virtio: Fix Clang warning (bugprone-sizeof-expression, cert-arr39-c)

In `virtqueue_read_indirect_desc()`, the pointer arithmetic involving
`desc` is intentional. We add the length in bytes (`read_len`)
divided by the size of `struct vring_desc` to `desc`, which is
an array of `struct vring_desc`. This correctly calculates the
offset in terms of the number of `struct vring_desc` elements.

Clang issues the following warning due to this explicit scaling:

virtio.c:238:8: error: suspicious usage of 'sizeof(...)' in pointer
arithmetic; this scaled value will be scaled again by the '+='
operator [bugprone-sizeof-expression,cert-arr39-c,-Werror]
  238 |         desc += read_len / sizeof(struct vring_desc);
      |               ^            ~~~~~~~~~~~~~~~~~~~~~~~~~
virtio.c:238:8: note: '+=' in pointer arithmetic internally scales
with 'sizeof(struct vring_desc)' == 16

This behavior is intended, so the warning can be considered a
false positive in this context. The code correctly advances the
pointer by the desired number of descriptor entries.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/virtio.c b/virtio.c
index bc2b89a..f7db007 100644
--- a/virtio.c
+++ b/virtio.c
@@ -235,6 +235,7 @@ static int virtqueue_read_indirect_desc(const struct vu_dev *dev,
 		memcpy(desc, orig_desc, read_len);
 		len -= read_len;
 		addr += read_len;
+		/* NOLINTNEXTLINE(bugprone-sizeof-expression,cert-arr39-c) */
 		desc += read_len / sizeof(struct vring_desc);
 	}