fs: WARN when f_count resurrection is attempted

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-8f34-0337890906fc@gmail.com/ [1]
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20240503201620.work.651-kees@kernel.orgSigned-off-by: Christian Brauner <brauner@kernel.org>

include/linux/fs.h

@@ -1034,7 +1034,8 @@ 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;
 }