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

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Date:   Tue, 22 Aug 2023 09:37:20 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.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 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.

-- 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;
> +}
> +