linux-kernel - Re: [PATCH v9 3.2 1/9] uprobes: Install and remove breakpoints.

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Date:	Mon, 30 Jan 2012 18:51:22 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-mm <linux-mm@...ck.org>, Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Roland McGrath <roland@...k.frob.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Anton Arapov <anton@...hat.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	yrl.pp-manager.tt@...achi.com
Subject: Re: [PATCH v9 3.2 1/9] uprobes: Install and remove breakpoints.

(2012/01/27 3:28), Denys Vlasenko wrote:
> On Thu, Jan 26, 2012 at 3:14 PM, Masami Hiramatsu
> <masami.hiramatsu.pt@...achi.com> wrote:
>> (2012/01/26 0:32), Denys Vlasenko wrote:
>>> On Wed, Jan 25, 2012 at 4:13 PM, Denys Vlasenko
>>> <vda.linux@...glemail.com> wrote:
>>>>> +       /*
>>>>> +        * Convert from rip-relative addressing to indirect addressing
>>>>> +        * via a scratch register.  Change the r/m field from 0x5 (%rip)
>>>>> +        * to 0x0 (%rax) or 0x1 (%rcx), and squeeze out the offset field.
>>>>> +        */
>>>>> +       reg = MODRM_REG(insn);
>>>>> +       if (reg == 0) {
>>>>> +               /*
>>>>> +                * The register operand (if any) is either the A register
>>>>> +                * (%rax, %eax, etc.) or (if the 0x4 bit is set in the
>>>>> +                * REX prefix) %r8.  In any case, we know the C register
>>>>> +                * is NOT the register operand, so we use %rcx (register
>>>>> +                * #1) for the scratch register.
>>>>> +                */
>>>>> +               uprobe->arch_info.fixups = UPROBES_FIX_RIP_CX;
>>>>> +               /* Change modrm from 00 000 101 to 00 000 001. */
>>>>> +               *cursor = 0x1;
>>>
>>> Hmm. I think we have a bug here.
>>>
>>> What if this instruction has REX.B = 1? Granted, REX.B = 1 has no effect on
>>> rip-relative addressing and therefore normally won't be generated by gcc/as,
>>> but still. If you replace md and r/m fields as above, you are trying to convert
>>> 0x12345678(%rip) reference to (%rcx), but if REX.B = 1, then you in fact
>>> converted it to (%r9)!
>>
>> Right, thanks for finding :)
>> And %rax register reference encoding has same problem, doesn't it?
>
> Yes.
>
> The solution is trivial: "if (REX pfx exists) REX.B = 0;"
>
> Also, I don't remember whether (%rip) addressing is
> affected by 0x67 (address size) prefix. If it is, then
> nothing needs to be done.

In Intel SDM vol.2, "2.2.1.6 RIP-Relative Addressing" explains
that as below;

---
RIP-relative addressing is enabled by 64-bit mode, not by a 64-bit address-size.
The use of the address-size prefix does not disable RIP-relative addressing. The
effect of the address-size prefix is to truncate and zero-extend the computed
effective address to 32 bits.
---

This means 0x67 pfx can be used with RIP-relative, and
effective address(EA) will be 32bit, as same as others.
So, I think it should work :)


Thank you!

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com
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