bpf: add arg:nullable tag to be combined with trusted pointers

Add ability to mark arg:trusted arguments with optional arg:nullable
tag to mark it as PTR_TO_BTF_ID_OR_NULL variant, which will allow
callers to pass NULL, and subsequently will force global subprog's code
to do NULL check. This allows to have "optional" PTR_TO_BTF_ID values
passed into global subprogs.

For now arg:nullable cannot be combined with anything else.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240130000648.2144827-3-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index ed7a058..c8c6e6c 100644 (file)
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -7056,6 +7056,7 @@ enum btf_arg_tag {
        ARG_TAG_CTX = 0x1,
        ARG_TAG_NONNULL = 0x2,
        ARG_TAG_TRUSTED = 0x4,
+       ARG_TAG_NULLABLE = 0x8,
 };
 
 /* Process BTF of a function to produce high-level expectation of function
@@ -7161,6 +7162,8 @@ int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog)
                                tags |= ARG_TAG_TRUSTED;
                        } else if (strcmp(tag, "nonnull") == 0) {
                                tags |= ARG_TAG_NONNULL;
+                       } else if (strcmp(tag, "nullable") == 0) {
+                               tags |= ARG_TAG_NULLABLE;
                        } else {
                                bpf_log(log, "arg#%d has unsupported set of tags\n", i);
                                return -EOPNOTSUPP;
@@ -7210,12 +7213,19 @@ int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog)
                                return kern_type_id;
 
                        sub->args[i].arg_type = ARG_PTR_TO_BTF_ID | PTR_TRUSTED;
+                       if (tags & ARG_TAG_NULLABLE)
+                               sub->args[i].arg_type |= PTR_MAYBE_NULL;
                        sub->args[i].btf_id = kern_type_id;
                        continue;
                }
                if (is_global) { /* generic user data pointer */
                        u32 mem_size;
 
+                       if (tags & ARG_TAG_NULLABLE) {
+                               bpf_log(log, "arg#%d has invalid combination of tags\n", i);
+                               return -EINVAL;
+                       }
+
                        t = btf_type_skip_modifiers(btf, t->type, NULL);
                        ref_t = btf_resolve_size(btf, t, &mem_size);
                        if (IS_ERR(ref_t)) {