bpf/verifier: introduce BPF_PTR_TO_MAP_VALUE

BPF_MAP_TYPE_CGROUP_STORAGE maps are special in a way
that the access from the bpf program side is lookup-free.
That means the result is guaranteed to be a valid
pointer to the cgroup storage; no NULL-check is required.

This patch introduces BPF_PTR_TO_MAP_VALUE return type,
which is required to cause the verifier accept programs,
which are not checking the map value pointer for being NULL.
Signed-off-by: NRoman Gushchin <guro@fb.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: NMartin KaFai Lau <kafai@fb.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

include/linux/bpf.h

@@ -155,6 +155,7 @@ enum bpf_arg_type {
 enum bpf_return_type {
 	RET_INTEGER,			/* function returns integer */
 	RET_VOID,			/* function doesn't return anything */
+	RET_PTR_TO_MAP_VALUE,		/* returns a pointer to map elem value */
 	RET_PTR_TO_MAP_VALUE_OR_NULL,	/* returns a pointer to map elem value or NULL */
 };
 

kernel/bpf/verifier.c

@@ -2545,7 +2545,11 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
 		mark_reg_unknown(env, regs, BPF_REG_0);
 	} else if (fn->ret_type == RET_VOID) {
 		regs[BPF_REG_0].type = NOT_INIT;
-	} else if (fn->ret_type == RET_PTR_TO_MAP_VALUE_OR_NULL) {
+	} else if (fn->ret_type == RET_PTR_TO_MAP_VALUE_OR_NULL ||
+		   fn->ret_type == RET_PTR_TO_MAP_VALUE) {
+		if (fn->ret_type == RET_PTR_TO_MAP_VALUE)
+			regs[BPF_REG_0].type = PTR_TO_MAP_VALUE;
+		else
 			regs[BPF_REG_0].type = PTR_TO_MAP_VALUE_OR_NULL;
 		/* There is no offset yet applied, variable or fixed */
 		mark_reg_known_zero(env, regs, BPF_REG_0);