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

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Message-ID: <CAK1hOcMeYDrLpgbyQXFAkrqk3Le-rLkoVwCN3wjkT61YJg0G6Q@mail.gmail.com>
Date:	Wed, 25 Jan 2012 16:32:08 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	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>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	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>
Subject: Re: [PATCH v9 3.2 1/9] uprobes: Install and remove breakpoints.

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)!

-- 
vda
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