Prevent abnormal termination of a child daemon process when created

by a parent that is a session leader (e.g., login shell) by ignoring
SIGHUP in before calling fork(2) and then restoring SIGHUP's action
after setsid(3).  Based on the patch by Martin Kammerhofer
<mkamm@gmx.net>.

PR:		bin/25462
Reviewed by:	bde, alex.neyman@auriga.ru

2 files changed, 35 insertions, 3 deletions

diff --git a/lib/libc/gen/daemon.3 b/lib/libc/gen/daemon.3
index a08d0df..97025e6 100644
--- a/lib/libc/gen/daemon.3
+++ b/lib/libc/gen/daemon.3
@@ -71,12 +71,13 @@ The
 function may fail and set
 .Va errno
 for any of the errors specified for the library functions
-.Xr fork 2
+.Xr fork 2 ,
 and
 .Xr setsid 2 .
 .Sh SEE ALSO
 .Xr fork 2 ,
-.Xr setsid 2
+.Xr setsid 2 ,
+.Xr sigaction 2
 .Sh HISTORY
 The
 .Fn daemon
@@ -101,3 +102,15 @@ should therefore either call
 .Fn daemon
 before opening any files or sockets, or verify that any file
 descriptors obtained have values greater than 2.
+.Pp
+The
+.Fn daemon
+function temporarily ignores
+.Dv SIGHUP
+while calling
+.Xr setsid 2
+to prevent a parent session group leader's calls to
+.Xr fork 2
+and then
+.Xr _exit 2
+from prematurely terminating the child process.
diff --git a/lib/libc/gen/daemon.c b/lib/libc/gen/daemon.c
index 315b827..0158976 100644
--- a/lib/libc/gen/daemon.c
+++ b/lib/libc/gen/daemon.c
@@ -38,8 +38,10 @@ static char sccsid[] = "@(#)daemon.c	8.1 (Berkeley) 6/4/93";
 __FBSDID("$FreeBSD$");
 
 #include "namespace.h"
+#include <errno.h>
 #include <fcntl.h>
 #include <paths.h>
+#include <signal.h>
 #include <unistd.h>
 #include "un-namespace.h"
 
@@ -47,7 +49,17 @@ int
 daemon(nochdir, noclose)
 	int nochdir, noclose;
 {
+	struct sigaction osa, sa;
 	int fd;
+	pid_t newgrp;
+	int oerrno;
+	int osa_ok;
+
+	/* A SIGHUP may be thrown when the parent exits below. */
+	sigemptyset(&sa.sa_mask);
+	sa.sa_handler = SIG_IGN;
+	sa.sa_flags = 0;
+	osa_ok = _sigaction(SIGHUP, &sa, &osa);
 
 	switch (fork()) {
 	case -1:
@@ -58,8 +70,15 @@ daemon(nochdir, noclose)
 		_exit(0);
 	}
 
-	if (setsid() == -1)
+	newgrp = setsid();
+	oerrno = errno;
+	if (osa_ok != -1)
+		_sigaction(SIGHUP, &osa, NULL);
+
+	if (newgrp == -1) {
+		errno = oerrno;
 		return (-1);
+	}
 
 	if (!nochdir)
 		(void)chdir("/");