[patch 42/45] fix writev regression: pan hanging unkillable and un-straceable

Greg KH
2008-02-07T21:14:36Z

[-- Attachment #1: fix-writev-regression-pan-hanging-unkillable-and-un-straceable.patch --]
[-- Type: text/plain, Size: 2088 bytes --]

2.6.24-stable review patch. If anyone has any objections, please let us know.

------------------
From: Nick Piggin <nickpiggin@yahoo.com.au>

patch 124d3b7041f9a0ca7c43a6293e1cae4576c32fd5 in mainline.

Frederik Himpe reported an unkillable and un-straceable pan process.

Zero length iovecs can go into an infinite loop in writev, because the
iovec iterator does not always advance over them.

The sequence required to trigger this is not trivial. I think it
requires that a zero-length iovec be followed by a non-zero-length iovec
which causes a pagefault in the atomic usercopy. This causes the writev
code to drop back into single-segment copy mode, which then tries to
copy the 0 bytes of the zero-length iovec; a zero length copy looks like
a failure though, so it loops.

Put a test into iov_iter_advance to catch zero-length iovecs. We could
just put the test in the fallback path, but I feel it is more robust to
skip over zero-length iovecs throughout the code (iovec iterator may be
used in filesystems too, so it should be robust).

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
mm/filemap.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1733,7 +1733,11 @@ static void __iov_iter_advance_iov(struc
const struct iovec *iov = i->iov;
size_t base = i->iov_offset;

- while (bytes) {
+ /*
+ * The !iov->iov_len check ensures we skip over unlikely
+ * zero-length segments.
+ */
+ while (bytes || !iov->iov_len) {
int copy = min(bytes, iov->iov_len - base);

bytes -= copy;
@@ -2251,6 +2255,7 @@ again:

cond_resched();

+ iov_iter_advance(i, copied);
if (unlikely(copied == 0)) {
/*
* If we were unable to copy any data at all, we must
@@ -2264,7 +2269,6 @@ again:
iov_iter_single_seg_count(i));
goto again;
}
- iov_iter_advance(i, copied);
pos += copied;
written += copied;

--