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

On 3/6/26 09:25, Stefano Brivio wrote:

On Fri, 6 Mar 2026 09:17:32 +0100 Laurent Vivier wrote:

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On 3/6/26 08:35, Stefano Brivio wrote:

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On Thu, 5 Mar 2026 13:56:48 +0100 Laurent Vivier wrote:

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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 ---

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:

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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)

Could you try: diff --git a/tcp_vu.c b/tcp_vu.c index 8ca4170f13f6..fd734e857b3b 100644 --- a/tcp_vu.c +++ b/tcp_vu.c @@ -440,10 +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 */ Coverity likes "ASSERT()"... Thanks, Laurent