linux-kernel - Re: test-case (Was: [PATCH 12/X] uprobes: x86: introduce abort

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Date:	Tue, 25 Oct 2011 17:49:06 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: test-case (Was: [PATCH 12/X] uprobes: x86: introduce
	abort_xol())

On 10/25, Srikar Dronamraju wrote:
> >
> > 	static inline void *uc_ip(struct ucontext *ctxt)
> > 	{
> > 		return (void*)ctxt->uc_mcontext.gregs[16];
> > 	}
> > ...
> >
> I have tested this on both x86_32 and x86_64 and can confirm that the
> behaviour is same with or without uprobes placed at that instruction.
> This is on the uprobes code with your changes.

Great, thanks.

> However on x86_32; the output is different from x86_64.
>
> On x86_32 (I have additionally printed the uc_ip and fault_insn.
>
> SIGSEGV! ip=0x10246 addr=0x12345678
> ERR!! wrong ip uc_ip(ctxt) = 10246 fault_insn = 804856c

Yep. uc_ip() is not correct on x86_32. Sorry, I forgot to mention this.

I was really surprised when I wrote this test. I simply can't understand
how can I play with ucontext in the user-space. I guess uc_ip() should use
REG_EIP instead of 16, but I wasn't able to compile it even if I added
__USE_GNU. It would be even better to use sigcontext instead of the ugly
mcontext_t, but this looks "impossible". The kernel is much simpler ;)


> I still trying to dig up what uc_ip is and why its different on x86_32.

See above. I guess it needs ctxt->uc_mcontext.gregs[14]. Or REG_EIP.

uc_ip() simply reads sigcontext->ip passed by setup_sigcontext().

> Also I was thinking on your suggestion of making abort_xol a weak
> function. In which case we could have architecture independent function
> in kernel/uprobes.c which is just a wrapper for set_instruction_pointer.
>
> void __weak abort_xol(struct pt_regs *regs, struct uprobe_task *utask)
> {
> 	set_instruction_pointer(regs, utask->vaddr);
> }
>
> where it would called  from uprobe_notify_resume() as
>
> 	abort_xol(regs, utask);
>
> If other archs would want to do something else, they could override
> abort_xol definition.

I didn't suggest this ;) But looks reasonable to me. And afaics x86_32
can use this arch-independent function.

Oleg.

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