passt: Fix error check for signal(), improve error messages

Valtteri reports that if SIGPIPE already has a disposition set by the
parent process, such as systemd with the default setting of
IgnoreSIGPIPE=yes, signal() will return the previous value, not zero,
and this is not an error: check for SIG_ERR instead.

While at it, split messages for failures of sigaction() and signal(),
and report the actual error.

Reported-by: Valtteri Vuorikoski <vuori@notcom.org>
Fixes: 8534be076c73 ("Catch failures when installing signal handlers")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 9 insertions, 2 deletions

diff --git a/passt.c b/passt.c
index 7fa1170..3b9b36b 100644
--- a/passt.c
+++ b/passt.c
@@ -204,8 +204,15 @@ int main(int argc, char **argv)
 	name = basename(argv0);
 	if (strstr(name, "pasta")) {
 		sa.sa_handler = pasta_child_handler;
-		if (sigaction(SIGCHLD, &sa, NULL) || signal(SIGPIPE, SIG_IGN))
-			die("Couldn't install signal handlers");
+		if (sigaction(SIGCHLD, &sa, NULL)) {
+			die("Couldn't install signal handlers: %s",
+			    strerror(errno));
+		}
+
+		if (signal(SIGPIPE, SIG_IGN) == SIG_ERR) {
+			die("Couldn't set disposition for SIGPIPE: %s",
+			    strerror(errno));
+		}
 
 		c.mode = MODE_PASTA;
 		log_name = "pasta";