selftests/powerpc: Add a test of SEGV error behaviour

Add a test case of the error code reported when we take a SEGV on a
mapped but inaccessible area. We broke this recently.

Based on a test case from John Sperbeck <jsperbeck@google.com>.

Acked-by: John Sperbeck <jsperbeck@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/mm/.gitignore

@@ -1,4 +1,5 @@
 hugetlb_vs_thp_test
 subpage_prot
 tempfile
-prot_sao
+prot_sao
+segv_errors

tools/testing/selftests/powerpc/mm/Makefile

@@ -2,7 +2,7 @@
 noarg:
 	$(MAKE) -C ../
 
-TEST_GEN_PROGS := hugetlb_vs_thp_test subpage_prot prot_sao
+TEST_GEN_PROGS := hugetlb_vs_thp_test subpage_prot prot_sao segv_errors
 TEST_GEN_FILES := tempfile
 
 include ../../lib.mk

tools/testing/selftests/powerpc/mm/segv_errors.c

@@ -0,0 +1,78 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright 2017 John Sperbeck
+ *
+ * Test that an access to a mapped but inaccessible area causes a SEGV and
+ * reports si_code == SEGV_ACCERR.
+ */
+
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <signal.h>
+#include <sys/mman.h>
+#include <assert.h>
+#include <ucontext.h>
+
+#include "utils.h"
+
+static bool faulted;
+static int si_code;
+
+static void segv_handler(int n, siginfo_t *info, void *ctxt_v)
+{
+	ucontext_t *ctxt = (ucontext_t *)ctxt_v;
+	struct pt_regs *regs = ctxt->uc_mcontext.regs;
+
+	faulted = true;
+	si_code = info->si_code;
+	regs->nip += 4;
+}
+
+int test_segv_errors(void)
+{
+	struct sigaction act = {
+		.sa_sigaction = segv_handler,
+		.sa_flags = SA_SIGINFO,
+	};
+	char c, *p = NULL;
+
+	p = mmap(NULL, getpagesize(), 0, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
+	FAIL_IF(p == MAP_FAILED);
+
+	FAIL_IF(sigaction(SIGSEGV, &act, NULL) != 0);
+
+	faulted = false;
+	si_code = 0;
+
+	/*
+	 * We just need a compiler barrier, but mb() works and has the nice
+	 * property of being easy to spot in the disassembly.
+	 */
+	mb();
+	c = *p;
+	mb();
+
+	FAIL_IF(!faulted);
+	FAIL_IF(si_code != SEGV_ACCERR);
+
+	faulted = false;
+	si_code = 0;
+
+	mb();
+	*p = c;
+	mb();
+
+	FAIL_IF(!faulted);
+	FAIL_IF(si_code != SEGV_ACCERR);
+
+	return 0;
+}
+
+int main(void)
+{
+	return test_harness(test_segv_errors, "segv_errors");
+}