socket-util: fix socket_xattr_supported() in initramfs

Don't test xattr support on socket created in filesystem. This doesn't
work in initramfs when /tmp doesn't have tmpfs mounted inside as
initramfs doesn't have xattr support. Setting socket xattr falls back to
parent directory xattr handlers when we work with FS based socket.

Let's test sockfs based socket instead so that the check is generic and
works in all environments.

src/basic/socket-util.c

@@ -22,7 +22,6 @@
 #include "fd-util.h"
 #include "format-ifname.h"
 #include "format-util.h"
-#include "fs-util.h"
 #include "in-addr-util.h"
 #include "io-util.h"
 #include "log.h"
@@ -38,7 +37,6 @@
 #include "string-util.h"
 #include "strv.h"
 #include "sysctl-util.h"
-#include "tmpfile-util.h"
 #include "user-util.h"
 #include "xattr-util.h"
 
@@ -1902,37 +1900,23 @@ int tos_to_priority(uint8_t tos) {
 int socket_xattr_supported(void) {
         int r;
 
-        // FIXME: Drop this check once Linux 7.0 becomes our baseline
-
-        /* Checks if socket inodes may have xattrs on this kernel. This should pass on kernel 7.0, fail on
-         * older kernels */
+        // FIXME: Drop this check once Linux 7.1 becomes our baseline
 
         static int cached = -1;
         if (cached >= 0)
                 return cached;
 
-        const char *t;
-        r = tmp_dir(&t);
-        if (r < 0)
-                return r;
-
-        _cleanup_free_ char *sp = NULL;
-        r = tempfn_random_child(t, "sockxattrtest", &sp);
-        if (r < 0)
-                return r;
-
-        if (mknod(sp, S_IFSOCK | 0600, /* dev= */ 0) < 0)
+        _cleanup_close_ int fd = socket(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC, /* protocol= */ 0);
+        if (fd < 0)
                 return -errno;
 
-        _cleanup_(unlink_and_freep) char *sp_destroy = TAKE_PTR(sp);
-
         /* Old kernels return EPERM. But let's also check for more appropriate error codes, to be friendly to
          * seccomp policies */
-        r = xsetxattr(AT_FDCWD, sp_destroy, /* at_flags= */ 0, "user.testxxx", "1");
+        r = xsetxattr(fd, /* path= */ NULL, AT_EMPTY_PATH, "user.testxxx", "1");
         if (ERRNO_IS_NEG_NOT_SUPPORTED(r) || r == -EPERM)
                 return (cached = false);
         if (r < 0)
-                return log_debug_errno(r, "Failed to set test xattr on socket inode '%s': %m", sp_destroy);
+                return log_debug_errno(r, "Failed to set test xattr on socket: %m");
 
         return (cached = true);
 }