[PATCH bpf-next v3] selftests/bpf: Add arena direct-value one-past-end reject test

[Woojin Ji <random6.xyz@gmail.com>]

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.

This is intentionally kept as a userspace raw-instruction test. I tried
expressing the same BPF_PSEUDO_MAP_VALUE + off == arena_size case in
verifier_arena.c with inline assembly. The only form that produces the
desired instruction bytes uses __imm_addr(arena), but that emits
R_BPF_64_NODYLD32, which the libbpf/bpftool link step rejects. Other
register, immediate, and memory constraints either fail in the BPF
backend or lower to a normal R_BPF_64_64 load followed by an ALU add,
which does not exercise arena_map_direct_value_addr() with the boundary
offset in the second ldimm64 slot.

A legacy test_verifier fixture can express the raw instruction directly,
but it needs arena map creation, mmap, and fixup plumbing in the legacy
runner. That is more intrusive than the small prog_tests raw-instruction
test.

Use the userspace raw-instruction test, following the existing selftests
pattern used for direct map-value pseudo loads, so insns[1].imm can be
set to arena_size precisely.

Assisted-by: ChatGPT:gpt-5.5
Signed-off-by: Woojin Ji <random6.xyz@gmail.com>
Cc: Emil Tsalapatis <emil@etsalapatis.com>
Cc: Junyoung Jang <graypanda.inzag@gmail.com>
---
Changes in v3:
- Document why verifier_arena.c inline assembly cannot express this case:
  __imm_addr emits R_BPF_64_NODYLD32, while other constraints either fail
  in the BPF backend or miss the pseudo-load boundary path.
- Mention that natural C forms lower to a normal load plus ALU add, so they
  do not exercise arena_map_direct_value_addr() with off == arena_size.
- Keep the userspace raw-instruction test as the least intrusive option;
  test_verifier was checked but would require legacy runner arena plumbing.
- Link to v2: https://patch.msgid.link/20260605-arena-direct-value-v1-v2-1-a92cb281e376@gmail.com

Changes in v2:
- Explain why this uses a userspace raw-instruction test rather than a
  verifier_arena.c failure program: the test must set the second
  BPF_PSEUDO_MAP_VALUE ldimm64 immediate to arena_size exactly. For
  arena globals, libbpf derives that immediate from the arena-relative
  symbol offset, so the boundary value cannot be represented directly as
  a verifier_arena.c C-level failure program.
- Link to v1: https://patch.msgid.link/20260603-arena-direct-value-v1-v1-1-7b31cc0a8cac@gmail.com

Tested on x86_64 with tools/testing/selftests/bpf/vmtest.sh:
  $ ./test_progs -t arena_direct_value
  #2/1     arena_direct_value/one_past_end:OK
  #2       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>