epoll: Keep a reference on files added to the check list (a9ed4a65) · Commits · 戴 / test

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: stable@vger.kernel.org Signed-off-by:

Marc Zyngier <maz@kernel.org> Signed-off-by:

Al Viro <viro@zeniv.linux.org.uk>

fs/eventpoll.c

+9 −2

Original line number	Diff line number	Diff line
		@@ -1994,11 +1994,13 @@ 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);

		return error;
		@@ -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)) {

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