dbus: fixes from a code review of the branch

A pass over the whole branch before merge, mostly in libink since
that is the new code and the part exposed to the wire.  Grouped here
rather than scattered so the review is easy to read in one place.

libink parser and dispatch:

 - Bound reader lengths so a 32-bit size_t can't wrap a wire length
   past the guard and read out of bounds.  Reachable pre-auth on any
   bus, so it matters on the 32-bit targets Finit runs on.
 - Drop a peer when a reply send fails instead of limping on with a
   half-written frame; a built-in whose send failed used to fall
   through and put a second frame on the wire.

initctl:

 - Copy a D-Bus error name out of the reply before closing the client;
   the reply points into memory the close frees.  Both error paths now
   share one helper so this can't creep back.

Authorization:

 - Take the caller's groups from the kernel (SO_PEERCRED plus
   SO_PEERGROUPS) rather than getpwuid()/getgrouplist(), which go
   through NSS and can block PID 1 on a slow LDAP or SSSD backend.
   The check is now a lookup against the group resolved once at init,
   with no NSS and no 256 KiB array on the stack.  A caller reaching
   us through a broker carries no group set, so system-bus privileged
   methods are root-only; the local bus keeps group support.  See
   libink/README.md for the note on lifting that.

Shutdown:

 - Call dbus_exit() from the shutdown path so the server, its peers,
   and the socket are let go cleanly.  The teardown existed but nobody
   called it.

Tests, CI, docs:

 - A fuzz target for the message parser, run as a quick sweep in the
   suite and properly under libFuzzer in CI, with the corpus carried
   between runs.  The -as-uid tests drop groups the way a login does
   so SO_PEERGROUPS sees the right set, and widen the test socket to
   reach the per-method check behind the 0660 gate.  Bring the GitHub
   actions up to versions that run on Node 24, and tidy a few small
   things a /simplify pass turned up.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

test/src/.gitignore

@@ -4,3 +4,4 @@
 /.deps/
 /serv
 /dbus-auth-client
+/fuzz-msg-parse

test/src/dbus-auth-client.c

@@ -42,6 +42,8 @@
  */
 
 #include <errno.h>
+#include <grp.h>
+#include <pwd.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -52,6 +54,17 @@
 
 #include "link.h"
 
+/* The -as-uid modes drop all privileges before exiting, and LeakSanitizer
+ * cannot run under that (it needs ptrace on itself).  Turn leak detection
+ * off for this test client via ASan's startup hook; it is a short-lived
+ * tool, and finit -- the process whose leaks actually matter -- is checked
+ * on its own.  Harmless when the build has no sanitizer. */
+const char *__asan_default_options(void);
+const char *__asan_default_options(void)
+{
+	return "detect_leaks=0";
+}
+
 /* ---------- manual SASL: only mode_auth uses this ---------- */
 
 static const char hex[] = "0123456789abcdef";
@@ -153,9 +166,15 @@ static int report_rc(link_client_t *c, int rc)
 	return 2;
 }
 
-/* Drop effective uid to argv-supplied value (decimal). */
+/* Become the argv-supplied uid (decimal), the way a login does: reset
+ * the group set from the target user's /etc/group entry before setuid,
+ * so the connection carries that user's real credentials.  A bare
+ * setuid() would keep root's groups, and SO_PEERGROUPS would then hand
+ * the server a caller who still holds root's privileges -- masking the
+ * very authorization the -as-uid tests check. */
 static int drop_uid(const char *uid_arg, const char *progname)
 {
+	struct passwd *pw;
 	char *ep = NULL;
 	long  v;
 
@@ -165,6 +184,22 @@ static int drop_uid(const char *uid_arg, const char *progname)
 		fprintf(stderr, "%s: bad uid: %s\n", progname, uid_arg);
 		return 2;
 	}
+
+	pw = getpwuid((uid_t)v);
+	if (pw) {
+		if (initgroups(pw->pw_name, pw->pw_gid) < 0) {
+			perror("initgroups");
+			return 2;
+		}
+		if (setgid(pw->pw_gid) < 0) {
+			perror("setgid");
+			return 2;
+		}
+	} else if (setgroups(0, NULL) < 0) {	/* no entry: drop all groups */
+		perror("setgroups");
+		return 2;
+	}
+
 	if (setuid((uid_t)v) < 0) {
 		perror("setuid");
 		return 2;

test/src/fuzz-msg-parse.c

@@ -27,7 +27,7 @@
 #include "internal.h"
 
 #define FRAME_MAX      512
-#define HDR_FIXED_SIZE  16	/* must agree with proto.c */
+#define HDR_FIXED_SIZE  LINK_HDR_FIXED_SIZE	/* from proto.h, no drift */
 
 static void fail(const char *what, size_t size)
 {