selftests/bpf: Silence array bounds warning in global_map_resize

When compiling BPF selftests with -O2, GCC reports an array bounds
violation warning in global_map_resize test:

    In function ‘global_map_resize_bss_subtest’,
        inlined from ‘test_global_map_resize’ at /bpf-next/tools/testing/selftests/bpf/prog_tests/global_map_resize.c:228:3:
    /bpf-next/tools/testing/selftests/bpf/prog_tests/global_map_resize.c:64:33: error: array subscript 1 is above array bounds of ‘int[1]’ [-Werror=array-bounds=]
       64 |                 skel->bss->array[i] = 1;
          |                 ~~~~~~~~~~~~~~~~^~~
    In file included from /bpf-next/tools/testing/selftests/bpf/prog_tests/global_map_resize.c:6:
    ./test_global_map_resize.skel.h: In function ‘test_global_map_resize’:
    ./test_global_map_resize.skel.h:44:21: note: while referencing ‘array’
       44 |                 int array[1];
          |                     ^~~~~

This is a false positive because `array` (a BPF map) has been resized
from within the BPF program. GCC doesn't know that so let us silence the
warning by accessing the array via a plain pointer.

Fixes: 08b089567573 ("libbpf: Selftests for resizing datasec maps")
Signed-off-by: Viktor Malik <vmalik@redhat.com>
Link: https://lore.kernel.org/bpf/57765bc465a27923c3c093eba222cc24d08d8c40.1784112948.git.vmalik@redhat.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/tools/testing/selftests/bpf/prog_tests/global_map_resize.c b/tools/testing/selftests/bpf/prog_tests/global_map_resize.c
index 56b5baef35c8..602ce30f1720 100644
--- a/tools/testing/selftests/bpf/prog_tests/global_map_resize.c
+++ b/tools/testing/selftests/bpf/prog_tests/global_map_resize.c
@@ -23,6 +23,7 @@ static void global_map_resize_bss_subtest(void)
 	struct bpf_map *map;
 	const __u32 desired_sz = sizeof(skel->bss->sum) + sysconf(_SC_PAGE_SIZE) * 2;
 	size_t array_len, actual_sz, new_sz;
+	int *array;
 
 	skel = test_global_map_resize__open();
 	if (!ASSERT_OK_PTR(skel, "test_global_map_resize__open"))
@@ -58,10 +59,13 @@ static void global_map_resize_bss_subtest(void)
 		goto teardown;
 
 	/* fill the newly resized array with ones,
-	 * skipping the first element which was previously set
+	 * skipping the first element which was previously set;
+	 * access through a plain pointer to avoid -Warray-bounds
+	 * since the array was resized beyond its declared length.
 	 */
+	array = skel->bss->array;
 	for (int i = 1; i < array_len; i++)
-		skel->bss->array[i] = 1;
+		array[i] = 1;
 
 	/* set global const values before loading */
 	skel->rodata->pid = getpid();