tests: add kill_child test

This tests repeatedly creates and kills children, so some corner
cases in handling of not-quite-existing processes can be observed.

Previously, strace was crashing in the following situation:

    13994 ????( <unfinished ...>
    ...
    13994 <... ???? resumed>) = ?

as tcp->s_ent wasn't initialised on syscall entering and
strace.c:print_event_exit segfaulted when tried to access
tcp->s_ent->sys_name.

* tests/kill_child.c: New file.
* tests/kill_child.test: New test.
* tests/Makefile.am (check_PROGRAMS): Add kill_child.
(MISC_TESTS): Add kill_child.test.

tests/Makefile.am

@@ -106,6 +106,7 @@ check_PROGRAMS = $(PURE_EXECUTABLES) \
 	ioctl_perf-success \
 	ioctl_rtc-v \
 	is_linux_mips_n64 \
+	kill_child \
 	ksysent \
 	list_sigaction_signum \
 	localtime \
@@ -307,6 +308,7 @@ MISC_TESTS = \
 	get_regs.test \
 	inject-nf.test \
 	interactive_block.test \
+	kill_child.test \
 	ksysent.test \
 	localtime.test \
 	looping_threads.test \

tests/kill_child.c

@@ -0,0 +1,46 @@
+/**
+ * Check for a corner case that previously lead to segfault due to an attempt
+ * to access unitialised tcp->s_ent.
+ *
+ * 13994 ????( <unfinished ...>
+ * ...
+ * 13994 <... ???? resumed>) = ?
+ */
+
+#include <signal.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+
+#include "tests.h"
+
+#define ITERS    10000
+#define SC_ITERS 10000
+
+int main(void)
+{
+	unsigned int i;
+
+	for (i = 0; i < ITERS; i++) {
+		int pid = fork();
+
+		if (pid < 0)
+			perror_msg_and_fail("fork");
+
+		if (!pid) {
+			unsigned int j;
+
+			for (j = 0; j < SC_ITERS; j++)
+				getuid();
+
+			pause();
+
+			return 0;
+		}
+
+		kill(pid, SIGKILL);
+		wait(NULL);
+	}
+
+	return 0;
+}

tests/kill_child.test

@@ -0,0 +1,17 @@
+#!/bin/sh
+#
+# Check whether repeated killing of just forked processes crashes strace.
+
+. "${srcdir=.}/init.sh"
+
+run_prog "../$NAME"
+
+# Run strace until the known corner case is observed
+while :; do
+	run_strace -f -qq -esignal=none $args
+
+	# Printing of "<... SYSCALL resumed>" in strace.c:print_event_exit
+	# previously led to segfault if the syscall number hadn't been obtained
+	# on syscall entering.
+	grep -qE '^[0-9]+ <\.\.\. \?\?\?\? resumed>\) += \?$' "$LOG" && exit 0
+done