having trouble setting socket timeout options

[dotysan]

If I do strace rsync --sockopts=SO_RCVBUF=1000000 ... 2>&1 |grep -E 'sock' it seems to work!

socket(AF_INET, SOCK_STREAM, IPPROTO_TCP) = 3
setsockopt(3, SOL_SOCKET, SO_RCVBUF, [1000000], 4) = 0
socketpair(AF_UNIX, SOCK_STREAM, 0, [4, 5]) = 0

However, when I try to adjust the timeouts with --sockopts=SO_RCVTIMEO=15, I get an "Invalid argument" error message. Additionally strace refers to a different syscall SO_RCVTIMEO_OLD.

socket(AF_INET, SOCK_STREAM, IPPROTO_TCP) = 3
setsockopt(3, SOL_SOCKET, SO_RCVTIMEO_OLD, [15], 4) = -1 EINVAL (Invalid argument)
write(2, "rsync: [Receiver] failed to set "..., 81rsync: [Receiver] failed to set socket option SO_RCVTIMEO: Invalid argument (22)
socketpair(AF_UNIX, SOCK_STREAM, 0, [4, 5]) = 0

What am I doing wrong?!?

Reproduced on a few different 5.15 linux kernels.