selftest/vm: verify mmap addr in mremap_test

Avoid calling mmap with requested addresses that are less than the
system's mmap_min_addr.  When run as root, mmap returns EACCES when
trying to map addresses < mmap_min_addr.  This is not one of the error
codes for the condition to retry the mmap in the test.

Rather than arbitrarily retrying on EACCES, don't attempt an mmap until
addr > vm.mmap_min_addr.

Add a munmap call after an alignment check as the mappings are retained
after the retry and can reach the vm.max_map_count sysctl.

Link: https://lkml.kernel.org/r/20220420215721.4868-1-sidhartha.kumar@oracle.com
Signed-off-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/vm/mremap_test.c

@@ -6,6 +6,7 @@
 
 #include <errno.h>
 #include <stdlib.h>
+#include <stdio.h>
 #include <string.h>
 #include <sys/mman.h>
 #include <time.h>
@@ -63,6 +64,35 @@ enum {
 	.expect_failure = should_fail				\
 }
 
+/* Returns mmap_min_addr sysctl tunable from procfs */
+static unsigned long long get_mmap_min_addr(void)
+{
+	FILE *fp;
+	int n_matched;
+	static unsigned long long addr;
+
+	if (addr)
+		return addr;
+
+	fp = fopen("/proc/sys/vm/mmap_min_addr", "r");
+	if (fp == NULL) {
+		ksft_print_msg("Failed to open /proc/sys/vm/mmap_min_addr: %s\n",
+			strerror(errno));
+		exit(KSFT_SKIP);
+	}
+
+	n_matched = fscanf(fp, "%llu", &addr);
+	if (n_matched != 1) {
+		ksft_print_msg("Failed to read /proc/sys/vm/mmap_min_addr: %s\n",
+			strerror(errno));
+		fclose(fp);
+		exit(KSFT_SKIP);
+	}
+
+	fclose(fp);
+	return addr;
+}
+
 /*
  * Returns the start address of the mapping on success, else returns
  * NULL on failure.
@@ -71,8 +101,15 @@ static void *get_source_mapping(struct config c)
 {
 	unsigned long long addr = 0ULL;
 	void *src_addr = NULL;
+	unsigned long long mmap_min_addr;
+
+	mmap_min_addr = get_mmap_min_addr();
+
 retry:
 	addr += c.src_alignment;
+	if (addr < mmap_min_addr)
+		goto retry;
+
 	src_addr = mmap((void *) addr, c.region_size, PROT_READ | PROT_WRITE,
 			MAP_FIXED_NOREPLACE | MAP_ANONYMOUS | MAP_SHARED,
 			-1, 0);
@@ -90,8 +127,10 @@ retry:
 	 * alignment in the tests.
 	 */
 	if (((unsigned long long) src_addr & (c.src_alignment - 1)) ||
-			!((unsigned long long) src_addr & c.src_alignment))
+			!((unsigned long long) src_addr & c.src_alignment)) {
+		munmap(src_addr, c.region_size);
 		goto retry;
+	}
 
 	if (!src_addr)
 		goto error;