selftests/powerpc: Add test of memcmp at end of page

Update our memcmp selftest, to test the case where we're comparing up
to the end of a page and the subsequent page is not mapped. We have to
make sure we don't read off the end of the page and cause a fault.

We had a bug there in the past, fixed in commit
d9470757 ("powerpc/64: Fix memcmp reading past the end of src/dest").
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200722055315.962391-1-mpe@ellerman.id.au

tools/testing/selftests/powerpc/stringloops/memcmp.c

@@ -2,6 +2,7 @@
 #include <malloc.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/mman.h>
 #include <time.h>
 #include "utils.h"
 
@@ -13,6 +14,9 @@
 #define LARGE_MAX_OFFSET 32
 #define LARGE_SIZE_START 4096
 
+/* This is big enough to fit LARGE_SIZE and works on 4K & 64K kernels */
+#define MAP_SIZE (64 * 1024)
+
 #define MAX_OFFSET_DIFF_S1_S2 48
 
 int vmx_count;
@@ -68,25 +72,25 @@ static void test_one(char *s1, char *s2, unsigned long max_offset,
 
 static int testcase(bool islarge)
 {
-	char *s1;
-	char *s2;
-	unsigned long i;
-
-	unsigned long comp_size = (islarge ? LARGE_SIZE : SIZE);
-	unsigned long alloc_size = comp_size + MAX_OFFSET_DIFF_S1_S2;
-	int iterations = islarge ? LARGE_ITERATIONS : ITERATIONS;
-
-	s1 = memalign(128, alloc_size);
-	if (!s1) {
-		perror("memalign");
-		exit(1);
-	}
+	unsigned long i, comp_size, alloc_size;
+	char *p, *s1, *s2;
+	int iterations;
 
-	s2 = memalign(128, alloc_size);
-	if (!s2) {
-		perror("memalign");
-		exit(1);
-	}
+	comp_size = (islarge ? LARGE_SIZE : SIZE);
+	alloc_size = comp_size + MAX_OFFSET_DIFF_S1_S2;
+	iterations = islarge ? LARGE_ITERATIONS : ITERATIONS;
+
+	p = mmap(NULL, 4 * MAP_SIZE, PROT_READ | PROT_WRITE,
+		 MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+	FAIL_IF(p == MAP_FAILED);
+
+	/* Put s1/s2 at the end of a page */
+	s1 = p + MAP_SIZE - alloc_size;
+	s2 = p + 3 * MAP_SIZE - alloc_size;
+
+	/* And unmap the subsequent page to force a fault if we overread */
+	munmap(p + MAP_SIZE, MAP_SIZE);
+	munmap(p + 3 * MAP_SIZE, MAP_SIZE);
 
 	srandom(time(0));