On Thu, 21 Aug 2014 11:09:29 -0500 Christoph Hellwig <hch@xxxxxx> wrote:
> When we do non-page sized reads we can underflow the extent_length variable
> and read incorrect data. Fix the extent_length calculation and change to
> defensive <= checks for the extent length in the read and write path.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
Hi Christoph,
I was reviewing this patch for possible backport.
As 'extent_length' is sector_t, it is unsigned (either u64 or unsigned long).
So comparing "<= 0" has the same effect as comparing "== 0".
So the new checks are not "defensive".
That doesn't mean they are wrong, but they could be misleading...
There may be nothing that needs to be done here, but I thought I should let
you know.
NeilBrown
> ---
> fs/nfs/blocklayout/blocklayout.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/fs/nfs/blocklayout/blocklayout.c b/fs/nfs/blocklayout/blocklayout.c
> index 5427ae7..87a633d 100644
> --- a/fs/nfs/blocklayout/blocklayout.c
> +++ b/fs/nfs/blocklayout/blocklayout.c
> @@ -272,7 +272,7 @@ bl_read_pagelist(struct nfs_pgio_header *hdr)
> isect = (sector_t) (f_offset >> SECTOR_SHIFT);
> /* Code assumes extents are page-aligned */
> for (i = pg_index; i < hdr->page_array.npages; i++) {
> - if (!extent_length) {
> + if (extent_length <= 0) {
> /* We've used up the previous extent */
> bl_put_extent(be);
> bl_put_extent(cow_read);
> @@ -303,6 +303,7 @@ bl_read_pagelist(struct nfs_pgio_header *hdr)
> f_offset += pg_len;
> bytes_left -= pg_len;
> isect += (pg_offset >> SECTOR_SHIFT);
> + extent_length -= (pg_offset >> SECTOR_SHIFT);
> } else {
> pg_offset = 0;
> pg_len = PAGE_CACHE_SIZE;
> @@ -333,7 +334,7 @@ bl_read_pagelist(struct nfs_pgio_header *hdr)
> }
> }
> isect += (pg_len >> SECTOR_SHIFT);
> - extent_length -= PAGE_CACHE_SECTORS;
> + extent_length -= (pg_len >> SECTOR_SHIFT);
> }
> if ((isect << SECTOR_SHIFT) >= header->inode->i_size) {
> hdr->res.eof = 1;
> @@ -797,7 +798,7 @@ next_page:
> /* Middle pages */
> pg_index = header->args.pgbase >> PAGE_CACHE_SHIFT;
> for (i = pg_index; i < header->page_array.npages; i++) {
> - if (!extent_length) {
> + if (extent_length <= 0) {
> /* We've used up the previous extent */
> bl_put_extent(be);
> bl_put_extent(cow_read);
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