[PATCH v4 bpf-next 06/14] bpf/verifier: introduce BPF_PTR_TO_MAP_VALUE

[Roman Gushchin]

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: Roman Gushchin <guro@xxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Acked-by: Martin KaFai Lau <kafai@xxxxxx>
---
 include/linux/bpf.h   | 1 +
 kernel/bpf/verifier.c | 8 ++++++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 16be67888c30..ca4ac2a39def 100644
--- a/include/linux/bpf.h
+++ b/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 */
 };
 
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 11d3669bf854..1d8d50b6a73d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2545,8 +2545,12 @@ 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) {
-		regs[BPF_REG_0].type = 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);
 		regs[BPF_REG_0].off = 0;
-- 
2.14.4