use an alternative stack for SIGSEGV and test for stack overflow

Previously, we could run out of stack handling stack overflows, leaving
users with unspecific SIGSEGV crashes and no panic message.

By providing a single alternative stack exclusively for SIGSEGV handling
where sigaltstack() is available, we increase chances for our signal handler
to finish successfully.

In particular, this will make it easier to diagnose stack overflows by
comparing the failing address with the stack info from the panic output.

This could be further improved by giving advise to increase thread_pool_stack
if si_addr is near the stack boundaries.

c00057.vtc now triggers a stack overflow instead of raising a SIGSEGV.

Merges #2396

bin/varnishd/mgt/mgt_child.c

@@ -392,9 +392,23 @@ mgt_launch_child(struct cli *cli)
 			memset(&sa, 0, sizeof sa);
 			sa.sa_sigaction = child_signal_handler;
 			sa.sa_flags = SA_SIGINFO;
-			(void)sigaction(SIGSEGV, &sa, NULL);
 			(void)sigaction(SIGBUS, &sa, NULL);
 			(void)sigaction(SIGABRT, &sa, NULL);
+
+#ifdef HAVE_SIGALTSTACK
+			stack_t ss;
+			size_t sz = SIGSTKSZ + 4096;
+			if (sz < mgt_param.wthread_stacksize)
+				sz = mgt_param.wthread_stacksize;
+			ss.ss_sp = malloc(sz);
+			AN(ss.ss_sp);
+			ss.ss_size = sz;
+			ss.ss_flags = 0;
+			AZ(sigaltstack(&ss, NULL));
+			sa.sa_flags |= SA_ONSTACK;
+#endif
+			(void)sigaction(SIGSEGV, &sa, NULL);
+
 		}
 		(void)signal(SIGINT, SIG_DFL);
 		(void)signal(SIGTERM, SIG_DFL);

bin/varnishtest/tests/c00057.vtc

@@ -5,13 +5,32 @@ server s1 {
 	txresp
 } -start
 
-varnish v1 -cliok "param.set vcc_allow_inline_c true"
-varnish v1 -vcl+backend {
+varnish v1 \
+	-arg "-p vcc_allow_inline_c=true" \
+	-arg "-p thread_pool_stack=48k" \
+	-vcl+backend {
 	C{
 #include <signal.h>
 #include <unistd.h>
+#include <stdio.h>
+
+static void _accessor(volatile char *p) {
+     p[0] = 'V'; p[1] = '\0';
+     fprintf(stderr, "%p %s\n", p, p);
+}
+void (*accessor)(volatile char *p) = _accessor;
+
 }C
-	sub vcl_recv { C{ raise(SIGSEGV); sleep(2); }C }
+	sub vcl_recv { C{
+	    int i;
+	    volatile char overflow[48*1024];
+
+	    /* for downwards stack, take care to hit a single guard page */
+	    for (i = 47*1024; i >= 0; i -= 1024)
+		accessor(overflow + i);
+	    /* NOTREACHED */
+	    sleep(2);
+	}C }
 } -start
 
 client c1 {

configure.ac

@@ -258,6 +258,7 @@ AC_CHECK_FUNCS([setppriv])
 AC_CHECK_FUNCS([fallocate])
 AC_CHECK_FUNCS([closefrom])
 AC_CHECK_FUNCS([vsyslog])
+AC_CHECK_FUNCS([sigaltstack])
 
 save_LIBS="${LIBS}"
 LIBS="${PTHREAD_LIBS}"