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

Masami Hiramatsu
2023-08-22T14:36:33Z

On Tue, 22 Aug 2023 09:37:20 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> 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.

OK, let me allocate an array then.

Thank you,

>
> -- 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;
> > +}
> > +

--
Masami Hiramatsu (Google) <mhiramat@kernel.org>