socket-util: add new helper socket_xattr_supported()

src/basic/socket-util.c

@@ -13,6 +13,7 @@
 #include <poll.h>
 #include <stdio.h>
 #include <sys/ioctl.h>
+#include <sys/stat.h>
 #include <unistd.h>
 
 #include "alloc-util.h"
@@ -21,6 +22,7 @@
 #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"
@@ -36,6 +38,8 @@
 #include "string-util.h"
 #include "strv.h"
 #include "sysctl-util.h"
+#include "tmpfile-util.h"
+#include "xattr-util.h"
 
 #if ENABLE_IDN
 #  define IDN_FLAGS NI_IDN
@@ -1891,3 +1895,41 @@ int tos_to_priority(uint8_t tos) {
                 return TC_PRIO_BESTEFFORT;
         }
 }
+
+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 */
+
+        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)
+                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");
+        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 (cached = true);
+}

src/basic/socket-util.h

@@ -268,3 +268,5 @@ int socket_get_cookie(int fd, uint64_t *ret);
 void cmsg_close_all(struct msghdr *mh);
 
 int tos_to_priority(uint8_t tos);
+
+int socket_xattr_supported(void);