raidz_test: use only async-signal-safe functions in signal handler

execl*() before glibc 2.24 could allocate, but only if called with at
least 1024 arguments, which five isn't

errno modification is also fine, so long as we restore it at the end

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12086

cmd/raidz_test/raidz_test.c

@@ -37,11 +37,11 @@
 static int *rand_data;
 raidz_test_opts_t rto_opts;
 
-static char gdb[256];
-static const char gdb_tmpl[] = "gdb -ex \"set pagination 0\" -p %d";
+static char pid_s[16];
 
 static void sig_handler(int signo)
 {
+	int old_errno = errno;
 	struct sigaction action;
 	/*
 	 * Restore default action and re-raise signal so SIGSEGV and
@@ -52,10 +52,19 @@ static void sig_handler(int signo)
 	action.sa_flags = 0;
 	(void) sigaction(signo, &action, NULL);
 
-	if (rto_opts.rto_gdb)
-		if (system(gdb)) { }
+	if (rto_opts.rto_gdb) {
+		pid_t pid = fork();
+		if (pid == 0) {
+			execlp("gdb", "gdb", "-ex", "set pagination 0",
+			    "-p", pid_s, NULL);
+			_exit(-1);
+		} else if (pid > 0)
+			while (waitpid(pid, NULL, 0) == -1 && errno == EINTR)
+				;
+	}
 
 	raise(signo);
+	errno = old_errno;
 }
 
 static void print_opts(raidz_test_opts_t *opts, boolean_t force)
@@ -978,8 +987,8 @@ main(int argc, char **argv)
 	struct sigaction action;
 	int err = 0;
 
-	/* init gdb string early */
-	(void) sprintf(gdb, gdb_tmpl, getpid());
+	/* init gdb pid string early */
+	(void) sprintf(pid_s, "%d", getpid());
 
 	action.sa_handler = sig_handler;
 	sigemptyset(&action.sa_mask);