[PATCH bpf 2/2] selftests/bpf: Test BTF walk into a flexible array of zero-sized elements

[Jiayuan Chen <jiayuan.chen@linux.dev>]

The program stashes a bpf_obj_new() object whose type ends with a flexible
array of empty structs, then reads it back as an untrusted kptr. Without
the previous patch this divides by zero in btf_struct_walk() instead of
being rejected.

  # ./test_progs -t verifier_btf_flex_array
  ...
  #602     verifier_btf_flex_array:OK
  Summary: 1/1 PASSED, 0 SKIPPED, 0/0 FAILED

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 .../selftests/bpf/prog_tests/verifier.c       |  2 +
 .../bpf/progs/verifier_btf_flex_array.c       | 56 +++++++++++++++++++
 2 files changed, 58 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_btf_flex_array.c

diff --git a/tools/testing/selftests/bpf/prog_tests/verifier.c b/tools/testing/selftests/bpf/prog_tests/verifier.c
index f7f94ccebce2..33a5da37e3ad 100644
--- a/tools/testing/selftests/bpf/prog_tests/verifier.c
+++ b/tools/testing/selftests/bpf/prog_tests/verifier.c
@@ -24,6 +24,7 @@
 #include "verifier_bpf_trap.skel.h"
 #include "verifier_bswap.skel.h"
 #include "verifier_btf_ctx_access.skel.h"
+#include "verifier_btf_flex_array.skel.h"
 #include "verifier_btf_unreliable_prog.skel.h"
 #include "verifier_call_large_imm.skel.h"
 #include "verifier_cfg.skel.h"
@@ -188,6 +189,7 @@ void test_verifier_bpf_get_stack(void)        { RUN(verifier_bpf_get_stack); }
 void test_verifier_bpf_trap(void)             { RUN(verifier_bpf_trap); }
 void test_verifier_bswap(void)                { RUN(verifier_bswap); }
 void test_verifier_btf_ctx_access(void)       { RUN(verifier_btf_ctx_access); }
+void test_verifier_btf_flex_array(void)       { RUN(verifier_btf_flex_array); }
 void test_verifier_btf_unreliable_prog(void)  { RUN(verifier_btf_unreliable_prog); }
 void test_verifier_call_large_imm(void)       { RUN(verifier_call_large_imm); }
 void test_verifier_cfg(void)                  { RUN(verifier_cfg); }
diff --git a/tools/testing/selftests/bpf/progs/verifier_btf_flex_array.c b/tools/testing/selftests/bpf/progs/verifier_btf_flex_array.c
new file mode 100644
index 000000000000..59b84261f622
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/verifier_btf_flex_array.c
@@ -0,0 +1,56 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+
+#include "bpf_experimental.h"
+#include "bpf_misc.h"
+
+struct test_empty_event {};
+
+struct test_flex_batch {
+	int nr;
+	struct test_empty_event events[];
+};
+
+struct map_value {
+	struct test_flex_batch __kptr *batch;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__type(key, int);
+	__type(value, struct map_value);
+	__uint(max_entries, 1);
+} batches SEC(".maps");
+
+SEC("syscall")
+__description("btf walk into flexible array of zero-sized elements")
+__failure __msg("access beyond struct test_flex_batch at off 4 size 1")
+int stash_and_peek(void *ctx)
+{
+	struct test_flex_batch *b, *old;
+	struct map_value *v;
+	int key = 0;
+
+	v = bpf_map_lookup_elem(&batches, &key);
+	if (!v)
+		return 0;
+
+	b = bpf_obj_new(struct test_flex_batch);
+	if (!b)
+		return 0;
+	b->nr = 1;
+
+	old = bpf_kptr_xchg(&v->batch, b);
+	if (old)
+		bpf_obj_drop(old);
+
+	b = v->batch;
+	if (!b)
+		return 0;
+
+	return b->nr + *(char *)&b->events[0];
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.43.0