linux-kernel - Re: [PATCH] tracing/probes: Allow use of BTF names to dereference pointers

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Message-ID: <aIvlrQEZQ6OTZxAY@krava>
Date: Thu, 31 Jul 2025 23:52:45 +0200
From: Jiri Olsa <olsajiri@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Linux trace kernel <linux-trace-kernel@...r.kernel.org>,
	bpf@...r.kernel.org, Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Mark Rutland <mark.rutland@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Takaya Saeki <takayas@...gle.com>,
	Douglas Raillard <douglas.raillard@....com>,
	Tom Zanussi <zanussi@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ian Rogers <irogers@...gle.com>, aahringo@...hat.com
Subject: Re: [PATCH] tracing/probes: Allow use of BTF names to dereference
 pointers

On Tue, Jul 29, 2025 at 11:33:35AM -0400, Steven Rostedt wrote:

SNIP

> +/**
> + * btf_find_offset - Find an offset of a member for a structure
> + * @arg: A structure name followed by one or more members
> + * @offset_p: A pointer to where to store the offset
> + *
> + * Will parse @arg with the expected format of: struct.member[[.member]..]
> + * It is delimited by '.'. The first item must be a structure type.
> + * The next are its members. If the member is also of a structure type it
> + * another member may follow ".member".
> + *
> + * Note, @arg is modified but will be put back to what it was on return.
> + *
> + * Returns: 0 on success and -EINVAL if no '.' is present
> + *    or -ENXIO if the structure or member is not found.
> + *    Returns -EINVAL if BTF is not defined.
> + *  On success, @offset_p will contain the offset of the member specified
> + *    by @arg.
> + */
> +int btf_find_offset(char *arg, long *offset_p)
> +{
> +	const struct btf_type *t;
> +	struct btf *btf;
> +	long offset = 0;
> +	char *ptr;
> +	int ret;
> +	s32 id;
> +
> +	ptr = strchr(arg, '.');
> +	if (!ptr)
> +		return -EINVAL;
> +
> +	*ptr = '\0';
> +
> +	id = bpf_find_btf_id(arg, BTF_KIND_STRUCT, &btf);

hi,
I think you need to call btf_put(btf) before return

jirka


> +	if (id < 0)
> +		goto error;
> +
> +	/* Get BTF_KIND_FUNC type */
> +	t = btf_type_by_id(btf, id);
> +
> +	/* May allow more than one member, as long as they are structures */
> +	do {
> +		if (!t || !btf_type_is_struct(t))
> +			goto error;
> +
> +		*ptr++ = '.';
> +		arg = ptr;
> +		ptr = strchr(ptr, '.');
> +		if (ptr)
> +			*ptr = '\0';
> +
> +		ret = find_member(arg, btf, &t, 0);
> +		if (ret < 0)
> +			goto error;
> +
> +		offset += ret;
> +
> +	} while (ptr);
> +
> +	*offset_p = offset;
> +	return 0;
> +
> +error:
> +	if (ptr)
> +		*ptr = '.';
> +	return -ENXIO;
> +}

SNIP