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

Steven Rostedt
2023-08-22T13:37:40Z

On Mon, 7 Aug 2023 11:54:58 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> --- 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.

-- Steve

> + 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;
> +}
> +