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SubjectRe: 2.6.27-rc1: IP: iov_iter_advance+0x2e/0x90
On Wed, Jul 30, 2008 at 01:09:21PM -0700, Linus Torvalds wrote:
> On Wed, 30 Jul 2008, Alexey Dobriyan wrote:
> >
> > Steps to reproduce:
> >
> > # while true; do ./ftest03; done
> >
> > ftest03 from LTP 20080603
>
> Hmm. The oops disassembles to
>
> -12: 4c 8b 07 mov (%rdi),%r8
> -9: 48 8b 4f 10 mov 0x10(%rdi),%rcx
> -5: 48 85 f6 test %rsi,%rsi
> -2: 75 17 jne 0x42
> 0: 49 83 78 08 00 cmpq $0x0,0x8(%r8) <---
> 5: 75 07 jne 0xe
> 7: 48 83 7f 18 00 cmpq $0x0,0x18(%rdi)
> c: 75 09 jne 0x17
>
> So it looks like we just overflowed %r8 to a new page and you presumably
> have DEBUG_PAGEALLOC on.
>
> (And yes, I see in the oops that you do)
>
> > RIP [<ffffffff8026190e>] iov_iter_advance+0x2e/0x90
> > Call Trace:
> > [<ffffffff80263452>] generic_file_buffered_write+0x1e2/0x710
> > [<ffffffff8040cfd0>] ? _spin_unlock+0x30/0x60
> > [<ffffffff80263e0f>] __generic_file_aio_write_nolock+0x29f/0x450
> > [<ffffffff80264026>] generic_file_aio_write+0x66/0xd0
> > [<ffffffff802c9506>] ext3_file_write+0x26/0xc0
> > [<ffffffff80264250>] ? generic_file_aio_read+0x0/0x670
> > [<ffffffff802c94e0>] ? ext3_file_write+0x0/0xc0
> > [<ffffffff8028921b>] do_sync_readv_writev+0xeb/0x130
> > [<ffffffff8025284d>] ? trace_hardirqs_on+0xd/0x10
> > [<ffffffff802449c0>] ? autoremove_wake_function+0x0/0x40
> > [<ffffffff80289055>] ? rw_copy_check_uvector+0x95/0x130
> > [<ffffffff80289953>] do_readv_writev+0xc3/0x120
> > [<ffffffff8025284d>] ? trace_hardirqs_on+0xd/0x10
> > [<ffffffff802527b5>] ? trace_hardirqs_on_caller+0xd5/0x160
> > [<ffffffff8025284d>] ? trace_hardirqs_on+0xd/0x10
> > [<ffffffff802899e9>] vfs_writev+0x39/0x60
> > [<ffffffff80289d60>] sys_writev+0x50/0x90
> > [<ffffffff8020b65b>] system_call_fastpath+0x16/0x1b
> >
> > 0xffffffff8026190e is in iov_iter_advance (mm/filemap.c:1882).
> > 1877
> > 1878 /*
> > 1879 * The !iov->iov_len check ensures we skip over unlikely
> > 1880 * zero-length segments (without overruning the iovec).
> > 1881 */
> > 1882 ===> while (bytes || unlikely(!iov->iov_len && i->count)) {
>
> And yes, that oopsing op would be the one that loads 'iov->iov_len'.
>
> So it very much looks like iov_iter_advance() advances past the end of the
> iov array. We've had issues like that before. And I bet it's due to a
> combination of Nick's commit f7009264c519603b8ec67c881bd368a56703cfc9
> ("iov_iter_advance() fix") and 124d3b7041f9a0ca7c43a6293e1cae4576c32fd5
> ("fix writev regression: pan hanging unkillable and un-straceable").
>
> It's simply _not_ acceptable to look at iov->iov_len when 'bytes' has gone
> down to zero, because there may be no 'iov' left!
>
> Nick?
>
> That said, I do think that we have another issue with iovec's - I think we
> should strive to always pass in the number of iovec's when we pass a
> pointer to an iovec, in addition to the bytes. The sad part is that
> 'iov_iter_advance' actually -has- the count, but it's the byte count
> remaining, not the iovec's remaining.
>
> In this particular case, the trivial fix _may_ be to just change the order
> of testing iov->iov_len && i->count, but I really think we should also
> count actual iov entries and pass them around (and keep them updated).
>
> So does this (hacky, ugly) patch fix it for you?

You forgot "untested". And, yes, it helps.

> --- 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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