[Patch V3 2/4] unshare: implement signal forwarding when --forward-signals is used

[Kiran Rangoon <kiranrangoon0@gmail.com>]

When --forward-signals is specified, install signal handlers for
SIGTERM and SIGINT that forward these signals to the child process.
This allows child processes with cleanup handlers to execute gracefully during shutdown scenarios like
system reboot.

Also fix waitpid() to handle EINTR properly when signals are being
forwarded, ensuring the parent waits for the child to complete
signal handling before exiting.

Signed-off-by: Kiran Rangoon <kiranrangoon0@gmail.com>
---
 sys-utils/unshare.c | 55 +++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 51 insertions(+), 4 deletions(-)

diff --git a/sys-utils/unshare.c b/sys-utils/unshare.c
index 8a7a26df2..a4d28892d 100644
--- a/sys-utils/unshare.c
+++ b/sys-utils/unshare.c
@@ -74,6 +74,9 @@ static struct namespace_file {
 
 static int npersists;	/* number of persistent namespaces */
 
+/* Global PID of child process for signal forwarding */
+static volatile pid_t child_pid = 0;
+
 enum {
 	SETGROUPS_NONE = -1,
 	SETGROUPS_DENY = 0,
@@ -135,6 +138,19 @@ static void map_id(const char *file, uint32_t from, uint32_t to)
 	close(fd);
 }
 
+/**
+ * forward_signal() - Forward signal to child process
+ * @sig: Signal number to forward
+ *
+ * Signal handler that forwards SIGTERM/SIGINT from parent to child.
+ * Only used when --forward-signals flag is enabled.
+ */
+static void forward_signal(int sig)
+{
+	if (child_pid > 0)
+		kill(child_pid, sig);
+}
+
 static unsigned long parse_propagation(const char *str)
 {
 	size_t i;
@@ -1107,11 +1123,32 @@ int main(int argc, char *argv[])
 		settime(monotonic, CLOCK_MONOTONIC);
 
 	if (forkit) {
+		if (forward_signals) {
+			/* Install signal handlers to forward signals to child */
+			struct sigaction sa;
+
+			memset(&sa, 0, sizeof(sa));
+			sa.sa_handler = forward_signal;
+			sigemptyset(&sa.sa_mask);
+			sa.sa_flags = SA_RESTART;
+
+			if (sigaction(SIGTERM, &sa, NULL) == -1)
+				err(EXIT_FAILURE, _("sigaction SIGTERM failed"));
+			if (sigaction(SIGINT, &sa, NULL) == -1)
+				err(EXIT_FAILURE, _("sigaction SIGINT failed"));
+
+		/* Save old mask for child to restore */
+		if (sigemptyset(&sigset) != 0 ||
+		    sigprocmask(SIG_SETMASK, NULL, &oldsigset) != 0)
+			err(EXIT_FAILURE, _("sigprocmask failed"));
+	} else {
+		/* Block signals to prevent "impatient parent" problem */
 		if (sigemptyset(&sigset) != 0 ||
-			sigaddset(&sigset, SIGINT) != 0 ||
-			sigaddset(&sigset, SIGTERM) != 0 ||
-			sigprocmask(SIG_BLOCK, &sigset, &oldsigset) != 0)
+		    sigaddset(&sigset, SIGINT) != 0 ||
+		    sigaddset(&sigset, SIGTERM) != 0 ||
+		    sigprocmask(SIG_BLOCK, &sigset, &oldsigset) != 0)
 			err(EXIT_FAILURE, _("sigprocmask block failed"));
+	}
 #ifdef HAVE_PIDFD_OPEN
 		if (kill_child_signo != 0) {
 			/* make a connection to the original process (parent) */
@@ -1128,6 +1165,7 @@ int main(int argc, char *argv[])
 		case -1:
 			err(EXIT_FAILURE, _("fork failed"));
 		case 0:	/* child */
+			child_pid = 0; /* Clear in child process */
 			if (sigprocmask(SIG_SETMASK, &oldsigset, NULL))
 				err(EXIT_FAILURE,
 					_("sigprocmask restore failed"));
@@ -1135,6 +1173,8 @@ int main(int argc, char *argv[])
 				close(fd_bind);
 			break;
 		default: /* parent */
+			if (forward_signals)
+				child_pid = pid; /* Store for signal handler */
 			break;
 		}
 	}
@@ -1149,7 +1189,14 @@ int main(int argc, char *argv[])
 	}
 
 	if (pid) {
-		if (waitpid(pid, &status, 0) == -1)
+		int rc;
+
+		/* Wait for child, handling EINTR from signal forwarding */
+		do {
+			rc = waitpid(pid, &status, 0);
+		} while (rc == -1 && errno == EINTR);
+
+		if (rc == -1)
 			err(EXIT_FAILURE, _("waitpid failed"));
 
 		if (WIFEXITED(status))
-- 
2.47.3