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

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Message-Id: <20230822233629.669b3a891a0155addf52f461@kernel.org>
Date:   Tue, 22 Aug 2023 23:36:29 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Martin KaFai Lau <martin.lau@...ux.dev>, bpf@...r.kernel.org,
        Sven Schnelle <svens@...ux.ibm.com>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH v5 3/9] tracing/probes: Add a function to search a
 member of a struct/union

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

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