util: add keyring_describe helper and move to basic

So that it can be used from libsystemd. No external dependencies.

src/basic/keyring-util.c

@@ -0,0 +1,66 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "keyring-util.h"
+#include "memory-util.h"
+#include "missing_syscall.h"
+
+int keyring_read(key_serial_t serial, void **ret, size_t *ret_size) {
+        size_t bufsize = 100;
+
+        for (;;) {
+                _cleanup_(erase_and_freep) uint8_t *buf = NULL;
+                long n;
+
+                buf = new(uint8_t, bufsize + 1);
+                if (!buf)
+                        return -ENOMEM;
+
+                n = keyctl(KEYCTL_READ, (unsigned long) serial, (unsigned long) buf, (unsigned long) bufsize, 0);
+                if (n < 0)
+                        return -errno;
+
+                if ((size_t) n <= bufsize) {
+                        buf[n] = 0; /* NUL terminate, just in case */
+
+                        if (ret)
+                                *ret = TAKE_PTR(buf);
+                        if (ret_size)
+                                *ret_size = n;
+
+                        return 0;
+                }
+
+                bufsize = (size_t) n;
+        }
+}
+
+int keyring_describe(key_serial_t serial, char **ret) {
+        _cleanup_free_ char *tuple = NULL;
+        size_t sz = 64;
+        int c = -1; /* Workaround for maybe-uninitialized false positive due to missing_syscall indirection */
+
+        assert(ret);
+
+        for (;;) {
+                tuple = new(char, sz);
+                if (!tuple)
+                        return log_oom_debug();
+
+                c = keyctl(KEYCTL_DESCRIBE, serial, (unsigned long) tuple, c, 0);
+                if (c < 0)
+                        return log_debug_errno(errno, "Failed to describe key id %d: %m", serial);
+
+                if ((size_t) c <= sz)
+                        break;
+
+                sz = c;
+                free(tuple);
+        }
+
+        /* The kernel returns a final NUL in the string, verify that. */
+        assert(tuple[c-1] == 0);
+
+        *ret = TAKE_PTR(tuple);
+
+        return 0;
+}

src/basic/keyring-util.h

@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <sys/types.h>
+
+#include "missing_keyctl.h"
+
+/* Like TAKE_PTR() but for key_serial_t, resetting them to -1 */
+#define TAKE_KEY_SERIAL(key_serial) TAKE_GENERIC(key_serial, key_serial_t, -1)
+
+int keyring_read(key_serial_t serial, void **ret, size_t *ret_size);
+int keyring_describe(key_serial_t serial, char **ret);

src/basic/meson.build

@@ -57,6 +57,7 @@ basic_sources = files(
         'lock-util.c',
         'log.c',
         'login-util.c',
+        'keyring-util.c',
         'memfd-util.c',
         'memory-util.c',
         'mempool.c',