Re: [PATCH 1/2] libvhost-user: Fix pointer arithmetic in indirect read

["Daniel P. Berrangé" <berrange@redhat.com>]

On Sat, Jan 13, 2024 at 04:27:40AM +0300, Temir Zharaspayev wrote:
> When zero-copy usage of indirect descriptors buffer table isn't
> possible, library gather scattered memory chunks in a local copy.
> This commit fixes the issue with pointer arithmetic for the local copy
> buffer.
> 
> Signed-off-by: Temir Zharaspayev <masscry@gmail.com>
> ---
>  subprojects/libvhost-user/libvhost-user.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/subprojects/libvhost-user/libvhost-user.c b/subprojects/libvhost-user/libvhost-user.c
> index 6684057370..e952c098a3 100644
> --- a/subprojects/libvhost-user/libvhost-user.c
> +++ b/subprojects/libvhost-user/libvhost-user.c
> @@ -2307,7 +2307,7 @@ static int
>  virtqueue_read_indirect_desc(VuDev *dev, struct vring_desc *desc,
>                               uint64_t addr, size_t len)
>  {
> -    struct vring_desc *ori_desc;
> +    uint8_t *src_cursor, *dst_cursor;
>      uint64_t read_len;
>  
>      if (len > (VIRTQUEUE_MAX_SIZE * sizeof(struct vring_desc))) {
> @@ -2318,17 +2318,18 @@ virtqueue_read_indirect_desc(VuDev *dev, struct vring_desc *desc,
>          return -1;
>      }
>  
> +    dst_cursor = (uint8_t *) desc;
>      while (len) {
>          read_len = len;
> -        ori_desc = vu_gpa_to_va(dev, &read_len, addr);
> -        if (!ori_desc) {
> +        src_cursor = vu_gpa_to_va(dev, &read_len, addr);
> +        if (!src_cursor) {
>              return -1;
>          }
>  
> -        memcpy(desc, ori_desc, read_len);
> +        memcpy(dst_cursor, src_cursor, read_len);
>          len -= read_len;
>          addr += read_len;
> -        desc += read_len;
> +        dst_cursor += read_len;

The ori_desc -> src_cursor changes don't look to have any functional
effect. Having that change present obscures the functional part of
the patch, which is this line. FWIW, it is generally preferrable to
not mix functional and non-functional changes in the same patch

It now interprets 'read_len' as the number of bytes to increment the
address by, rather than incrementing by the number of elements of
size 'sizeof(struct vring_desc)'.

I don't know enough about this area of QEMU code to say which
semantics were desired, so I'll defer to the Michael as maintainer
to give a formal review.


With regards,
Daniel
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