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

@@ -33,3 +33,34 @@ int keyring_read(key_serial_t serial, void **ret, size_t *ret_size) {
                 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

@@ -9,3 +9,4 @@
 #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',

src/libsystemd/sd-id128/sd-id128.c

@@ -13,6 +13,7 @@
 #include "hmac.h"
 #include "id128-util.h"
 #include "io-util.h"
+#include "keyring-util.h"
 #include "macro.h"
 #include "missing_syscall.h"
 #include "missing_threads.h"
@@ -202,7 +203,6 @@ static int get_invocation_from_keyring(sd_id128_t *ret) {
         char *d, *p, *g, *u, *e;
         unsigned long perms;
         key_serial_t key;
-        size_t sz = 256;
         uid_t uid;
         gid_t gid;
         int r, c;
@@ -221,24 +221,9 @@ static int get_invocation_from_keyring(sd_id128_t *ret) {
                 return -errno;
         }
 
-        for (;;) {
-                description = new(char, sz);
-                if (!description)
-                        return -ENOMEM;
-
-                c = keyctl(KEYCTL_DESCRIBE, key, (unsigned long) description, sz, 0);
-                if (c < 0)
-                        return -errno;
-
-                if ((size_t) c <= sz)
-                        break;
-
-                sz = c;
-                free(description);
-        }
-
-        /* The kernel returns a final NUL in the string, verify that. */
-        assert(description[c-1] == 0);
+        r = keyring_describe(key, &description);
+        if (r < 0)
+                return r;
 
         /* Chop off the final description string */
         d = strrchr(description, ';');

src/shared/meson.build

@@ -100,7 +100,6 @@ shared_sources = files(
         'kbd-util.c',
         'kernel-config.c',
         'kernel-image.c',
-        'keyring-util.c',
         'killall.c',
         'label-util.c',
         'libarchive-util.c',