bpf: prepare btf_prepare_func_args() for handling static subprogs

Generalize btf_prepare_func_args() to support both global and static
subprogs. We are going to utilize this property in the next patch,
reusing btf_prepare_func_args() for subprog call logic instead of
reparsing BTF information in a completely separate implementation.

btf_prepare_func_args() now detects whether subprog is global or static
makes slight logic adjustments for static func cases, like not failing
fatally (-EFAULT) for conditions that are allowable for static subprogs.

Somewhat subtle (but major!) difference is the handling of pointer arguments.
Both global and static functions need to handle special context
arguments (which are pointers to predefined type names), but static
subprogs give up on any other pointers, falling back to marking subprog
as "unreliable", disabling the use of BTF type information altogether.

For global functions, though, we are assuming that such pointers to
unrecognized types are just pointers to fixed-sized memory region (or
error out if size cannot be established, like for `void *` pointers).

This patch accommodates these small differences and sets up a stage for
refactoring in the next patch, eliminating a separate BTF-based parsing
logic in btf_check_func_arg_match().

Acked-by: Eduard Zingerman <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: anakryiko

include/linux/bpf_verifier.h

@@ -738,6 +738,11 @@ struct bpf_verifier_env {
 	char tmp_str_buf[TMP_STR_BUF_LEN];
 };
 
+static inline struct bpf_func_info_aux *subprog_aux(struct bpf_verifier_env *env, int subprog)
+{
+	return &env->prog->aux->func_info_aux[subprog];
+}
+
 static inline struct bpf_subprog_info *subprog_info(struct bpf_verifier_env *env, int subprog)
 {
 	return &env->subprog_info[subprog];

kernel/bpf/btf.c

@@ -6914,6 +6914,7 @@ int btf_check_subprog_call(struct bpf_verifier_env *env, int subprog,
  */
 int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog)
 {
+	bool is_global = subprog_aux(env, subprog)->linkage == BTF_FUNC_GLOBAL;
 	struct bpf_subprog_info *sub = subprog_info(env, subprog);
 	struct bpf_verifier_log *log = &env->log;
 	struct bpf_prog *prog = env->prog;
@@ -6927,14 +6928,15 @@ int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog)
 	if (sub->args_cached)
 		return 0;
 
-	if (!prog->aux->func_info ||
-	    prog->aux->func_info_aux[subprog].linkage != BTF_FUNC_GLOBAL) {
+	if (!prog->aux->func_info) {
 		bpf_log(log, "Verifier bug\n");
 		return -EFAULT;
 	}
 
 	btf_id = prog->aux->func_info[subprog].type_id;
 	if (!btf_id) {
+		if (!is_global) /* not fatal for static funcs */
+			return -EINVAL;
 		bpf_log(log, "Global functions need valid BTF\n");
 		return -EFAULT;
 	}
@@ -6990,16 +6992,14 @@ int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog)
 			sub->args[i].arg_type = ARG_ANYTHING;
 			continue;
 		}
-		if (btf_type_is_ptr(t)) {
+		if (btf_type_is_ptr(t) && btf_get_prog_ctx_type(log, btf, t, prog_type, i)) {
+			sub->args[i].arg_type = ARG_PTR_TO_CTX;
+			continue;
+		}
+		if (is_global && btf_type_is_ptr(t)) {
 			u32 mem_size;
 
-			if (btf_get_prog_ctx_type(log, btf, t, prog_type, i)) {
-				sub->args[i].arg_type = ARG_PTR_TO_CTX;
-				continue;
-			}
-
 			t = btf_type_skip_modifiers(btf, t->type, NULL);
-
 			ref_t = btf_resolve_size(btf, t, &mem_size);
 			if (IS_ERR(ref_t)) {
 				bpf_log(log,

kernel/bpf/verifier.c

@@ -437,11 +437,6 @@ static const char *subprog_name(const struct bpf_verifier_env *env, int subprog)
 	return btf_type_name(env->prog->aux->btf, info->type_id);
 }
 
-static struct bpf_func_info_aux *subprog_aux(const struct bpf_verifier_env *env, int subprog)
-{
-	return &env->prog->aux->func_info_aux[subprog];
-}
-
 static void mark_subprog_exc_cb(struct bpf_verifier_env *env, int subprog)
 {
 	struct bpf_subprog_info *info = subprog_info(env, subprog);