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Date:   Wed, 19 Jul 2023 13:36:48 +0100
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Subject: Re: [PATCH v2 2/9] bpf/btf: tracing: Move finding func-proto API and
 getting func-param API to BTF
To:     "Masami Hiramatsu (Google)" <mhiramat@kernel.org>,
        linux-trace-kernel@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
        Martin KaFai Lau <martin.lau@linux.dev>, bpf@vger.kernel.org,
        Sven Schnelle <svens@linux.ibm.com>,
        Alexei Starovoitov <ast@kernel.org>
References: <168960739768.34107.15145201749042174448.stgit@devnote2>
 <168960741686.34107.6330273416064011062.stgit@devnote2>
Content-Language: en-GB
From:   Alan Maguire <alan.maguire@oracle.com>
In-Reply-To: <168960741686.34107.6330273416064011062.stgit@devnote2>
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On 17/07/2023 16:23, Masami Hiramatsu (Google) wrote:
> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> Move generic function-proto find API and getting function parameter API
> to BTF library code from trace_probe.c. This will avoid redundant efforts
> on different feature.
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> ---
>  include/linux/btf.h        |    4 ++++
>  kernel/bpf/btf.c           |   45 ++++++++++++++++++++++++++++++++++++++++
>  kernel/trace/trace_probe.c |   50 +++++++++++++-------------------------------
>  3 files changed, 64 insertions(+), 35 deletions(-)
> 
> diff --git a/include/linux/btf.h b/include/linux/btf.h
> index cac9f304e27a..98fbbcdd72ec 100644
> --- a/include/linux/btf.h
> +++ b/include/linux/btf.h
> @@ -221,6 +221,10 @@ const struct btf_type *
>  btf_resolve_size(const struct btf *btf, const struct btf_type *type,
>  		 u32 *type_size);
>  const char *btf_type_str(const struct btf_type *t);
> +const struct btf_type *btf_find_func_proto(struct btf *btf,
> +					   const char *func_name);
> +const struct btf_param *btf_get_func_param(const struct btf_type *func_proto,
> +					   s32 *nr);
>  
>  #define for_each_member(i, struct_type, member)			\
>  	for (i = 0, member = btf_type_member(struct_type);	\
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 817204d53372..e015b52956cb 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -1947,6 +1947,51 @@ btf_resolve_size(const struct btf *btf, const struct btf_type *type,
>  	return __btf_resolve_size(btf, type, type_size, NULL, NULL, NULL, NULL);
>  }
>  
> +/*
> + * Find a functio proto type by name, and return it.
> + * Return NULL if not found, or return -EINVAL if parameter is invalid.
> + */
> +const struct btf_type *btf_find_func_proto(struct btf *btf, const char *func_name)
> +{
> +	const struct btf_type *t;
> +	s32 id;
> +
> +	if (!btf || !func_name)
> +		return ERR_PTR(-EINVAL);
> +
> +	id = btf_find_by_name_kind(btf, func_name, BTF_KIND_FUNC);

as mentioned in my other mail, there are cases where the function name
may have a .isra.0 suffix, but the BTF representation will not. I looked
at this and it seems like symbol names are validated via
traceprobe_parse_event_name() - will this validation allow a "."-suffix
name? I tried the following (with pahole v1.25 that emits BTF for
schedule_work.isra.0):

[45454] FUNC 'schedule_work' type_id=45453 linkage=static

$ echo 'f schedule_work.isra.0 $arg*' >> dynamic_events
bash: echo: write error: No such file or directory

So presuming that such "."-suffixed names are allowed, would it make
sense to fall back to search BTF for the prefix ("schedule_work")
instead of the full name ("schedule_work.isra.0"), as the former is what
makes it into the BTF representation? Thanks!

