Re: [PATCH 1/4] selftests/arm: Add mm test

From: Dev Jain
Date: Wed Apr 17 2024 - 00:54:21 EST



On 4/10/24 09:45, Dev Jain wrote:

On 4/7/24 02:53, Muhammad Usama Anjum wrote:
On 4/5/24 1:44 PM, Dev Jain wrote:
This patch tests the 4GB VA restriction for 32-bit processes; it is required
to test the compat layer, whether the kernel knows that it is running a 32-bit
process or not. Chunks are allocated until the VA gets exhausted; mmap must
fail beyond 4GB. This is asserted against the VA mappings found
in /proc/self/maps.

Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
---
  tools/testing/selftests/arm/mm/compat_va.c | 94 ++++++++++++++++++++++
  1 file changed, 94 insertions(+)
  create mode 100644 tools/testing/selftests/arm/mm/compat_va.c

diff --git a/tools/testing/selftests/arm/mm/compat_va.c b/tools/testing/selftests/arm/mm/compat_va.c
new file mode 100644
index 000000000000..3a78f240bc87
--- /dev/null
+++ b/tools/testing/selftests/arm/mm/compat_va.c
@@ -0,0 +1,94 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2024 ARM Limited
+ *
+ * Author : Dev Jain <dev.jain@xxxxxxx>
+ *
+ * Tests 4GB VA restriction for 32 bit process
+ */
+
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/mman.h>
+
+#include <linux/sizes.h>
+#include <kselftest.h>
+
+#define MAP_CHUNK_SIZE    SZ_1M
+#define NR_CHUNKS_4G    (SZ_1G / MAP_CHUNK_SIZE) * 4    /* prevent overflow */
+
+static int validate_address_hint(void)
+{
+    char *ptr;
+
+    ptr = mmap((void *) (1UL << 29), MAP_CHUNK_SIZE, PROT_READ |
+           PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+
+    if (ptr == MAP_FAILED)
+        return 0;
+
+    return 1;
Usually we return negative value instead of positive one which indicates
error situation.

+}
+
+int main(int argc, char *argv[])
+{
+    char *ptr[NR_CHUNKS_4G + 3];
+    char line[1000];
+    const char *file_name;
+    int chunks;
+    FILE *file;
+    int i;
+
+    ksft_print_header();
+    ksft_set_plan(1);
There are multiple test cases. Instead of saying there is only 1 test.
There should be multiple ksft_test_result{_pass,_fail} statements for each
sub-tests.


My initial idea was to treat this as a single logical test, as I

am asserting the restriction on the number of chunks against

the VMAs. I guess your approach is cleaner; thanks.


Thinking again, using a lot of return statements is in fact making the code easier

to follow. If I just set a variable ret = 0/1 and use it to pass or fail, I'll have to

unnecessarily use a lot of if/else statements. Take a look at the examples below.



+
+    /* try allocation beyond 4 GB */
+    for (i = 0; i < NR_CHUNKS_4G + 3; ++i) {
+        ptr[i] = mmap(NULL, MAP_CHUNK_SIZE, PROT_READ | PROT_WRITE,
+                  MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+
+        if (ptr[i] == MAP_FAILED) {
+            if (validate_address_hint())
+                ksft_exit_fail_msg("VA exhaustion failed\n");
+            break;


I will have to set ret value here, forcing two statements inside the if block.

+        }
+    }
+
+    chunks = i;
+    if (chunks >= NR_CHUNKS_4G) {
+        ksft_test_result_fail("mmapped chunks beyond 4GB\n");
+        ksft_finished();
+    }
+
+    /* parse /proc/self/maps, confirm 32 bit VA mappings */
+    file_name = "/proc/self/maps";
+    file = fopen(file_name, "r");
+    if (file == NULL)
+        ksft_exit_fail_msg("/proc/self/maps cannot be opened\n");
+


I will have to set ret here, and enclose the below while statement inside

an else block. In short, saving the value of ret will require if/else blocks

if I were to use it in the end in ksft_test_result(). When I use ksft_exit

statements, it is clear that a problem was spotted here, and there is

no need to study the remaining code.

+    while (fgets(line, sizeof(line), file)) {
+        const char *whitespace_loc, *hyphen_loc;
+
+        hyphen_loc = strchr(line, '-');
+        whitespace_loc = strchr(line, ' ');
+
+        if (!(hyphen_loc && whitespace_loc)) {
+            ksft_test_result_skip("Unexpected format");
+            ksft_finished();
I'm unable to follow as there are too many return statements. If you divide
the test into multiple sub-tests, you can skip/pass/fail each sub-test easily.

+        }
+
+        if ((hyphen_loc - line > 8) ||
+            (whitespace_loc - hyphen_loc) > 9) {
+            ksft_test_result_fail("Memory map more than 32 bits\n");
+            ksft_finished();
+        }
+    }
+
+    for (int i = 0; i < chunks; ++i)
+        munmap(ptr[i], MAP_CHUNK_SIZE);
+
+    ksft_test_result_pass("Test\n");
+    ksft_finished();
+}

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