[PATCH v5 3/9] tracing/probes: Add a function to search a member of a struct/union

Masami Hiramatsu (Google)
2023-08-07T02:55:03Z

Add btf_find_struct_member() API to search a member of a given data structure
or union from the member's name.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
---
Changes in v3:
- Remove simple input check.
- Fix unneeded IS_ERR_OR_NULL() check for btf_type_by_id().
- Move the code next to btf_get_func_param().
- Use for_each_member() macro instead of for-loop.
- Use btf_type_skip_modifiers() instead of btf_type_by_id().
Changes in v4:
- Use a stack for searching in anonymous members instead of nested call.
Changes in v5:
- Move the generic functions into a new file, trace_btf.c, for avoiding
tree-merge conflict.
- Return anon_offset for calculating correct offset from root struct.
---
kernel/trace/trace_btf.c | 57 ++++++++++++++++++++++++++++++++++++++++++++++
kernel/trace/trace_btf.h | 4 +++
2 files changed, 61 insertions(+)

diff --git a/kernel/trace/trace_btf.c b/kernel/trace/trace_btf.c
index 82cbbeeda071..109a238437cf 100644
--- a/kernel/trace/trace_btf.c
+++ b/kernel/trace/trace_btf.c
@@ -50,3 +50,60 @@ const struct btf_param *btf_get_func_param(const struct btf_type *func_proto, s3
return NULL;
}

+#define BTF_ANON_STACK_MAX 16
+
+/*
+ * Find a member of data structure/union by name and return it.
+ * Return NULL if not found, or -EINVAL if parameter is invalid.
+ * If the member is an member of anonymous union/structure, the offset
+ * of that anonymous union/structure is stored into @anon_offset. Caller
+ * can calculate the correct offset from the root data structure by
+ * adding anon_offset to the member's offset.
+ */
+const struct btf_member *btf_find_struct_member(struct btf *btf,
+ const struct btf_type *type,
+ const char *member_name,
+ u32 *anon_offset)
+{
+ struct {
+ u32 tid;
+ u32 offset;
+ } anon_stack[BTF_ANON_STACK_MAX];
+ const struct btf_member *member;
+ u32 tid, cur_offset = 0;
+ const char *name;
+ int i, top = 0;
+
+retry:
+ if (!btf_type_is_struct(type))
+ return ERR_PTR(-EINVAL);
+
+ for_each_member(i, type, member) {
+ if (!member->name_off) {
+ /* Anonymous union/struct: push it for later use */
+ type = btf_type_skip_modifiers(btf, member->type, &tid);
+ if (type && top < BTF_ANON_STACK_MAX) {
+ anon_stack[top].tid = tid;
+ anon_stack[top++].offset =
+ cur_offset + member->offset;
+ }
+ } else {
+ name = btf_name_by_offset(btf, member->name_off);
+ if (name && !strcmp(member_name, name)) {
+ if (anon_offset)
+ *anon_offset = cur_offset;
+ return member;
+ }
+ }
+ }
+ if (top > 0) {
+ /* Pop from the anonymous stack and retry */
+ tid = anon_stack[--top].tid;
+ cur_offset = anon_stack[top].offset;
+ type = btf_type_by_id(btf, tid);
+ goto retry;
+ }
+
+ return NULL;
+}
+
diff --git a/kernel/trace/trace_btf.h b/kernel/trace/trace_btf.h
index 2ef1b2367e1e..fb2919a74225 100644
--- a/kernel/trace/trace_btf.h
+++ b/kernel/trace/trace_btf.h
@@ -5,3 +5,7 @@ const struct btf_type *btf_find_func_proto(const char *func_name,
struct btf **btf_p);
const struct btf_param *btf_get_func_param(const struct btf_type *func_proto,
s32 *nr);
+const struct btf_member *btf_find_struct_member(struct btf *btf,
+ const struct btf_type *type,
+ const char *member_name,
+ u32 *anon_offset);