Commit 296291cd authored Oct 22, 2015 by Jan Kara Committed by Linus Torvalds Oct 23, 2015

mm: make sendfile(2) killable



Currently a simple program below issues a sendfile(2) system call which
takes about 62 days to complete in my test KVM instance.

        int fd;
        off_t off = 0;

        fd = open("file", O_RDWR | O_TRUNC | O_SYNC | O_CREAT, 0644);
        ftruncate(fd, 2);
        lseek(fd, 0, SEEK_END);
        sendfile(fd, fd, &off, 0xfffffff);

Now you should not ask kernel to do a stupid stuff like copying 256MB in
2-byte chunks and call fsync(2) after each chunk but if you do, sysadmin
should have a way to stop you.

We actually do have a check for fatal_signal_pending() in
generic_perform_write() which triggers in this path however because we
always succeed in writing something before the check is done, we return
value > 0 from generic_perform_write() and thus the information about
signal gets lost.

Fix the problem by doing the signal check before writing anything.  That
way generic_perform_write() returns -EINTR, the error gets propagated up
and the sendfile loop terminates early.

Signed-off-by: Jan Kara <jack@suse.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 47aee4d8

mm/filemap.c

+5 −4

Original line number	Diff line number	Diff line
		@@ -2488,6 +2488,11 @@ again:
		break;
		}

		if (fatal_signal_pending(current)) {
		status = -EINTR;
		break;
		}

		status = a_ops->write_begin(file, mapping, pos, bytes, flags,
		&page, &fsdata);
		if (unlikely(status < 0))
		@@ -2525,10 +2530,6 @@ again:
		written += copied;

		balance_dirty_pages_ratelimited(mapping);
		if (fatal_signal_pending(current)) {
		status = -EINTR;
		break;
		}
		} while (iov_iter_count(i));

		return written ? written : status;

Admin message