Alan

> +	if (id <= 0)
> +		return NULL;
> +
> +	/* Get BTF_KIND_FUNC type */
> +	t = btf_type_by_id(btf, id);
> +	if (!t || !btf_type_is_func(t))
> +		return NULL;
> +
> +	/* The type of BTF_KIND_FUNC is BTF_KIND_FUNC_PROTO */
> +	t = btf_type_by_id(btf, t->type);
> +	if (!t || !btf_type_is_func_proto(t))
> +		return NULL;
> +
> +	return t;
> +}
> +
> +/*
> + * Get function parameter with the number of parameters.
> + * This can return NULL if the function has no parameters.
> + */
> +const struct btf_param *btf_get_func_param(const struct btf_type *func_proto, s32 *nr)
> +{
> +	if (!func_proto || !nr)
> +		return ERR_PTR(-EINVAL);
> +
> +	*nr = btf_type_vlen(func_proto);
> +	if (*nr > 0)
> +		return (const struct btf_param *)(func_proto + 1);
> +	else
> +		return NULL;
> +}
> +
>  static u32 btf_resolved_type_id(const struct btf *btf, u32 type_id)
>  {
>  	while (type_id < btf->start_id)
> diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
> index c68a72707852..cd89fc1ebb42 100644
> --- a/kernel/trace/trace_probe.c
> +++ b/kernel/trace/trace_probe.c
> @@ -371,47 +371,23 @@ static const char *type_from_btf_id(struct btf *btf, s32 id)
>  	return NULL;
>  }
>  
> -static const struct btf_type *find_btf_func_proto(const char *funcname)
> -{
> -	struct btf *btf = traceprobe_get_btf();
> -	const struct btf_type *t;
> -	s32 id;
> -
> -	if (!btf || !funcname)
> -		return ERR_PTR(-EINVAL);
> -
> -	id = btf_find_by_name_kind(btf, funcname, BTF_KIND_FUNC);
> -	if (id <= 0)
> -		return ERR_PTR(-ENOENT);
> -
> -	/* Get BTF_KIND_FUNC type */
> -	t = btf_type_by_id(btf, id);
> -	if (!t || !btf_type_is_func(t))
> -		return ERR_PTR(-ENOENT);
> -
> -	/* The type of BTF_KIND_FUNC is BTF_KIND_FUNC_PROTO */
> -	t = btf_type_by_id(btf, t->type);
> -	if (!t || !btf_type_is_func_proto(t))
> -		return ERR_PTR(-ENOENT);
> -
> -	return t;
> -}
> -
>  static const struct btf_param *find_btf_func_param(const char *funcname, s32 *nr,
>  						   bool tracepoint)
>  {
> +	struct btf *btf = traceprobe_get_btf();
>  	const struct btf_param *param;
>  	const struct btf_type *t;
>  
> -	if (!funcname || !nr)
> +	if (!funcname || !nr || !btf)
>  		return ERR_PTR(-EINVAL);
>  
> -	t = find_btf_func_proto(funcname);
> -	if (IS_ERR(t))
> +	t = btf_find_func_proto(btf, funcname);
> +	if (IS_ERR_OR_NULL(t))
>  		return (const struct btf_param *)t;
>  
> -	*nr = btf_type_vlen(t);
> -	param = (const struct btf_param *)(t + 1);
> +	param = btf_get_func_param(t, nr);
> +	if (IS_ERR_OR_NULL(param))
> +		return param;
>  
>  	/* Hide the first 'data' argument of tracepoint */
>  	if (tracepoint) {
> @@ -490,8 +466,8 @@ static const struct fetch_type *parse_btf_retval_type(
>  	const struct btf_type *t;
>  
>  	if (btf && ctx->funcname) {
> -		t = find_btf_func_proto(ctx->funcname);
> -		if (!IS_ERR(t))
> +		t = btf_find_func_proto(btf, ctx->funcname);
> +		if (!IS_ERR_OR_NULL(t))
>  			typestr = type_from_btf_id(btf, t->type);
>  	}
>  
> @@ -500,10 +476,14 @@ static const struct fetch_type *parse_btf_retval_type(
>  
>  static bool is_btf_retval_void(const char *funcname)
>  {
> +	struct btf *btf = traceprobe_get_btf();
>  	const struct btf_type *t;
>  
> -	t = find_btf_func_proto(funcname);
> -	if (IS_ERR(t))
> +	if (!btf)
> +		return false;
> +
> +	t = btf_find_func_proto(btf, funcname);
> +	if (IS_ERR_OR_NULL(t))
>  		return false;
>  
>  	return t->type == 0;
> 
>