[PATCH 2/4] selftest/vm: verify remap destination address in mremap_test

[Sidhartha Kumar]

Because mremap does not have a NOREPLACE flag,
it can destroy existing mappings. This can
cause a segfault if regions such as text are
destroyed. Verify the requested mremap destination
address does not overlap any existing mappings
by using mmap's FIXED_NOREPLACE flag and checking
for the EEXIST error code. Keep incrementing the
destination address until a valid mapping is found
or max address is reached.

Signed-off-by: Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx>
---
 tools/testing/selftests/vm/mremap_test.c | 36 ++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/tools/testing/selftests/vm/mremap_test.c b/tools/testing/selftests/vm/mremap_test.c
index 58600fee4b81..98e9cff34aa7 100644
--- a/tools/testing/selftests/vm/mremap_test.c
+++ b/tools/testing/selftests/vm/mremap_test.c
@@ -10,6 +10,7 @@
 #include <string.h>
 #include <sys/mman.h>
 #include <time.h>
+#include <limits.h>
 
 #include "../kselftest.h"
 
@@ -65,6 +66,34 @@ enum {
 	.expect_failure = should_fail				\
 }
 
+/*
+ * Returns 0 if the requested remap region overlaps with an
+ * existing mapping (e.g text, stack) else returns 1.
+ */
+static int remap_region_valid(void *addr, unsigned long long size)
+{
+	void *remap_addr = NULL;
+	int ret = 1;
+
+	if ((unsigned long long) addr > ULLONG_MAX - size) {
+		ksft_print_msg("Can't find a valid region to remap to\n");
+		exit(KSFT_SKIP);
+	}
+
+	/* Use MAP_FIXED_NOREPLACE flag to ensure region is not mapped */
+	remap_addr = mmap(addr, size, PROT_READ | PROT_WRITE,
+			MAP_FIXED_NOREPLACE | MAP_ANONYMOUS | MAP_SHARED,
+			-1, 0);
+	if (remap_addr == MAP_FAILED) {
+		if (errno == EEXIST)
+			ret = 0;
+	} else {
+		munmap(remap_addr, size);
+	}
+
+	return ret;
+}
+
 /* Returns mmap_min_addr sysctl */
 static unsigned long long get_mmap_min_addr(void)
 {
@@ -180,6 +209,13 @@ static long long remap_region(struct config c, unsigned int threshold_mb,
 	if (!((unsigned long long) addr & c.dest_alignment))
 		addr = (void *) ((unsigned long long) addr | c.dest_alignment);
 
+	/* Don't destroy existing mappings unless expected to overlap */
+	while (!remap_region_valid(addr, c.region_size)) {
+		if (c.overlapping)
+			break;
+		addr += c.src_alignment;
+	}
+
 	clock_gettime(CLOCK_MONOTONIC, &t_start);
 	dest_addr = mremap(src_addr, c.region_size, c.region_size,
 			MREMAP_MAYMOVE|MREMAP_FIXED, (char *) addr);
-- 
2.27.0