From 51954a3b812c57c57835be1b4b06c5c9918c5783 Mon Sep 17 00:00:00 2001
From: Taylor R Campbell <riastradh@NetBSD.org>
Date: Sat, 15 Jul 2023 20:02:28 +0000
Subject: [PATCH] system(3): Change SIG_IGN to SIG_DFL for SIGCHLD while we
 work.

PR kern/57527
---
 lib/libc/stdlib/system.c | 32 ++++++++++++++++++++++++++++++--
 1 file changed, 30 insertions(+), 2 deletions(-)

diff --git a/lib/libc/stdlib/system.c b/lib/libc/stdlib/system.c
index 5a19bc9e0568..4d3e45f1892a 100644
--- a/lib/libc/stdlib/system.c
+++ b/lib/libc/stdlib/system.c
@@ -54,7 +54,7 @@ int
 system(const char *command)
 {
 	pid_t pid;
-	struct sigaction intsa, quitsa, sa;
+	struct sigaction chldsa, intsa, quitsa, sa;
 	sigset_t nmask, omask, sigdefault;
 	int pstat;
 	const char *argp[] = {"sh", "-c", "--", command, NULL};
@@ -90,6 +90,32 @@ system(const char *command)
 		return -1;
 	}
 
+	/*
+	 * Make sure SIGCHLD is not set to SIG_IGN, because that means
+	 * waitpid will wait until _all_ children have exited, under a
+	 * peculiar clause of POSIX enshrining a quirk of SysV
+	 * semantics.
+	 *
+	 * If SIGCHLD was set to anything other than SIG_IGN, we must
+	 * not interfere with it -- changing the action may clear any
+	 * queued signals for for processes _other_ than the one that
+	 * system(3) itself spawns.
+	 */
+	if (sigaction(SIGCHLD, NULL, &chldsa) == -1) {
+		(void)sigprocmask(SIG_SETMASK, &omask, NULL);
+		sigaction(SIGINT, &intsa, NULL);
+		sigaction(SIGQUIT, &quitsa, NULL);
+		return -1;
+	}
+	if (chldsa.sa_handler == SIG_IGN &&
+	    sigaction(SIGCHLD, &(struct sigaction){ .sa_handler = SIG_DFL },
+		NULL) == -1) {
+		(void)sigprocmask(SIG_SETMASK, &omask, NULL);
+		sigaction(SIGINT, &intsa, NULL);
+		sigaction(SIGQUIT, &quitsa, NULL);
+		return -1;
+	}
+
 	/*
 	 * We arrange to inherit all signal handlers from the caller by
 	 * default, except possibly SIGINT and SIGQUIT.  These we have
@@ -133,7 +159,9 @@ system(const char *command)
 		}
 	}
 
-out:	sigaction(SIGINT, &intsa, NULL);
+out:	if (chldsa.sa_handler == SIG_IGN)
+		sigaction(SIGCHLD, &chldsa, NULL);
+	sigaction(SIGINT, &intsa, NULL);
 	sigaction(SIGQUIT, &quitsa, NULL);
 	(void)sigprocmask(SIG_SETMASK, &omask, NULL);