[PATCH v2 tip/perf/core 3/5] fs: add back RCU-delayed freeing of FMODE_BACKING file

[Andrii Nakryiko]

6cf41fcfe099 ("backing file: free directly") switched FMODE_BACKING
files to direct freeing as back then there were no use cases requiring
RCU protected access to such files.

Now, with speculative lockless VMA-to-uprobe lookup logic, we do need to
have a guarantee that struct file memory is not going to be freed from
under us during speculative check. So add back RCU-delayed freeing
logic.

We use headless kfree_rcu_mightsleep() variant, as file_free() is only
called for FMODE_BACKING files in might_sleep() context.

Suggested-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Amir Goldstein <amir73il@xxxxxxxxx>
Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
---
 fs/file_table.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/file_table.c b/fs/file_table.c
index ca7843dde56d..257691d358ee 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -68,7 +68,7 @@ static inline void file_free(struct file *f)
 	put_cred(f->f_cred);
 	if (unlikely(f->f_mode & FMODE_BACKING)) {
 		path_put(backing_file_user_path(f));
-		kfree(backing_file(f));
+		kfree_rcu_mightsleep(backing_file(f));
 	} else {
 		kmem_cache_free(filp_cachep, f);
 	}
-- 
2.43.5