linux-kernel - Re: [PATCH 08/17] tracing/probes: Split [ku]probes_fetch_type

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Date:	Mon, 2 Dec 2013 18:09:52 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Hyeoncheol Lee <cheol.lee@....com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Hemant Kumar <hkshaw@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Namhyung Kim <namhyung.kim@....com>
Subject: Re: [PATCH 08/17] tracing/probes: Split [ku]probes_fetch_type_table

On 12/02, Oleg Nesterov wrote:
>
> On 11/27, Namhyung Kim wrote:
> >
> > Use separate fetch_type_table for kprobes and uprobes.  It currently
> > shares all fetch methods but some of them will be implemented
> > differently later.
>
> Hmm. This looks wrong, afaics...
>
> >  static int parse_probe_arg(char *arg, const struct fetch_type *t,
> >  		     struct fetch_param *f, bool is_return, bool is_kprobe)
> >  {
> > +	const struct fetch_type *ftbl;
> >  	unsigned long param;
> >  	long offset;
> >  	char *tmp;
> > -	int ret;
> > +	int ret = 0;
> >
> > -	ret = 0;
> > +	ftbl = is_kprobe ? kprobes_fetch_type_table : uprobes_fetch_type_table;
>
> OK, but what if, say, CONFIG_KPROBE_EVENT && !CONFIG_UPROBE_EVENT ?
> The kernel won't compile in this case?

Ah, wait, probably I was wrong. I didn't noticee that this patch
does

	extern __weak const struct fetch_type kprobes_fetch_type_table[];
	extern __weak const struct fetch_type uprobes_fetch_type_table[];

Is it the reason for "weak" ?

If yes, perhaps this deserves a comment or at least a note in the changelog.
Or is there another reason?

I am wondering if this should always work, with any toolchain. I simply
do not know what is the documented behaviour if a "weak" symbol is never
defined.

Oleg.

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