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

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Message-ID: <CAK1hOcP-+gXcfCV6zJFEu381R9XPR=qxNFQTOT8Y2o4L-1mwhA@mail.gmail.com>
Date:	Thu, 26 Jan 2012 19:28:28 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.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.

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.

But if there is an exception and CPU ignores 0x67 pfx
for (%rip) addressing, we will need to check for and
remove this pfx.

Otherwise, it'll instruct to use 32-bit registers
for addressing, thus it will turn our (%rax) operand
into (%eax). Not good.

-- 
vda
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