linux-kernel - [PATCH 4.2.y-ckt 054/305] vfs: Make sendfile(2) killable even better

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Message-Id: <1452902519-2754-55-git-send-email-kamal@canonical.com>
Date:	Fri, 15 Jan 2016 15:57:48 -0800
From:	Kamal Mostafa <kamal@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>,
	Kamal Mostafa <kamal@...onical.com>
Subject: [PATCH 4.2.y-ckt 054/305] vfs: Make sendfile(2) killable even better

4.2.8-ckt2 -stable review patch.  If anyone has any objections, please let me know.

---8<------------------------------------------------------------

From: Jan Kara <jack@...e.cz>

commit c725bfce7968009756ed2836a8cd7ba4dc163011 upstream.

Commit 296291cdd162 (mm: make sendfile(2) killable) fixed an issue where
sendfile(2) was doing a lot of tiny writes into a filesystem and thus
was unkillable for a long time. However sendfile(2) can be (mis)used to
issue lots of writes into arbitrary file descriptor such as evenfd or
similar special file descriptors which never hit the standard filesystem
write path and thus are still unkillable. E.g. the following example
from Dmitry burns CPU for ~16s on my test system without possibility to
be killed:

        int r1 = eventfd(0, 0);
        int r2 = memfd_create("", 0);
        unsigned long n = 1<<30;
        fallocate(r2, 0, 0, n);
        sendfile(r1, r2, 0, n);

There are actually quite a few tests for pending signals in sendfile
code however we data to write is always available none of them seems to
trigger. So fix the problem by adding a test for pending signal into
splice_from_pipe_next() also before the loop waiting for pipe buffers to
be available. This should fix all the lockup issues with sendfile of the
do-ton-of-tiny-writes nature.

Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
Signed-off-by: Jan Kara <jack@...e.cz>
Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Kamal Mostafa <kamal@...onical.com>
---
 fs/splice.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/splice.c b/fs/splice.c
index 5fc1e50..882e5aa 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -809,6 +809,13 @@ static int splice_from_pipe_feed(struct pipe_inode_info *pipe, struct splice_des
  */
 static int splice_from_pipe_next(struct pipe_inode_info *pipe, struct splice_desc *sd)
 {
+	/*
+	 * Check for signal early to make process killable when there are
+	 * always buffers available
+	 */
+	if (signal_pending(current))
+		return -ERESTARTSYS;
+
 	while (!pipe->nrbufs) {
 		if (!pipe->writers)
 			return 0;
-- 
1.9.1