Spotted this so-far harmless, but technically undefined behaviour while looking at other things. Fixed one other nit at the same time. David Gibson (2): tcp: Remove redundant initialisation of iov[TCP_IOV_ETH].iov_base tcp: Avoid overlapping memcpy() in DUP_ACK handling tcp_buf.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) -- 2.46.0
This initialisation for IPv4 flags buffers is redundant with the very next line which sets both iov_base and iov_len. Signed-off-by: David Gibson <david(a)gibson.dropbear.id.au> --- tcp_buf.c | 1 - 1 file changed, 1 deletion(-) diff --git a/tcp_buf.c b/tcp_buf.c index c31e9f31..2e044b27 100644 --- a/tcp_buf.c +++ b/tcp_buf.c @@ -168,7 +168,6 @@ void tcp_sock4_iov_init(const struct ctx *c) iov = tcp4_l2_flags_iov[i]; iov[TCP_IOV_TAP] = tap_hdr_iov(c, &tcp4_flags_tap_hdr[i]); - iov[TCP_IOV_ETH].iov_base = &tcp4_eth_src; iov[TCP_IOV_ETH] = IOV_OF_LVALUE(tcp4_eth_src); iov[TCP_IOV_IP] = IOV_OF_LVALUE(tcp4_flags_ip[i]); iov[TCP_IOV_PAYLOAD].iov_base = &tcp4_flags[i]; -- 2.46.0
When handling the DUP_ACK flag, we copy all the buffers making up the ack
frame. However, all our frames share the same buffer for the Ethernet
header (tcp4_eth_src or tcp6_eth_src), so copying the TCP_IOV_ETH will
result in a (perfectly) overlapping memcpy(). This seems to have been
harmless so far, but overlapping ranges to memcpy() is undefined behaviour,
so we really should avoid it.
Signed-off-by: David Gibson <david(a)gibson.dropbear.id.au>
---
tcp_buf.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)
diff --git a/tcp_buf.c b/tcp_buf.c
index 2e044b27..1a398461 100644
--- a/tcp_buf.c
+++ b/tcp_buf.c
@@ -332,9 +332,13 @@ int tcp_buf_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int
flags)
else
dup_iov = tcp6_l2_flags_iov[tcp6_flags_used++];
- for (i = 0; i < TCP_NUM_IOVS; i++)
- memcpy(dup_iov[i].iov_base, iov[i].iov_base,
- iov[i].iov_len);
+ for (i = 0; i < TCP_NUM_IOVS; i++) {
+ /* All frames share the same ethernet header buffer */
+ if (i != TCP_IOV_ETH) {
+ memcpy(dup_iov[i].iov_base, iov[i].iov_base,
+ iov[i].iov_len);
+ }
+ }
dup_iov[TCP_IOV_PAYLOAD].iov_len = iov[TCP_IOV_PAYLOAD].iov_len;
}
--
2.46.0
On Thu, 12 Sep 2024 16:59:38 +1000 David Gibson <david(a)gibson.dropbear.id.au> wrote:Spotted this so-far harmless, but technically undefined behaviour while looking at other things. Fixed one other nit at the same time. David Gibson (2): tcp: Remove redundant initialisation of iov[TCP_IOV_ETH].iov_base tcp: Avoid overlapping memcpy() in DUP_ACK handling tcp_buf.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-)Applied. -- Stefano