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

[Alexey Dobriyan]

On Wed, Jul 30, 2008 at 03:02:32PM -0700, Linus Torvalds wrote:
> 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.

2.6.26 fails the same way and patch also seems to help.

> commit 94ad374a0751f40d25e22e036c37f7263569d24c
> Author: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> 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 <adobriyan@xxxxxxxxx>
>     Cc: Nick Piggin <npiggin@xxxxxxx>
>     Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>     Cc: <stable@xxxxxxxxxx> [2.6.25.x, 2.6.26.x]
>     Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> ---
>  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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