Re: [PATCH v3] iov: Add iov_truncate() helper and use it in vu handlers

[Stefano Brivio <sbrivio@redhat.com>]

On Thu,  5 Mar 2026 13:56:48 +0100
Laurent Vivier <lvivier@redhat.com> 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.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
> 
> Notes:
>     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 | 14 +++-----------
>  udp_vu.c | 12 +++---------
>  4 files changed, 29 insertions(+), 20 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..8ca4170f13f6 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);

I had a quick look, but I couldn't figure this out. This causes
Coverity Scan to report:

/home/sbrivio/passt/tcp_vu.c:457:3:
  Type: Overflowed constant (INTEGER_OVERFLOW)

/home/sbrivio/passt/tcp_vu.c:355:2:
  1. path: Condition "!vu_queue_enabled(vq)", taking false branch.
/home/sbrivio/passt/tcp_vu.c:355:2:
  2. path: Condition "!vu_queue_started(vq)", taking false branch.
/home/sbrivio/passt/tcp_vu.c:362:2:
  3. path: Condition "0U /* (uint32_t)0 */ - (uint32_t)already_sent - 1 < (16777216U /* 1 << 16 + 8 */)", taking false branch.
/home/sbrivio/passt/tcp_vu.c:374:2:
  4. path: Condition "!wnd_scaled", taking false branch.
/home/sbrivio/passt/tcp_vu.c:374:2:
  5. path: Condition "already_sent >= wnd_scaled", taking false branch.
/home/sbrivio/passt/tcp_vu.c:388:2:
  6. path: Condition "v6", taking true branch.
/home/sbrivio/passt/tcp_vu.c:390:2:
  7. path: Condition "len < 0", taking false branch.
/home/sbrivio/passt/tcp_vu.c:402:2:
  8. path: Condition "!len", taking false branch.
/home/sbrivio/passt/tcp_vu.c:425:2:
  9. path: Condition "log_trace", taking true branch.
/home/sbrivio/passt/tcp_vu.c:426:2:
  10. path: Condition "log_trace", taking true branch.
/home/sbrivio/passt/tcp_vu.c:439:2:
  11. path: Condition "v6", taking true branch.
/home/sbrivio/passt/tcp_vu.c:439:2:
  12. function_return: Function "tcp_vu_hdrlen(v6)" returns 86.
/home/sbrivio/passt/tcp_vu.c:439:2:
  13. known_value_assign: "hdrlen" = "tcp_vu_hdrlen(v6)", its value is now 86.
/home/sbrivio/passt/tcp_vu.c:440:2:
  14. path: Condition "i < head_cnt", taking true branch.
/home/sbrivio/passt/tcp_vu.c:443:3:
  15. function_return: Function "iov_size(iov, buf_cnt)" returns 0.
/home/sbrivio/passt/tcp_vu.c:443:3:
  16. known_value_assign: "dlen" = "iov_size(iov, buf_cnt) - hdrlen", its value is now 18446744073709551530.
/home/sbrivio/passt/tcp_vu.c:450:3:
  17. path: Condition "previous_dlen != dlen", taking true branch.
/home/sbrivio/passt/tcp_vu.c:454:3:
  18. path: Condition "!*c->pcap", taking false branch.
/home/sbrivio/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".

-- 
Stefano