[PATCH] selftests/mm: Handle implicit read permissions in madv_populate

[Xionggui Chen <evers_chen@163.com>]

From: Xionggui Chen <chenxionggui@lanxincomputing.com>

The madv_populate selftest assumes that a mapping created with
PROT_WRITE cannot be read, and expects MADV_POPULATE_READ to fail
with EINVAL.

However, some architectures, including RISC-V, may implicitly grant
read permission to PROT_WRITE mappings. On such architectures,
MADV_POPULATE_READ succeeds, causing the test to fail incorrectly.

Check the actual VMA permissions through /proc/self/maps and adjust
the expected result accordingly.

Add a PROT_NONE test to verify that MADV_POPULATE_READ still fails
with EINVAL for mappings without read permission.

On RISC-V with Linux 7.2.0, the original test fails 1 out of 16
tests, while the updated test passes all 17 tests.

Signed-off-by: Xionggui Chen <chenxionggui@lanxincomputing.com>
---
 tools/testing/selftests/mm/madv_populate.c | 67 ++++++++++++++++++++--
 1 file changed, 63 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/mm/madv_populate.c b/tools/testing/selftests/mm/madv_populate.c
index 7fce5d0b622b..818f99ee2004 100644
--- a/tools/testing/selftests/mm/madv_populate.c
+++ b/tools/testing/selftests/mm/madv_populate.c
@@ -69,10 +69,38 @@ static void test_prot_read(void)
 	munmap(addr, SIZE);
 }
 
+static int mapping_is_readable(void *addr)
+{
+	FILE *maps;
+	char line[256];
+	unsigned long start, end;
+	char perms[5];
+	int readable = -1;
+
+	maps = fopen("/proc/self/maps", "r");
+	if (!maps)
+		return -1;
+
+	while (fgets(line, sizeof(line), maps)) {
+		if (sscanf(line, "%lx-%lx %4s",
+			   &start, &end, perms) != 3)
+			continue;
+
+		if (start <= (unsigned long)addr &&
+		    (unsigned long)addr < end) {
+			readable = perms[0] == 'r';
+			break;
+		}
+	}
+
+	fclose(maps);
+	return readable;
+}
+
 static void test_prot_write(void)
 {
 	char *addr;
-	int ret;
+	int ret, readable;
 
 	ksft_print_msg("[RUN] %s\n", __func__);
 
@@ -80,9 +108,19 @@ static void test_prot_write(void)
 	if (addr == MAP_FAILED)
 		ksft_exit_fail_msg("mmap failed\n");
 
+	readable = mapping_is_readable(addr);
+	if (readable < 0)
+		ksft_exit_fail_msg("Cannot determine VMA permissions\n");
+
+	errno = 0;
 	ret = madvise(addr, SIZE, MADV_POPULATE_READ);
-	ksft_test_result(ret == -1 && errno == EINVAL,
-			 "MADV_POPULATE_READ with PROT_WRITE\n");
+
+	if (readable)
+		ksft_test_result(ret == 0,
+				 "MADV_POPULATE_READ with readable PROT_WRITE mapping\n");
+	else
+		ksft_test_result(ret == -1 && errno == EINVAL,
+				 "MADV_POPULATE_READ with write-only mapping\n");
 
 	ret = madvise(addr, SIZE, MADV_POPULATE_WRITE);
 	ksft_test_result(!ret, "MADV_POPULATE_WRITE with PROT_WRITE\n");
@@ -90,6 +128,26 @@ static void test_prot_write(void)
 	munmap(addr, SIZE);
 }
 
+static void test_prot_none(void)
+{
+	char *addr;
+	int ret;
+
+	ksft_print_msg("[RUN] %s\n", __func__);
+
+	addr = mmap(0, SIZE, PROT_NONE,
+		    MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
+	if (addr == MAP_FAILED)
+		ksft_exit_fail_msg("mmap failed\n");
+
+	errno = 0;
+	ret = madvise(addr, SIZE, MADV_POPULATE_READ);
+	ksft_test_result(ret == -1 && errno == EINVAL,
+			 "MADV_POPULATE_READ with PROT_NONE\n");
+
+	munmap(addr, SIZE);
+}
+
 static void test_holes(void)
 {
 	char *addr;
@@ -266,7 +324,7 @@ static void test_softdirty(void)
 
 int main(int argc, char **argv)
 {
-	int nr_tests = 16;
+	int nr_tests = 17;
 	int err;
 
 	pagesize = getpagesize();
@@ -280,6 +338,7 @@ int main(int argc, char **argv)
 	sense_support();
 	test_prot_read();
 	test_prot_write();
+	test_prot_none();
 	test_holes();
 	test_populate_read();
 	test_populate_write();
-- 
2.53.0