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

[Stefano Brivio <sbrivio@redhat.com>]

On Fri, 6 Mar 2026 09:17:32 +0100
Laurent Vivier <lvivier@redhat.com> wrote:

> On 3/6/26 08:35, Stefano Brivio wrote:
> > 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".
> >   
> 
> if iov_size(iov, buf_cnt) = 0 and hdrlen = 86 (all unsigned)
> "dlen" = "iov_size(iov, buf_cnt) - hdrlen", its value is now 18446744073709551530 (see 
> 16.) (i.e. -hdrlen, unsigned -86)
> so "dlen + hdrlen" overflows (I guess).

I was also thinking it was something like that but:

> 
> Try:
> diff --git a/tcp_vu.c b/tcp_vu.c
> index 8ca4170f13f6..787ee004a66a 100644
> --- a/tcp_vu.c
> +++ b/tcp_vu.c
> @@ -440,7 +440,7 @@ 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;
> +               ssize_t dlen = (ssize_t)iov_size(iov, buf_cnt) - hdrlen;
>                  bool push = i == head_cnt - 1;
>                  size_t l2len;

...still not happy because of how we use dlen later (I think?):

/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:440:2:
  12. path: Condition "i < head_cnt", taking true branch.
/home/sbrivio/passt/tcp_vu.c:443:3:
  13. known_value_assign: "dlen" = "(ssize_t)iov_size(iov, buf_cnt) - hdrlen", its value is now 18446744073709551530.
/home/sbrivio/passt/tcp_vu.c:450:3:
  14. path: Condition "previous_dlen != dlen", taking true branch.
/home/sbrivio/passt/tcp_vu.c:454:3:
  15. path: Condition "!*c->pcap", taking false branch.
/home/sbrivio/passt/tcp_vu.c:457:3:
  16. 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".

/home/sbrivio/passt/conf.c:2373:4:
  Type: Untrusted value as argument (TAINTED_SCALAR)

-- 
Stefano