linux-kernel - Re: [PATCHSET 00/13] tracing/uprobes: Add support for more fetch methods (v6)

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Message-ID: <20131105164102.GC2460@redhat.com>
Date:	Tue, 5 Nov 2013 17:41:02 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Namhyung Kim <namhyung.kim@....com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Hyeoncheol Lee <cheol.lee@....com>,
	Hemant Kumar <hkshaw@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCHSET 00/13] tracing/uprobes: Add support for more fetch
	methods (v6)

On 11/05, Namhyung Kim wrote:
>
> On Mon, 4 Nov 2013 19:57:54 +0100, Oleg Nesterov wrote:
> >>
> >> 	static void __user *get_user_vaddr(struct pt_regs *regs, unsigned long addr)
> >> 	{
> >> 		return (void __force __user *)addr + instruction_pointer(regs);
> >> 	}
> >>
> >> ?
> >>
> >> This should solve the problems with relocations/randomization/bss.
> >>
> >> The obvious disadvantage is that it is not easy to calculate the
> >> offset we need to pass as an argument, it depends on the probed
> >> function.
> >
> > forgot to mention... and instruction_pointer() can't work in ret-probe,
> > we need to pass the "unsigned long func" arg somehow...
>
> Hmm.. what's the value of tu->offset in this case?  Does it have the
> offset of the return address or the start of the function?

It is the offest of function. IOW, it is the same regardless of
is_ret_probe().

Ignoring probes_seq_show() we only need it for uprobe_register().

(yes, it is also used in uprobe_unregister/apply, but this is only
 because this API ugly and should be cleanuped).

Oleg.

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