FAILED: patch "[PATCH] epoll: Keep a reference on files added to the check list" failed to apply to 5.4-stable tree

PATCH [email protected] 1 participant 1 message Aug 24, 2020

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2020-08-24 07:24 UTC #0 lore raw

The patch below does not apply to the 5.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From a9ed4a6560b8562b7e2e2bed9527e88001f7b682 Mon Sep 17 00:00:00 2001
From: Marc Zyngier <[email protected]>
Date: Wed, 19 Aug 2020 17:12:17 +0100
Subject: [PATCH] epoll: Keep a reference on files added to the check list

When adding a new fd to an epoll, and that this new fd is an epoll fd itself, we recursively scan the fds attached to it to detect cycles, and add non-epool files to a "check list" that gets subsequently parsed.

However, this check list isn't completely safe when deletions can happen concurrently. To sidestep the issue, make sure that a struct file placed on the check list sees its f_count increased, ensuring that a concurrent deletion won't result in the file disapearing from under our feet.

Cc [email protected]

Signed-off-by Marc Zyngier <[email protected]>

Signed-off-by Al Viro <[email protected]>

fs/eventpoll.c +9 -2

index 12eebcdea9c8..196003d9242c 100644
@@ -1994,9 +1994,11 @@ static int ep_loop_check_proc(void *priv, void *cookie, int call_nests)
 			 * not already there, and calling reverse_path_check()
 			 * during ep_insert().
 			 */
-			if (list_empty(&epi->ffd.file->f_tfile_llink))
+			if (list_empty(&epi->ffd.file->f_tfile_llink)) {
+				get_file(epi->ffd.file);
 				list_add(&epi->ffd.file->f_tfile_llink,
 					 &tfile_check_list);
+			}
 		}
 	}
 	mutex_unlock(&ep->mtx);

@@ -2040,6 +2042,7 @@ static void clear_tfile_check_list(void)
 		file = list_first_entry(&tfile_check_list, struct file,
 					f_tfile_llink);
 		list_del_init(&file->f_tfile_llink);
+		fput(file);
 	}
 	INIT_LIST_HEAD(&tfile_check_list);
 }

@@ -2204,13 +2207,17 @@ int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
 					clear_tfile_check_list();
 					goto error_tgt_fput;
 				}
-			} else
+			} else {
+				get_file(tf.file);
 				list_add(&tf.file->f_tfile_llink,
 							&tfile_check_list);
+			}
 			error = epoll_mutex_lock(&ep->mtx, 0, nonblock);
 			if (error) {
 out_del:
 				list_del(&tf.file->f_tfile_llink);
+				if (!is_file_epoll(tf.file))
+					fput(tf.file);
 				goto error_tgt_fput;
 			}
 			if (is_file_epoll(tf.file)) {