netcat: switch mktemp() → mkstemp() to silence GCC/ld warning on Linux

[catatsuy]

apps/nc/netcat.c still calls mktemp().

LibreSSL already keeps a small set of local patches on top of the upstream OpenBSD source, so this change can be added to the same patch block.

On Linux (GCC + glibc) the linker prints:

…/netcat.c:470: warning: the use of `mktemp' is dangerous, better use `mkstemp' or `mkdtemp'

Suggested patch (fits into the existing netcat.c patch block)

-           if (mktemp(unix_dg_tmp_socket_buf) == NULL)
-                   err(1, "mktemp");
+           int fd = mkstemp(unix_dg_tmp_socket_buf);
+           if (fd == -1)
+                   err(1, "mkstemp");
+           close(fd);
+           unlink(unix_dg_tmp_socket_buf);

[botovq]

It is a known issue and it is annoying. I do not want to add this patch since it does not fix anything. There is no problem that is solved with this patch. The linker warning is a warning and it does not apply in this case and in no way is using one of the suggested alternatives better.

[botovq]

People have tried to upstream similar changes and they were rejected: https://marc.info/?l=openbsd-tech&m=173056731630093&w=2

[botovq]

The way I believe we should try to solve this is as follows: we have a mktemp implementation in openrsync that can create a socket: https://github.com/openbsd/src/blob/7639f02327c4dddc3a3d0bd41f882fd3514a054a/usr.bin/rsync/mktemp.c#L162 if we can drop this into netcat, we get rid of the races and the warnings. It is of course going to be a bit more complicated than your patch.