Re: [PATCH v2 15/16] xen-blkback: Minor cleanups

Roger Pau Monné
2023-06-06T08:37:15Z

On Tue, May 30, 2023 at 04:31:15PM -0400, Demi Marie Obenour wrote:
> This adds a couple of BUILD_BUG_ON()s and moves some arithmetic after
> the validation code that checks the arithmetic’s preconditions. The
> previous code was correct but could potentially trip sanitizers that
> check for unsigned integer wraparound.
>
> Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
> ---
> drivers/block/xen-blkback/blkback.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> index c362f4ad80ab07bfb58caff0ed7da37dc1484fc5..ac760a08d559085ab875784f1c58cdf2ead95a43 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -1342,6 +1342,8 @@ static int dispatch_rw_block_io(struct xen_blkif_ring *ring,
> nseg = req->operation == BLKIF_OP_INDIRECT ?
> req->u.indirect.nr_segments : req->u.rw.nr_segments;
>
> + BUILD_BUG_ON(offsetof(struct blkif_request, u.rw.id) != 8);
> + BUILD_BUG_ON(offsetof(struct blkif_request, u.indirect.id) != 8);

Won't it be clearer as:

offsetof(struct blkif_request, u.rw.id) != offsetof(struct blkif_request, u.indirect.id)

We don't really care about the specific offset value, but both layouts
must match.

Also, you likely want to check for all requests types, not just rw and
indirect (discard and other).

> if (unlikely(nseg == 0 && operation_flags != REQ_PREFLUSH) ||
> unlikely((req->operation != BLKIF_OP_INDIRECT) &&
> (nseg > BLKIF_MAX_SEGMENTS_PER_REQUEST)) ||
> @@ -1365,13 +1367,13 @@ static int dispatch_rw_block_io(struct xen_blkif_ring *ring,
> preq.sector_number = req->u.rw.sector_number;
> for (i = 0; i < nseg; i++) {
> pages[i]->gref = req->u.rw.seg[i].gref;
> - seg[i].nsec = req->u.rw.seg[i].last_sect -
> - req->u.rw.seg[i].first_sect + 1;
> - seg[i].offset = (req->u.rw.seg[i].first_sect << 9);
> if ((req->u.rw.seg[i].last_sect >= (XEN_PAGE_SIZE >> 9)) ||
> (req->u.rw.seg[i].last_sect <
> req->u.rw.seg[i].first_sect))
> goto fail_response;
> + seg[i].nsec = req->u.rw.seg[i].last_sect -
> + req->u.rw.seg[i].first_sect + 1;
> + seg[i].offset = (req->u.rw.seg[i].first_sect << 9);

Parentheses here are unneeded. If we do that move, we might as well
move the assignation of pages[i]->gref as well, just to avoid
assigning to gref to take the failure path.

I do think however wraparound is not an issue here, as we will discard
the result.

Thanks, Roger.