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net: UDP recv with MSG_TRUNC must return the datagram length (fixes -u --skip-rx-copy throughput) #2075

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net: UDP recv with MSG_TRUNC must return the datagram length (fixes -u --skip-rx-copy throughput) #2075

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e804e3d
net: UDP recv with MSG_TRUNC must return the datagram length
Sep 10, 2026
9999350
net: reject UDP datagrams shorter than the header under MSG_TRUNC
Sep 10, 2026
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22 changes: 22 additions & 0 deletions src/net.c
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Original file line number Diff line number Diff line change
Expand Up @@ -459,6 +459,17 @@ Nrecv(int fd, char *buf, size_t count, int prot, int sock_opt)
break;

if (sock_opt & MSG_TRUNC) {
/*
* On a datagram socket MSG_TRUNC returns the full datagram length
* even though only `count` bytes were copied. That length is the
* value the caller must account (fastpath fix: upstream returned
* `count`, so UDP --skip-rx-copy under-reported throughput ~100x).
*/
if (prot == SOCK_DGRAM) { /* Pudp; iperf_api.h is not included here */
if (r < count)
return NET_HARDERROR; /* at least the UDP message header must be present */
return r;
}
size_t bytes_copied = (r > nleft)? nleft: r;
nleft -= bytes_copied;
buf += bytes_copied;
Expand Down Expand Up @@ -538,6 +549,17 @@ Nrecv_no_select(int fd, char *buf, size_t count, int prot, int sock_opt)
break;

if (sock_opt & MSG_TRUNC) {
/*
* On a datagram socket MSG_TRUNC returns the full datagram length
* even though only `count` bytes were copied. That length is the
* value the caller must account (fastpath fix: upstream returned
* `count`, so UDP --skip-rx-copy under-reported throughput ~100x).
*/
if (prot == SOCK_DGRAM) { /* Pudp; iperf_api.h is not included here */
if (r < count)
return NET_HARDERROR; /* at least the UDP message header must be present */
return r;
}
size_t bytes_copied = (r > nleft)? nleft: r;
nleft -= bytes_copied;
buf += bytes_copied;
Expand Down