Re: [PATCH v13 09/12] tracing/probes: Add BTF retval type support

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

On Mon, 12 Jun 2023 15:29:00 +0800
Donglin Peng <pengdonglin@sangfor.com.cn> wrote:

> On 2023/5/26 12:19, Masami Hiramatsu (Google) wrote:
> > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > 
> > Check the target function has non-void retval type and set the correct
> > fetch type if user doesn't specify it.
> > If the function returns void, $retval is rejected as below;
> > 
> >   # echo 'f unregister_kprobes%return $retval' >> dynamic_events
> > sh: write error: No such file or directory
> >   # cat error_log
> > [   37.488397] trace_fprobe: error: This function returns 'void' type
> >    Command: f unregister_kprobes%return $retval
> >                                         ^
> > 
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > ---
> > Changes in v8:
> >   - Fix wrong indentation.
> > Changes in v7:
> >   - Introduce this as a new patch.
> > ---
> >   kernel/trace/trace_probe.c |   69 ++++++++++++++++++++++++++++++++++++++++----
> >   kernel/trace/trace_probe.h |    1 +
> >   2 files changed, 63 insertions(+), 7 deletions(-)
> > 
> > diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
> > index 7318642aceb3..dfe3e1823eec 100644
> > --- a/kernel/trace/trace_probe.c
> > +++ b/kernel/trace/trace_probe.c
> > @@ -371,15 +371,13 @@ static const char *type_from_btf_id(struct btf *btf, s32 id)
> >   	return NULL;
> >   }
> >   
> > -static const struct btf_param *find_btf_func_param(const char *funcname, s32 *nr,
> > -						   bool tracepoint)
> > +static const struct btf_type *find_btf_func_proto(const char *funcname)
> >   {
> >   	struct btf *btf = traceprobe_get_btf();
> > -	const struct btf_param *param;
> >   	const struct btf_type *t;
> >   	s32 id;
> >   
> > -	if (!btf || !funcname || !nr)
> > +	if (!btf || !funcname)
> >   		return ERR_PTR(-EINVAL);
> >   
> >   	id = btf_find_by_name_kind(btf, funcname, BTF_KIND_FUNC);
> > @@ -396,6 +394,22 @@ static const struct btf_param *find_btf_func_param(const char *funcname, s32 *nr
> >   	if (!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)
> > +{
> > +	const struct btf_param *param;
> > +	const struct btf_type *t;
> > +
> > +	if (!funcname || !nr)
> > +		return ERR_PTR(-EINVAL);
> > +
> > +	t = find_btf_func_proto(funcname);
> > +	if (IS_ERR(t))
> > +		return (const struct btf_param *)t;
> > +
> >   	*nr = btf_type_vlen(t);
> >   	param = (const struct btf_param *)(t + 1);
> >   
> > @@ -462,6 +476,32 @@ static const struct fetch_type *parse_btf_arg_type(int arg_idx,
> >   	return find_fetch_type(typestr, ctx->flags);
> >   }
> >   
> > +static const struct fetch_type *parse_btf_retval_type(
> > +					struct traceprobe_parse_context *ctx)
> > +{
> 
> Can we make this a common interface so that the funcgraph-retval can
>   also use it to get the return type?

It is possible to expose BTF part as an independent file so that
other ftrace tracers reuse it.
But you might need to store the result for each function somewhere
in the kernel. Would you have any idea?

Thank you,

> 
> -- Donglin Peng
> 
> > +	struct btf *btf = traceprobe_get_btf();
> > +	const char *typestr = NULL;
> > +	const struct btf_type *t;
> > +
> > +	if (btf && ctx->funcname) {
> > +		t = find_btf_func_proto(ctx->funcname);
> > +		if (!IS_ERR(t))
> > +			typestr = type_from_btf_id(btf, t->type);
> > +	}
> > +
> > +	return find_fetch_type(typestr, ctx->flags);
> > +}
> > +
> > +static bool is_btf_retval_void(const char *funcname)
> > +{
> > +	const struct btf_type *t;
> > +
> > +	t = find_btf_func_proto(funcname);
> > +	if (IS_ERR(t))
> > +		return false;
> > +
> > +	return t->type == 0;
> > +}
> >   #else
> >   static struct btf *traceprobe_get_btf(void)
> >   {
> > @@ -480,8 +520,15 @@ static int parse_btf_arg(const char *varname, struct fetch_insn *code,
> >   	trace_probe_log_err(ctx->offset, NOSUP_BTFARG);
> >   	return -EOPNOTSUPP;
> >   }
> > +
> >   #define parse_btf_arg_type(idx, ctx)		\
> >   	find_fetch_type(NULL, ctx->flags)
> > +
> > +#define parse_btf_retval_type(ctx)		\
> > +	find_fetch_type(NULL, ctx->flags)
> > +
> > +#define is_btf_retval_void(funcname)	(false)
> > +
> >   #endif
> >   
> >   #define PARAM_MAX_STACK (THREAD_SIZE / sizeof(unsigned long))
> > @@ -512,6 +559,11 @@ static int parse_probe_vars(char *arg, const struct fetch_type *t,
> >   
> >   	if (strcmp(arg, "retval") == 0) {
> >   		if (ctx->flags & TPARG_FL_RETURN) {
> > +			if ((ctx->flags & TPARG_FL_KERNEL) &&
> > +			    is_btf_retval_void(ctx->funcname)) {
> > +				err = TP_ERR_NO_RETVAL;
> > +				goto inval;
> > +			}
> >   			code->op = FETCH_OP_RETVAL;
> >   			return 0;
> >   		}
> > @@ -912,9 +964,12 @@ static int traceprobe_parse_probe_arg_body(const char *argv, ssize_t *size,
> >   		goto fail;
> >   
> >   	/* Update storing type if BTF is available */
> > -	if (IS_ENABLED(CONFIG_PROBE_EVENTS_BTF_ARGS) &&
> > -	    !t && code->op == FETCH_OP_ARG)
> > -		parg->type = parse_btf_arg_type(code->param, ctx);
> > +	if (IS_ENABLED(CONFIG_PROBE_EVENTS_BTF_ARGS) && !t) {
> > +		if (code->op == FETCH_OP_ARG)
> > +			parg->type = parse_btf_arg_type(code->param, ctx);
> > +		else if (code->op == FETCH_OP_RETVAL)
> > +			parg->type = parse_btf_retval_type(ctx);
> > +	}
> >   
> >   	ret = -EINVAL;
> >   	/* Store operation */
> > diff --git a/kernel/trace/trace_probe.h b/kernel/trace/trace_probe.h
> > index c864e6dea10f..eb43bea5c168 100644
> > --- a/kernel/trace/trace_probe.h
> > +++ b/kernel/trace/trace_probe.h
> > @@ -449,6 +449,7 @@ extern int traceprobe_define_arg_fields(struct trace_event_call *event_call,
> >   	C(BAD_EVENT_NAME,	"Event name must follow the same rules as C identifiers"), \
> >   	C(EVENT_EXIST,		"Given group/event name is already used by another event"), \
> >   	C(RETVAL_ON_PROBE,	"$retval is not available on probe"),	\
> > +	C(NO_RETVAL,		"This function returns 'void' type"),	\
> >   	C(BAD_STACK_NUM,	"Invalid stack number"),		\
> >   	C(BAD_ARG_NUM,		"Invalid argument number"),		\
> >   	C(BAD_VAR,		"Invalid $-valiable specified"),	\
> > 
> > 
> > 
> 


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