Re: [PATCH v4] iov: Add iov_truncate() helper and use it in vu handlers

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

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On Fri, Mar 06, 2026 at 12:51:19PM +0100, Laurent Vivier wrote:
> Add a generic iov_truncate() function that truncates an IO vector to a
> given number of bytes, returning the number of iov entries that contain
> data after truncation.
> 
> Use it in udp_vu_sock_recv() and tcp_vu_sock_recv() to replace the
> open-coded truncation logic that adjusted iov entries after recvmsg().
> Also convert the direct iov_len assignment in tcp_vu_send_flag() to use
> iov_truncate() for consistency.
> 
> Add an ASSERT() in tcp_vu_data_from_sock() to quiet the Coverity error:
> 
> passt/tcp_vu.c:457:3:
>   19. overflow_const: Expression "dlen + hdrlen", where "dlen" is known to
>       be equal to -86,  and "hdrlen" is known to be equal to 86, underflows
>       the type of "dlen + hdrlen", which is type "unsigned long".
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

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

> ---
> 
> Notes:
>     v4: add ASSERT() to quiet the Coverity error
>     v3: use in tcp_vu_send_flag() too
>     v2: use iov_truncate() in udp_vu_sock_recv() too
> 
>  iov.c    | 22 ++++++++++++++++++++++
>  iov.h    |  1 +
>  tcp_vu.c | 20 ++++++++------------
>  udp_vu.c | 12 +++---------
>  4 files changed, 34 insertions(+), 21 deletions(-)
> 
> diff --git a/iov.c b/iov.c
> index ad726daa4cd8..31a3f5bc29e5 100644
> --- a/iov.c
> +++ b/iov.c
> @@ -147,6 +147,28 @@ size_t iov_size(const struct iovec *iov, size_t iov_cnt)
>  	return len;
>  }
>  
> +/**
> + * iov_truncate() - Truncate an IO vector to a given number of bytes
> + * @iov:	IO vector (modified)
> + * @iov_cnt:	Number of entries in @iov
> + * @size:	Total number of bytes to keep
> + *
> + * Return: number of iov entries that contain data after truncation
> + */
> +size_t iov_truncate(struct iovec *iov, size_t iov_cnt, size_t size)
> +{
> +	size_t i, offset;
> +
> +	i = iov_skip_bytes(iov, iov_cnt, size, &offset);
> +
> +	if (i < iov_cnt) {
> +		iov[i].iov_len = offset;
> +		i += !!offset;
> +	}
> +
> +	return i;
> +}
> +
>  /**
>   * iov_tail_prune() - Remove any unneeded buffers from an IOV tail
>   * @tail:	IO vector tail (modified)
> diff --git a/iov.h b/iov.h
> index d1ab91a94e22..b4e50b0fca5a 100644
> --- a/iov.h
> +++ b/iov.h
> @@ -29,6 +29,7 @@ size_t iov_from_buf(const struct iovec *iov, size_t iov_cnt,
>  size_t iov_to_buf(const struct iovec *iov, size_t iov_cnt,
>  		  size_t offset, void *buf, size_t bytes);
>  size_t iov_size(const struct iovec *iov, size_t iov_cnt);
> +size_t iov_truncate(struct iovec *iov, size_t iov_cnt, size_t size);
>  
>  /*
>   * DOC: Theory of Operation, struct iov_tail
> diff --git a/tcp_vu.c b/tcp_vu.c
> index 88be232dca66..fd734e857b3b 100644
> --- a/tcp_vu.c
> +++ b/tcp_vu.c
> @@ -131,7 +131,7 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
>  		return ret;
>  	}
>  
> -	flags_elem[0].in_sg[0].iov_len = hdrlen + optlen;
> +	iov_truncate(&flags_iov[0], 1, hdrlen + optlen);
>  	payload = IOV_TAIL(flags_elem[0].in_sg, 1, hdrlen);
>  
>  	if (flags & KEEPALIVE)
> @@ -192,9 +192,9 @@ static ssize_t tcp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq,
>  	struct msghdr mh_sock = { 0 };
>  	uint16_t mss = MSS_GET(conn);
>  	int s = conn->sock;
> -	ssize_t ret, len;
>  	size_t hdrlen;
>  	int elem_cnt;
> +	ssize_t ret;
>  	int i;
>  
>  	*iov_cnt = 0;
> @@ -247,15 +247,7 @@ static ssize_t tcp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq,
>  		ret -= already_sent;
>  
>  	/* adjust iov number and length of the last iov */
> -	len = ret;
> -	for (i = 0; len && i < elem_cnt; i++) {
> -		struct iovec *iov = &elem[i].in_sg[0];
> -
> -		if (iov->iov_len > (size_t)len)
> -			iov->iov_len = len;
> -
> -		len -= iov->iov_len;
> -	}
> +	i = iov_truncate(&iov_vu[DISCARD_IOV_NUM], elem_cnt, ret);
>  
>  	/* adjust head count */
>  	while (*head_cnt > 0 && head[*head_cnt - 1] >= i)
> @@ -448,10 +440,14 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
>  	for (i = 0, previous_dlen = -1, check = NULL; i < head_cnt; i++) {
>  		struct iovec *iov = &elem[head[i]].in_sg[0];
>  		int buf_cnt = head[i + 1] - head[i];
> -		ssize_t dlen = iov_size(iov, buf_cnt) - hdrlen;
> +		size_t frame_size = iov_size(iov, buf_cnt);
>  		bool push = i == head_cnt - 1;
> +		ssize_t dlen;
>  		size_t l2len;
>  
> +		ASSERT(frame_size >= hdrlen);
> +
> +		dlen = frame_size - hdrlen;
>  		vu_set_vnethdr(iov->iov_base, buf_cnt);
>  
>  		/* The IPv4 header checksum varies only with dlen */
> diff --git a/udp_vu.c b/udp_vu.c
> index 3520f89e5671..5effca777e0a 100644
> --- a/udp_vu.c
> +++ b/udp_vu.c
> @@ -71,9 +71,9 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
>  			    bool v6, ssize_t *dlen)
>  {
>  	const struct vu_dev *vdev = c->vdev;
> -	int iov_cnt, idx, iov_used;
> -	size_t off, hdrlen, l2len;
>  	struct msghdr msg  = { 0 };
> +	int iov_cnt, iov_used;
> +	size_t hdrlen, l2len;
>  
>  	ASSERT(!c->no_udp);
>  
> @@ -115,13 +115,7 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
>  	iov_vu[0].iov_base = (char *)iov_vu[0].iov_base - hdrlen;
>  	iov_vu[0].iov_len += hdrlen;
>  
> -	/* count the numbers of buffer filled by recvmsg() */
> -	idx = iov_skip_bytes(iov_vu, iov_cnt, *dlen + hdrlen, &off);
> -
> -	/* adjust last iov length */
> -	if (idx < iov_cnt)
> -		iov_vu[idx].iov_len = off;
> -	iov_used = idx + !!off;
> +	iov_used = iov_truncate(iov_vu, iov_cnt, *dlen + hdrlen);
>  
>  	/* pad frame to 60 bytes: first buffer is at least ETH_ZLEN long */
>  	l2len = *dlen + hdrlen - VNET_HLEN;
> -- 
> 2.53.0
> 

-- 
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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