bpf: selftests: verifier: Add nullness elision tests

Test that nullness elision works for common use cases. For example, we
want to check that both constant scalar spills and STACK_ZERO functions.
As well as when there's both const and non-const values of R2 leading up
to a lookup. And obviously some bound checks.

Particularly tricky are spills both smaller or larger than key size. For
smaller, we need to ensure verifier doesn't let through a potential read
into unchecked bytes. For larger, endianness comes into play, as the
native endian value tracked in the verifier may not be the bytes the
kernel would have read out of the key pointer. So check that we disallow
both.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>

tools/testing/selftests/bpf/progs/verifier_array_access.c

@@ -28,6 +28,20 @@ struct {
 	__uint(map_flags, BPF_F_WRONLY_PROG);
 } map_array_wo SEC(".maps");
 
+struct {
+	__uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
+	__uint(max_entries, 2);
+	__type(key, __u32);
+	__type(value, struct test_val);
+} map_array_pcpu SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 2);
+	__type(key, __u32);
+	__type(value, struct test_val);
+} map_array SEC(".maps");
+
 struct {
 	__uint(type, BPF_MAP_TYPE_HASH);
 	__uint(max_entries, 1);
@@ -525,4 +539,158 @@ l0_%=:	exit;						\
 	: __clobber_all);
 }
 
+SEC("socket")
+__description("valid map access into an array using constant without nullness")
+__success __retval(4)
+unsigned int an_array_with_a_constant_no_nullness(void)
+{
+	/* Need 8-byte alignment for spill tracking */
+	__u32 __attribute__((aligned(8))) key = 1;
+	struct test_val *val;
+
+	val = bpf_map_lookup_elem(&map_array, &key);
+	val->index = offsetof(struct test_val, foo);
+
+	return val->index;
+}
+
+SEC("socket")
+__description("valid multiple map access into an array using constant without nullness")
+__success __retval(8)
+unsigned int multiple_array_with_a_constant_no_nullness(void)
+{
+	__u32 __attribute__((aligned(8))) key = 1;
+	__u32 __attribute__((aligned(8))) key2 = 0;
+	struct test_val *val, *val2;
+
+	val = bpf_map_lookup_elem(&map_array, &key);
+	val->index = offsetof(struct test_val, foo);
+
+	val2 = bpf_map_lookup_elem(&map_array, &key2);
+	val2->index = offsetof(struct test_val, foo);
+
+	return val->index + val2->index;
+}
+
+SEC("socket")
+__description("valid map access into an array using natural aligned 32-bit constant 0 without nullness")
+__success __retval(4)
+unsigned int an_array_with_a_32bit_constant_0_no_nullness(void)
+{
+	/* Unlike the above tests, 32-bit zeroing is precisely tracked even
+	 * if writes are not aligned to BPF_REG_SIZE. This tests that our
+	 * STACK_ZERO handling functions.
+	 */
+	struct test_val *val;
+	__u32 key = 0;
+
+	val = bpf_map_lookup_elem(&map_array, &key);
+	val->index = offsetof(struct test_val, foo);
+
+	return val->index;
+}
+
+SEC("socket")
+__description("valid map access into a pcpu array using constant without nullness")
+__success __retval(4)
+unsigned int a_pcpu_array_with_a_constant_no_nullness(void)
+{
+	__u32 __attribute__((aligned(8))) key = 1;
+	struct test_val *val;
+
+	val = bpf_map_lookup_elem(&map_array_pcpu, &key);
+	val->index = offsetof(struct test_val, foo);
+
+	return val->index;
+}
+
+SEC("socket")
+__description("invalid map access into an array using constant without nullness")
+__failure __msg("R0 invalid mem access 'map_value_or_null'")
+unsigned int an_array_with_a_constant_no_nullness_out_of_bounds(void)
+{
+	/* Out of bounds */
+	__u32 __attribute__((aligned(8))) key = 3;
+	struct test_val *val;
+
+	val = bpf_map_lookup_elem(&map_array, &key);
+	val->index = offsetof(struct test_val, foo);
+
+	return val->index;
+}
+
+SEC("socket")
+__description("invalid map access into an array using constant smaller than key_size")
+__failure __msg("R0 invalid mem access 'map_value_or_null'")
+unsigned int an_array_with_a_constant_too_small(void)
+{
+	__u32 __attribute__((aligned(8))) key;
+	struct test_val *val;
+
+	/* Mark entire key as STACK_MISC */
+	bpf_probe_read_user(&key, sizeof(key), NULL);
+
+	/* Spilling only the bottom byte results in a tnum const of 1.
+	 * We want to check that the verifier rejects it, as the spill is < 4B.
+	 */
+	*(__u8 *)&key = 1;
+	val = bpf_map_lookup_elem(&map_array, &key);
+
+	/* Should fail, as verifier cannot prove in-bound lookup */
+	val->index = offsetof(struct test_val, foo);
+
+	return val->index;
+}
+
+SEC("socket")
+__description("invalid map access into an array using constant larger than key_size")
+__failure __msg("R0 invalid mem access 'map_value_or_null'")
+unsigned int an_array_with_a_constant_too_big(void)
+{
+	struct test_val *val;
+	__u64 key = 1;
+
+	/* Even if the constant value is < max_entries, if the spill size is
+	 * larger than the key size, the set bits may not be where we expect them
+	 * to be on different endian architectures.
+	 */
+	val = bpf_map_lookup_elem(&map_array, &key);
+	val->index = offsetof(struct test_val, foo);
+
+	return val->index;
+}
+
+SEC("socket")
+__description("invalid elided lookup using const and non-const key")
+__failure __msg("R0 invalid mem access 'map_value_or_null'")
+unsigned int mixed_const_and_non_const_key_lookup(void)
+{
+	__u32 __attribute__((aligned(8))) key;
+	struct test_val *val;
+	__u32 rand;
+
+	rand = bpf_get_prandom_u32();
+	key = rand > 42 ? 1 : rand;
+	val = bpf_map_lookup_elem(&map_array, &key);
+
+	return val->index;
+}
+
+SEC("socket")
+__failure __msg("invalid read from stack R2 off=4096 size=4")
+__naked void key_lookup_at_invalid_fp(void)
+{
+	asm volatile ("					\
+	r1 = %[map_array] ll;				\
+	r2 = r10;					\
+	r2 += 4096;					\
+	call %[bpf_map_lookup_elem];			\
+	r0 = *(u64*)(r0 + 0);				\
+	exit;						\
+"	:
+	: __imm(bpf_map_lookup_elem),
+	  __imm_addr(map_array)
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";