Re: [PATCH v3 07/10] pcap: Pass explicit L2 length to pcap_iov()

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Apr 16, 2026 at 05:57:18PM +0200, Laurent Vivier wrote:
> With vhost-user multibuffer frames, the iov can be larger than the
> actual L2 frame. The previous approach of computing L2 length as
> iov_size() - offset would overcount and write extra bytes into the
> pcap file.
> 
> Pass the L2 frame length explicitly to pcap_frame() and pcap_iov(),
> and write exactly that many bytes instead of the full iov remainder.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

[snip]
> @@ -500,7 +500,8 @@ static size_t tap_send_frames_passt(const struct ctx *c,
>  		/* Number of unsent or partially sent buffers for the frame */
>  		size_t rembufs = bufs_per_frame - (i % bufs_per_frame);
>  
> -		if (write_remainder(c->fd_tap, &iov[i], rembufs, buf_offset) < 0) {
> +		if (write_remainder(c->fd_tap, &iov[i], rembufs, buf_offset,
> +				    SIZE_MAX) < 0) {
>  			err_perror("tap: partial frame send");
>  			return i;
[snip]
> @@ -722,31 +722,54 @@ int do_clone(int (*fn)(void *), char *stack_area, size_t stack_size, int flags,
>   * @iov:	IO vector
>   * @iovcnt:	Number of entries in @iov
>   * @skip:	Number of bytes of the vector to skip writing
> + * @length:	Number of bytes of the vector to write

Nit: This is the maximum number of bytes to write, not the exact
number, which we rely on in tap_send_frames_passt() above.  That makes
the comment slightly misleading.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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