Kernel does not expect VAR entries to have types starting from
BTF_TYPE_TAG. Specifically, the code like below will be rejected:
struct rq __percpu runqueues;
...
rq = (struct rq *)bpf_per_cpu_ptr(&runqueues, cpu);
... rq->cpu ... // rq type is now PTR_TO_BTF_ID
The access to 'rq->cpu' would be checked by a call to
kernel/bpf/btf.c:btf_struct_access() which invokes btf_struct_walk(),
using rq's type as a starting point. The btf_struct_walk() wants the
first type in a chain to be STRUCT or UNION and does not skip modifiers.
Before introduction of support for 'btf:type_tag' such situations were
not possible, as TYPE_TAG entries were always preceded by PTR entries.
Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
---
btf_encoder.c | 30 +++++++++++++++++++++++++++++-
1 file changed, 29 insertions(+), 1 deletion(-)
diff --git a/btf_encoder.c b/btf_encoder.c
index 65f6e71..300d9c2 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -1498,6 +1498,19 @@ static bool ftype__has_arg_names(const struct ftype *ftype)
return true;
}
+static type_id_t skip_btf_type_tags(struct cu *cu, type_id_t id)
+{
+ for (;;) {
+ struct tag *tag = cu__type(cu, id);
+
+ if (tag == NULL || tag->tag != DW_TAG_LLVM_annotation)
+ break;
+ id = tag->type;
+ }
+
+ return id;
+}
+
static int btf_encoder__encode_cu_variables(struct btf_encoder *encoder)
{
struct cu *cu = encoder->cu;
@@ -1583,7 +1596,22 @@ static int btf_encoder__encode_cu_variables(struct btf_encoder *encoder)
continue;
}
- type = var->ip.tag.type + encoder->type_id_off;
+ /* Kernel does not expect VAR entries to have types starting from BTF_TYPE_TAG.
+ * Specifically, the code like below will be rejected:
+ *
+ * struct rq __percpu runqueues;
+ * ...
+ * rq = (struct rq *)bpf_per_cpu_ptr(&runqueues, cpu);
+ * ... rq->cpu ... // rq type is now PTR_TO_BTF_ID
+ *
+ * The access to 'rq->cpu' would be checked by a call to
+ * kernel/bpf/btf.c:btf_struct_access() which invokes btf_struct_walk(),
+ * using rq's type as a starting point. The btf_struct_walk() wants the
+ * first type in a chain to be STRUCT or UNION and does not skip modifiers.
+ *
+ * Thus, call skip_btf_type_tags() here.
+ */
+ type = skip_btf_type_tags(cu, var->ip.tag.type) + encoder->type_id_off;
linkage = var->external ? BTF_VAR_GLOBAL_ALLOCATED : BTF_VAR_STATIC;
if (encoder->verbose) {
--
2.40.1
