Re: [PATCH bpf-next] selftests/bpf: Add arena direct-value one-past-end reject test

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Wed Jun 3, 2026 at 8:09 AM EDT, Woojin Ji wrote:
> BPF_MAP_TYPE_ARENA supports direct-value pseudo loads, but unlike array
> maps its map value_size is zero and the valid direct-value range is the
> arena mmap size, max_entries * PAGE_SIZE.
>
> Commit 3ac1a467e376 ("bpf: Fix off-by-one boundary validation in arena
> direct-value access") fixed arena_map_direct_value_addr() to reject an
> offset exactly at the end of the arena mapping. Add a regression test
> that loads a BPF_PSEUDO_MAP_VALUE with off == arena_size and verifies
> that the verifier rejects it with the expected offset in the log.
>
> Unlike __arena global relocation, where insn[1].imm depends on
> compiler/layout, this raw instruction directly drives
> map_direct_value_addr(map, &addr, off) with off == arena_size, so the
> boundary check is exercised directly.
>
> Assisted-by: ChatGPT:gpt-5.5
> Signed-off-by: Woojin Ji <random6.xyz@gmail.com>
> ---

Makes sense to add this, but I don't see why this isn't a single test
program in verifier_arena.c and requires a whole new userspace test.

> Tested with a reduced test_progs runner because the local source/output
> combination has unrelated build failures in progs/bpf_cubic.c and
> test_kmods/bpf_testmod.ko, so the stock full test_progs binary is not
> produced.
>
> Before fix, on host Arch kernel 7.0.10-arch1-1:
>   arena_direct_value/one_past_end: FAIL, prog_load returned fd 5
>
> After fix, on QEMU guest 7.1.0-rc6-00063-gba3e43a9e601:
>   #1/1     arena_direct_value/one_past_end:OK
>   #1       arena_direct_value:OK
>   Summary: 1/1 PASSED, 0 SKIPPED, 0 FAILED
> ---
>  .../selftests/bpf/prog_tests/arena_direct_value.c  | 73 ++++++++++++++++++++++
>  1 file changed, 73 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/arena_direct_value.c b/tools/testing/selftests/bpf/prog_tests/arena_direct_value.c
> new file mode 100644
> index 000000000000..b7760b021670
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/arena_direct_value.c
> @@ -0,0 +1,73 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <test_progs.h>
> +#include <bpf/bpf.h>
> +#include <errno.h>
> +#include <sys/mman.h>
> +#include <unistd.h>
> +
> +#define ARENA_PAGES 32
> +
> +static void test_arena_direct_value_one_past_end(void)
> +{
> +	char log_buf[16384] = {}, expected[128];
> +	__u32 arena_sz = ARENA_PAGES * getpagesize();
> +	struct bpf_insn insns[] = {
> +		BPF_LD_IMM64_RAW(BPF_REG_1, BPF_PSEUDO_MAP_VALUE, 0),
> +		BPF_MOV64_IMM(BPF_REG_0, 0),
> +		BPF_EXIT_INSN(),
> +	};
> +	LIBBPF_OPTS(bpf_map_create_opts, map_opts);
> +	LIBBPF_OPTS(bpf_prog_load_opts, prog_opts);
> +	void *arena = MAP_FAILED;
> +	int map_fd, prog_fd;
> +
> +	map_opts.map_flags = BPF_F_MMAPABLE;
> +	prog_opts.log_buf = log_buf;
> +	prog_opts.log_size = sizeof(log_buf);
> +	prog_opts.log_level = 1;
> +
> +	map_fd = bpf_map_create(BPF_MAP_TYPE_ARENA, "arena_direct_value",
> +				0, 0, ARENA_PAGES, &map_opts);
> +	if (map_fd < 0) {
> +		if (errno == EOPNOTSUPP || errno == EINVAL) {
> +			test__skip();
> +			return;
> +		}
> +		ASSERT_GE(map_fd, 0, "bpf_map_create");
> +		return;
> +	}
> +
> +	arena = mmap(NULL, arena_sz, PROT_READ | PROT_WRITE, MAP_SHARED, map_fd, 0);
> +	if (!ASSERT_NEQ(arena, MAP_FAILED, "arena_mmap"))
> +		goto cleanup;
> +
> +	insns[0].imm = map_fd;
> +	insns[1].imm = arena_sz;
> +
> +	prog_fd = bpf_prog_load(BPF_PROG_TYPE_RAW_TRACEPOINT,
> +				"arena_direct_value", "GPL", insns,
> +				ARRAY_SIZE(insns), &prog_opts);
> +	if (!ASSERT_LT(prog_fd, 0, "prog_load")) {
> +		if (prog_fd >= 0)
> +			close(prog_fd);
> +		prog_fd = -1;
> +		goto cleanup;
> +	}
> +
> +	snprintf(expected, sizeof(expected),
> +		 "invalid access to map value pointer, value_size=0 off=%u",
> +		 arena_sz);
> +	ASSERT_HAS_SUBSTR(log_buf, expected, "verifier_log");
> +
> +cleanup:
> +	if (arena != MAP_FAILED)
> +		munmap(arena, arena_sz);
> +	close(map_fd);
> +}
> +
> +void test_arena_direct_value(void)
> +{
> +	if (test__start_subtest("one_past_end"))
> +		test_arena_direct_value_one_past_end();
> +}
>
> ---
> base-commit: ba3e43a9e601636f5edb54e259a74f96ca3b8fd8
> change-id: 20260603-arena-direct-value-v1-ef4df857b98b
>
> Best regards,
> --  
> Woojin Ji <random6.xyz@gmail.com>