On 4/18/22 3:47 PM, Kumar Kartikeya Dwivedi wrote:
It is guaranteed that for modifiers, clang always places type tags
before other modifiers, and then the base type. We would like to rely on
this guarantee inside the kernel to make it simple to parse type tags
from BTF.
However, a user would be allowed to construct a BTF without such
guarantees. Hence, add a pass to check that in modifier chains, type
tags only occur at the head of the chain, and then don't occur later in
the chain.
If we see a type tag, we can have one or more type tags preceding other
modifiers that then never have another type tag. If we see other
modifiers, all modifiers following them should never be a type tag.
Instead of having to walk chains we verified previously, we can remember
the last good modifier type ID which headed a good chain. At that point,
we must have verified all other chains headed by type IDs less than it.
This makes the verification process less costly, and it becomes a simple
O(n) pass.
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
Ack with a nit below.
Acked-by: Yonghong Song <yhs@xxxxxx>
---
kernel/bpf/btf.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 54 insertions(+)
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 0918a39279f6..c015ccd1c741 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -4541,6 +4541,48 @@ static int btf_parse_hdr(struct btf_verifier_env *env)
return 0;
}
+static int btf_check_type_tags(struct btf_verifier_env *env,
+ struct btf *btf, int start_id)
+{
+ int i, n, good_id = start_id - 1;
+ bool in_tags;
+
+ n = btf_nr_types(btf);
+ for (i = start_id; i < n; i++) {
+ const struct btf_type *t;
+ u32 cur_id = i;
+
+ t = btf_type_by_id(btf, i);
+ if (!t)
+ return -EINVAL;
+ if (!btf_type_is_modifier(t))
+ continue;
+
+ cond_resched();
+
+ in_tags = btf_type_is_type_tag(t);
+ while (btf_type_is_modifier(t)) {
+ if (btf_type_is_type_tag(t)) {
+ if (!in_tags) {
+ btf_verifier_log(env, "Type tags don't precede modifiers");
+ return -EINVAL;
+ }
+ } else if (in_tags) {
+ in_tags = false;
+ }
+ if (cur_id <= good_id)
+ break;
+ /* Move to next type */
+ cur_id = t->type;
+ t = btf_type_by_id(btf, t->type);
t->type can be replaced with cur_id.
+ if (!t)
+ return -EINVAL;
+ }
+ good_id = i;
+ }
+ return 0;
+}
+
[...]
