files' fops

[PATCH] staging/android: sync_debug: unproxify debugfs files' fops

[Nicolai Stange]

Since commit 49d200deaa68 ("debugfs: prevent access to removed files'
private data"), a debugfs file's file_operations methods get proxied
through lifetime aware wrappers.

However, only a certain subset of the file_operations members is supported
by debugfs and ->compat_ioctl isn't among them -- it appears to be NULL
from the VFS layer's perspective.

This behaviour breaks the /sys/kernel/debug/sync/sw_sync file introduced
concurrently with commit a44eb74cd413 ("staging/android: move SW_SYNC_USER
to a debugfs file").

Since that file never gets removed, there is no file removal race and thus,
a lifetime checking proxy isn't needed.

Avoid the proxying for /sys/kernel/debug/sync/sw_sync by creating it via
debugfs_create_file_unsafe() rather than debugfs_create_file().
For consistency, do the same for /sys/kernel/debug/sync/info.

Fixes: 49d200deaa68 ("debugfs: prevent access to removed files' private
                      data")
Fixes: a44eb74cd413 ("staging/android: move SW_SYNC_USER to a debugfs
                      file")
Signed-off-by: Nicolai Stange <nicstange@gmail.com>
---
 Applicable to linux-next-20160527.
 x86_64 allmodconfig build succeeds w/o any new warnings.

 Related discussion can be found here:
 http://lkml.kernel.org/g/878tz22peb.fsf@gmail.com

 drivers/staging/android/sync_debug.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/staging/android/sync_debug.c b/drivers/staging/android/sync_debug.c
index 5f57499..1197f9e 100644
--- a/drivers/staging/android/sync_debug.c
+++ b/drivers/staging/android/sync_debug.c
@@ -326,9 +326,15 @@ static __init int sync_debugfs_init(void)
 {
 	dbgfs = debugfs_create_dir("sync", NULL);
 
-	debugfs_create_file("info", 0444, dbgfs, NULL, &sync_info_debugfs_fops);
-	debugfs_create_file("sw_sync", 0644, dbgfs, NULL,
-			    &sw_sync_debugfs_fops);
+	/*
+	 * The debugfs files won't ever get removed and thus, there is
+	 * no need to protect it against removal races. The use of
+	 * debugfs_create_file_unsafe() is actually safe here.
+	 */
+	debugfs_create_file_unsafe("info", 0444, dbgfs, NULL,
+				   &sync_info_debugfs_fops);
+	debugfs_create_file_unsafe("sw_sync", 0644, dbgfs, NULL,
+				   &sw_sync_debugfs_fops);
 
 	return 0;
 }
-- 
2.8.2

Re: [PATCH] staging/android: sync_debug: unproxify debugfs files' fops

[Greg Kroah-Hartman]

On Fri, May 27, 2016 at 08:03:54PM +0200, Nicolai Stange wrote:
> Since commit 49d200deaa68 ("debugfs: prevent access to removed files'
> private data"), a debugfs file's file_operations methods get proxied
> through lifetime aware wrappers.
> 
> However, only a certain subset of the file_operations members is supported
> by debugfs and ->compat_ioctl isn't among them -- it appears to be NULL
> from the VFS layer's perspective.
> 
> This behaviour breaks the /sys/kernel/debug/sync/sw_sync file introduced
> concurrently with commit a44eb74cd413 ("staging/android: move SW_SYNC_USER
> to a debugfs file").
> 
> Since that file never gets removed, there is no file removal race and thus,
> a lifetime checking proxy isn't needed.
> 
> Avoid the proxying for /sys/kernel/debug/sync/sw_sync by creating it via
> debugfs_create_file_unsafe() rather than debugfs_create_file().
> For consistency, do the same for /sys/kernel/debug/sync/info.
> 
> Fixes: 49d200deaa68 ("debugfs: prevent access to removed files' private
>                       data")
> Fixes: a44eb74cd413 ("staging/android: move SW_SYNC_USER to a debugfs
>                       file")
> Signed-off-by: Nicolai Stange <nicstange@gmail.com>
> ---
>  Applicable to linux-next-20160527.
>  x86_64 allmodconfig build succeeds w/o any new warnings.
> 
>  Related discussion can be found here:
>  http://lkml.kernel.org/g/878tz22peb.fsf@gmail.com
> 
>  drivers/staging/android/sync_debug.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)

Thanks, will queue this up after 4.7-rc1 is out.

greg k-h