update explicit_bzero test to not assume SIGSTKSZ to be constant

[Brent Cook]

In the next version of Linux glibc, SIGSTKSZ is defined at runtime if
source is built with _GNU_SOURCE. On LibreSSL-portable, this is set to
bring in asprintf/vasprintf, which causes the explicit_bzero test to
fail to compile since the size of SIGSTKSZ is no longer known at compile
time. This adjusts the test to treat SIGSTKSZ as a runtime variable.

See http://patches-tcwg.linaro.org/patch/48127/ and
https://github.com/libressl-portable/portable/issues/653 for the
LibreSSL build failure report on Fedora Rawhide.

ok?

Index: explicit_bzero.c
===================================================================
RCS file: /cvs/src/regress/lib/libc/explicit_bzero/explicit_bzero.c,v
retrieving revision 1.6
diff -u -p -u -p -r1.6 explicit_bzero.c
--- explicit_bzero.c    11 Jul 2014 01:10:35 -0000      1.6
+++ explicit_bzero.c    23 Mar 2021 01:32:21 -0000
@@ -18,6 +18,7 @@
 #include <assert.h>
 #include <errno.h>
 #include <signal.h>
+#include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
 
@@ -36,16 +37,20 @@ enum {
        SECRETBYTES = SECRETCOUNT * sizeof(secret)
 };
 
-static char altstack[SIGSTKSZ + SECRETBYTES];
+static char *altstack;
+#define ALTSTACK_SIZE (SIGSTKSZ + SECRETBYTES)
 
 static void
 setup_stack(void)
 {
+       altstack = malloc(ALTSTACK_SIZE);
+
        const stack_t sigstk = {
                .ss_sp = altstack,
-               .ss_size = sizeof(altstack),
+               .ss_size = ALTSTACK_SIZE
        };
 
+       ASSERT_NE(NULL, altstack);
        ASSERT_EQ(0, sigaltstack(&sigstk, NULL));
 }
 
@@ -129,7 +134,7 @@ test_without_bzero()
        char buf[SECRETBYTES];
        assert_on_stack();
        populate_secret(buf, sizeof(buf));
-       char *res = memmem(altstack, sizeof(altstack), buf, sizeof(buf));
+       char *res = memmem(altstack, ALTSTACK_SIZE, buf, sizeof(buf));
        ASSERT_NE(NULL, res);
        return (res);
 }
@@ -140,7 +145,7 @@ test_with_bzero()
        char buf[SECRETBYTES];
        assert_on_stack();
        populate_secret(buf, sizeof(buf));
-       char *res = memmem(altstack, sizeof(altstack), buf, sizeof(buf));
+       char *res = memmem(altstack, ALTSTACK_SIZE, buf, sizeof(buf));
        ASSERT_NE(NULL, res);
        explicit_bzero(buf, sizeof(buf));
        return (res);
@@ -183,14 +188,14 @@ main()
         * on the stack.  This sanity checks that call_on_stack() and
         * populate_secret() work as intended.
         */
-       memset(altstack, 0, sizeof(altstack));
+       memset(altstack, 0, ALTSTACK_SIZE);
        call_on_stack(do_test_without_bzero);
 
        /*
         * Now test with a call to explicit_bzero() and check that we
         * *don't* find any instances of the secret data.
         */
-       memset(altstack, 0, sizeof(altstack));
+       memset(altstack, 0, ALTSTACK_SIZE);
        call_on_stack(do_test_with_bzero);
 
        return (0);