It should never happen that get_file() is called on a file with f_count equal to zero. If this happens, a use-after-free condition has happened[1], and we need to attempt a best-effort reporting of the situation to help find the root cause more easily. Additionally, this serves as a data corruption indicator that system owners using warn_limit or panic_on_warn would like to have detected.
Link: https://lore.kernel.org/lkml/7c41cf3c-2a71-4dbb-8f34-033789090...@gmail.com/ [1] Suggested-by: Peter Zijlstra <pet...@infradead.org> Signed-off-by: Kees Cook <keesc...@chromium.org> --- Cc: Christian Brauner <brau...@kernel.org> Cc: Alexander Viro <v...@zeniv.linux.org.uk> Cc: Jens Axboe <ax...@kernel.dk> Cc: Jann Horn <ja...@google.com> Cc: Jan Kara <j...@suse.cz> Cc: linux-fsde...@vger.kernel.org --- include/linux/fs.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/include/linux/fs.h b/include/linux/fs.h index 00fc429b0af0..fa9ea5390f33 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -1038,7 +1038,8 @@ struct file_handle { static inline struct file *get_file(struct file *f) { - atomic_long_inc(&f->f_count); + long prior = atomic_long_fetch_inc_relaxed(&f->f_count); + WARN_ONCE(!prior, "struct file::f_count incremented from zero; use-after-free condition present!\n"); return f; } -- 2.34.1
