x/sys/unix: syscall_unix.go - Recvmsg reports empty/wrong error value

[rinor]

Does this issue reproduce with the latest release?

• yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOVERSION="go1.20.1"

What did you do?

import (
	"golang.org/x/sys/unix"
)
...
retn, _, _, _, err = unix.Recvmsg(tunfd, msg[:], nil, 0)

What did you expect to see?

retn == -1
err != nil

What did you see instead?

retn == -1
err == nil

---

https://github.com/golang/sys/blob/494aa493ccb0c797af52f75ff7839a241bb26cdd/unix/syscall_unix.go#L356 is lacking error check

	n, oobn, recvflags, err = recvmsgRaw(fd, iov[:], oob, flags, &rsa)
	if rsa.Addr.Family != AF_UNSPEC {
		from, err = anyToSockaddr(fd, &rsa)
	}

vs

	n, oobn, recvflags, err = recvmsgRaw(fd, iov[:], oob, flags, &rsa)
	if err == nil && rsa.Addr.Family != AF_UNSPEC {
		from, err = anyToSockaddr(fd, &rsa)
	}

[ianlancetaylor]

Can you show us a complete program that recreates the problem? Thanks.

[rinor]

I'll try and figure out how to recreate that from the environment it happens since it depends on the socket state that affects rsa.Addr.Family , but from tests the behavior changes in this commit golang/sys@51cd998. Before that rsa.Addr.Family != AF_UNSPEC was no checked in case of an error returned from recvmsg and probably with the code reorg that check was lost. FWIW the other functions "similar" to Recvmsg there have the correct check in place

• Recvfrom - https://github.com/golang/sys/blob/494aa493ccb0c797af52f75ff7839a241bb26cdd/unix/syscall_unix.go#L325
• RecvmsgBuffers - https://github.com/golang/sys/blob/494aa493ccb0c797af52f75ff7839a241bb26cdd/unix/syscall_unix.go#L377

[rinor]

This case was triggered by a "kernel" implementation bug that was wrongly changing the socket address family also in case of errors.
While I still think would be better to add that error check and unify the handling, for the time being I'm unable to provide correct code example that can trigger that branch.

Thank you.