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

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Message-ID: <20131104184741.GA15945@redhat.com>
Date:	Mon, 4 Nov 2013 19:47:41 +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 11/04, Oleg Nesterov wrote:
>
> On 11/04, Oleg Nesterov wrote:
> >
> > But in any case, I strongly believe that it doesn't make any sense to
> > rely on tu->inode in get_user_vaddr().
>
> Hmm. But I forgot about the case when you probe the function in libc
> and want to dump the variable in libc...
>
> So probably I was wrong and this all needs more thinking. Damn.
> Perhaps we really need to pass @file/offset, but it is not clear what
> we can do with bss/anon-mapping.

Or. Not that I really like this, but just for discussion...

How about

	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.

And this still doesn't allow to, say, probe the executable but read
the data from libc. Unless, again, we attach to the running process
or randomize_va_space = 0, so we can know it in advance. But otherwise
I do not think there is any solution.

Oleg.

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