On Tue, 22 Aug 2023 09:37:20 -0400
Steven Rostedt [off-list ref] wrote:

On Mon,  7 Aug 2023 11:54:58 +0900
"Masami Hiramatsu (Google)" [off-list ref] wrote:

quoted

--- 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];

Where is this called as the above is 128 bytes, which is a bit large
for the stack. It may not be bad if it's not that generic of a
function. But if the stack is getting tight, this could still be an
issue.

OK, let me allocate an array then.

Thank you,

-- Steve

quoted

+	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;
+}
+

-- 
Masami Hiramatsu (Google) [off-list ref]