Give the server its own session keyring on Linux

The server otherwise keeps the session keyring of the login session that
started it, which pam_keyinit(8) may revoke when that session ends,
leaving the server and its panes unable to use the kernel keyring. Join
a new session keyring with the user keyring linked in once the
configuration file has been loaded, and revoke it when the server exits.

This is controlled by a new session-keyring server option, on by
default.

compat/keyring-linux.c

@@ -0,0 +1,42 @@
+#include <sys/types.h>
+#include <sys/syscall.h>
+
+#include <linux/keyctl.h>
+#include <errno.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "tmux.h"
+
+/* Serial of the session keyring we joined, or 0 if none. */
+static long	keyring_serial;
+
+/*
+ * Create a new session keyring since the current one belongs to the login
+ * session which we're meant to outlive (and pam_keyinit(8) is likely to revoke
+ * the login session's keyring when it ends).
+ */
+void
+keyring_join(void)
+{
+	long	serial;
+
+	serial = syscall(SYS_keyctl, KEYCTL_JOIN_SESSION_KEYRING, NULL);
+	if (serial == -1) {
+		log_debug("%s: join failed: %s", __func__, strerror(errno));
+		return;
+	}
+	if (syscall(SYS_keyctl, KEYCTL_LINK, KEY_SPEC_USER_KEYRING,
+	    KEY_SPEC_SESSION_KEYRING) == -1)
+		log_debug("%s: link failed: %s", __func__, strerror(errno));
+	keyring_serial = serial;
+}
+
+void
+keyring_revoke(void)
+{
+	if (keyring_serial == 0)
+		return;
+	if (syscall(SYS_keyctl, KEYCTL_REVOKE, keyring_serial) == -1)
+		log_debug("%s: revoke failed: %s", __func__, strerror(errno));
+}