tracefs: Set all files to the same group ownership as the mount option
As people have been asking to allow non-root processes to have access to the tracefs directory, it was considered best to only allow groups to have access to the directory, where it is easier to just set the tracefs file system to a specific group (as other would be too dangerous), and that way the admins could pick which processes would have access to tracefs. Unfortunately, this broke tooling on Android that expected the other bit to be set. For some special cases, for non-root tools to trace the system, tracefs would be mounted and change the permissions of the top level directory which gave access to all running tasks permission to the tracing directory. Even though this would be dangerous to do in a production environment, for testing environments this can be useful. Now with the new changes to not allow other (which is still the proper thing to do), it breaks the testing tooling. Now more code needs to be loaded on the system to change ownership of the tracing directory. The real solution is to have tracefs honor the gid=xxx option when mounting. That is, (tracing group tracing has value 1003) mount -t tracefs -o gid=1003 tracefs /sys/kernel/tracing should have it that all files in the tracing directory should be of the given group. Copy the logic from d_walk() from dcache.c and simplify it for the mount case of tracefs if gid is set. All the files in tracefs will be walked and their group will be set to the value passed in. Link: https://lkml.kernel.org/r/20211207171729.2a54e1b3@gandalf.local.home Cc: Ingo Molnar <mingo@kernel.org> Cc: Kees Cook <keescook@chromium.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: linux-fsdevel@vger.kernel.org Cc: Al Viro <viro@ZenIV.linux.org.uk> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Reported-by:Kalesh Singh <kaleshsingh@google.com> Reported-by:
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