selftests/bpf: Add test cases with CONST_PTR_TO_MAP null checks

A test requires the following to happen:
  * CONST_PTR_TO_MAP value is checked for null
  * the code in the null branch fails verification

Add test cases:
* direct global map_ptr comparison to null
* lookup inner map, then two checks (the first transforms
  map_value_or_null into map_ptr)
* lookup inner map, spill-fill it, then check for null
* use an array of ringbufs to recreate a common coding pattern [1]

[1] https://lore.kernel.org/bpf/CAEf4BzZNU0gX_sQ8k8JaLe1e+Veth3Rk=4x7MDhv=hQxvO8EDw@mail.gmail.com/

Suggested-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Ihor Solodrai <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: theihor

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

@@ -139,4 +139,122 @@ __naked void on_the_inner_map_pointer(void)
 	: __clobber_all);
 }
 
+SEC("socket")
+__description("map_ptr is never null")
+__success
+__naked void map_ptr_is_never_null(void)
+{
+	asm volatile ("					\
+	r0 = 0;						\
+	r1 = %[map_in_map] ll;				\
+	if r1 != 0 goto l0_%=;				\
+	r10 = 42;					\
+l0_%=:	exit;						\
+"	:
+	: __imm(bpf_map_lookup_elem),
+	  __imm_addr(map_in_map)
+	: __clobber_all);
+}
+
+SEC("socket")
+__description("map_ptr is never null inner")
+__success
+__naked void map_ptr_is_never_null_inner(void)
+{
+	asm volatile ("					\
+	r1 = 0;						\
+	*(u32*)(r10 - 4) = r1;				\
+	r2 = r10;					\
+	r2 += -4;					\
+	r1 = %[map_in_map] ll;				\
+	call %[bpf_map_lookup_elem];			\
+	if r0 == 0 goto l0_%=;				\
+	if r0 != 0 goto l0_%=;				\
+	r10 = 42;					\
+l0_%=:  exit;						\
+"	:
+	: __imm(bpf_map_lookup_elem),
+	  __imm_addr(map_in_map)
+	: __clobber_all);
+}
+
+SEC("socket")
+__description("map_ptr is never null inner spill fill")
+__success
+__naked void map_ptr_is_never_null_inner_spill_fill(void)
+{
+	asm volatile ("					\
+	r1 = 0;						\
+	*(u32*)(r10 - 4) = r1;				\
+	r2 = r10;					\
+	r2 += -4;					\
+	r1 = %[map_in_map] ll;				\
+	call %[bpf_map_lookup_elem];			\
+	if r0 != 0 goto l0_%=;				\
+	exit;						\
+l0_%=:	*(u64 *)(r10 -16) = r0;				\
+	r1 = *(u64 *)(r10 -16);				\
+	if r1 == 0 goto l1_%=;				\
+	exit;						\
+l1_%=:	r10 = 42;					\
+	exit;						\
+"	:
+	: __imm(bpf_map_lookup_elem),
+	  __imm_addr(map_in_map)
+	: __clobber_all);
+}
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY_OF_MAPS);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, int);
+	__array(values, struct {
+		__uint(type, BPF_MAP_TYPE_RINGBUF);
+		__uint(max_entries, 64 * 1024);
+	});
+} rb_in_map SEC(".maps");
+
+struct rb_ctx {
+	void *rb;
+	struct bpf_dynptr dptr;
+};
+
+static __always_inline struct rb_ctx __rb_event_reserve(__u32 sz)
+{
+	struct rb_ctx rb_ctx = {};
+	void *rb;
+	__u32 cpu = bpf_get_smp_processor_id();
+	__u32 rb_slot = cpu & 1;
+
+	rb = bpf_map_lookup_elem(&rb_in_map, &rb_slot);
+	if (!rb)
+		return rb_ctx;
+
+	rb_ctx.rb = rb;
+	bpf_ringbuf_reserve_dynptr(rb, sz, 0, &rb_ctx.dptr);
+
+	return rb_ctx;
+}
+
+static __noinline void __rb_event_submit(struct rb_ctx *ctx)
+{
+	if (!ctx->rb)
+		return;
+
+	/* If the verifier (incorrectly) concludes that ctx->rb can be
+	 * NULL at this point, we'll get "BPF_EXIT instruction in main
+	 * prog would lead to reference leak" error
+	 */
+	bpf_ringbuf_submit_dynptr(&ctx->dptr, 0);
+}
+
+SEC("socket")
+int map_ptr_is_never_null_rb(void *ctx)
+{
+	struct rb_ctx event_ctx = __rb_event_reserve(256);
+	__rb_event_submit(&event_ctx);
+	return 0;
+}
+
 char _license[] SEC("license") = "GPL";