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SubjectRe: 2.6.27-rc1: IP: iov_iter_advance+0x2e/0x90

On Wed, 30 Jul 2008, Linus Torvalds wrote:
> Ok, I'll commit it as obvious, ugly as it may be. And hope that Nick or
> some other god of iov_iter will think about trying to make these things
> more reliable and easier to use.

Oh, and I think it's stable material. Unless I misread something, this
area hasn't changed since 2.6.25. But somebody should double-check me,
that just sounds wrong.

stable added to cc, with patch appended.

commit 94ad374a0751f40d25e22e036c37f7263569d24c
Author: Linus Torvalds <>
Date: Wed Jul 30 14:45:12 2008 -0700

Fix off-by-one error in iov_iter_advance()

The iov_iter_advance() function would look at the iov->iov_len entry
even though it might have iterated over the whole array, and iov was
pointing past the end. This would cause DEBUG_PAGEALLOC to trigger a
kernel page fault if the allocation was at the end of a page, and the
next page was unallocated.

The quick fix is to just change the order of the tests: check that there
is any iovec data left before we check the iov entry itself.

Thanks to Alexey Dobriyan for finding this case, and testing the fix.

Reported-and-tested-by: Alexey Dobriyan <>
Cc: Nick Piggin <>
Cc: Andrew Morton <>
Cc: <> [2.6.25.x, 2.6.26.x]
Signed-off-by: Linus Torvalds <>
mm/filemap.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 42bbc69..d97d1ad 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1879,7 +1879,7 @@ void iov_iter_advance(struct iov_iter *i, size_t bytes)
* The !iov->iov_len check ensures we skip over unlikely
* zero-length segments (without overruning the iovec).
- while (bytes || unlikely(!iov->iov_len && i->count)) {
+ while (bytes || unlikely(i->count && !iov->iov_len)) {
int copy;

copy = min(bytes, iov->iov_len - base);

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