bpf: Introduce KF_ARG_PTR_TO_CONST_STR

Similar to ARG_PTR_TO_CONST_STR for BPF helpers, KF_ARG_PTR_TO_CONST_STR
specifies kfunc args that point to const strings. Annotation "__str" is
used to specify kfunc arg of type KF_ARG_PTR_TO_CONST_STR. Also, add
documentation for the "__str" annotation.

bpf_get_file_xattr() will be the first kfunc that uses this type.

Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://lore.kernel.org/bpf/20231107045725.2278852-4-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/kfuncs.rst

@@ -135,6 +135,30 @@ Either way, the returned buffer is either NULL, or of size buffer_szk. Without t
 annotation, the verifier will reject the program if a null pointer is passed in with
 a nonzero size.
 
+2.2.5 __str Annotation
+----------------------------
+This annotation is used to indicate that the argument is a constant string.
+
+An example is given below::
+
+        __bpf_kfunc bpf_get_file_xattr(..., const char *name__str, ...)
+        {
+        ...
+        }
+
+In this case, ``bpf_get_file_xattr()`` can be called as::
+
+        bpf_get_file_xattr(..., "xattr_name", ...);
+
+Or::
+
+        const char name[] = "xattr_name";  /* This need to be global */
+        int BPF_PROG(...)
+        {
+                ...
+                bpf_get_file_xattr(..., name, ...);
+                ...
+        }
 
 .. _BPF_kfunc_nodef:
 

kernel/bpf/verifier.c

@@ -10810,6 +10810,11 @@ static bool is_kfunc_arg_nullable(const struct btf *btf, const struct btf_param
 	return __kfunc_param_match_suffix(btf, arg, "__nullable");
 }
 
+static bool is_kfunc_arg_const_str(const struct btf *btf, const struct btf_param *arg)
+{
+	return __kfunc_param_match_suffix(btf, arg, "__str");
+}
+
 static bool is_kfunc_arg_scalar_with_name(const struct btf *btf,
 					  const struct btf_param *arg,
 					  const char *name)
@@ -10953,6 +10958,7 @@ enum kfunc_ptr_arg_type {
 	KF_ARG_PTR_TO_RB_ROOT,
 	KF_ARG_PTR_TO_RB_NODE,
 	KF_ARG_PTR_TO_NULL,
+	KF_ARG_PTR_TO_CONST_STR,
 };
 
 enum special_kfunc_type {
@@ -11103,6 +11109,9 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
 	if (is_kfunc_arg_rbtree_node(meta->btf, &args[argno]))
 		return KF_ARG_PTR_TO_RB_NODE;
 
+	if (is_kfunc_arg_const_str(meta->btf, &args[argno]))
+		return KF_ARG_PTR_TO_CONST_STR;
+
 	if ((base_type(reg->type) == PTR_TO_BTF_ID || reg2btf_ids[base_type(reg->type)])) {
 		if (!btf_type_is_struct(ref_t)) {
 			verbose(env, "kernel function %s args#%d pointer type %s %s is not supported\n",
@@ -11734,6 +11743,7 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 		case KF_ARG_PTR_TO_MEM_SIZE:
 		case KF_ARG_PTR_TO_CALLBACK:
 		case KF_ARG_PTR_TO_REFCOUNTED_KPTR:
+		case KF_ARG_PTR_TO_CONST_STR:
 			/* Trusted by default */
 			break;
 		default:
@@ -12005,6 +12015,15 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 			meta->arg_btf = reg->btf;
 			meta->arg_btf_id = reg->btf_id;
 			break;
+		case KF_ARG_PTR_TO_CONST_STR:
+			if (reg->type != PTR_TO_MAP_VALUE) {
+				verbose(env, "arg#%d doesn't point to a const string\n", i);
+				return -EINVAL;
+			}
+			ret = check_reg_const_str(env, reg, regno);
+			if (ret)
+				return ret;
+			break;
 		}
 	}