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From: Denys Vlasenko <vda.linux@googlemail.com>
Date: Wed, 25 Jan 2012 16:32:08 +0100
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Subject: Re: [PATCH v9 3.2 1/9] uprobes: Install and remove breakpoints.
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To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Linus Torvalds <torvalds@linux-foundation.org>, Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>, LKML <linux-kernel@vger.kernel.org>, Linux-mm <linux-mm@kvack.org>, Andi Kleen <andi@firstfloor.org>, Christoph Hellwig <hch@infradead.org>, Steven Rostedt <rostedt@goodmis.org>, Roland McGrath <roland@hack.frob.com>, Thomas Gleixner <tglx@linutronix.de>, Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>, Arnaldo Carvalho de Melo <acme@infradead.org>, Anton Arapov <anton@redhat.com>, Ananth N Mavinakayanahalli <ananth@in.ibm.com>, Jim Keniston <jkenisto@linux.vnet.ibm.com>, Stephen Rothwell <sfr@canb.auug.org.au>

On Wed, Jan 25, 2012 at 4:13 PM, Denys Vlasenko
<vda.linux@googlemail.com> wrote:
>> + =A0 =A0 =A0 /*
>> + =A0 =A0 =A0 =A0* Convert from rip-relative addressing to indirect addr=
essing
>> + =A0 =A0 =A0 =A0* via a scratch register. =A0Change the r/m field from =
0x5 (%rip)
>> + =A0 =A0 =A0 =A0* to 0x0 (%rax) or 0x1 (%rcx), and squeeze out the offs=
et field.
>> + =A0 =A0 =A0 =A0*/
>> + =A0 =A0 =A0 reg =3D MODRM_REG(insn);
>> + =A0 =A0 =A0 if (reg =3D=3D 0) {
>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 /*
>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0* The register operand (if any) is eith=
er the A register
>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0* (%rax, %eax, etc.) or (if the 0x4 bit=
 is set in the
>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0* REX prefix) %r8. =A0In any case, we k=
now the C register
>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0* is NOT the register operand, so we us=
e %rcx (register
>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0* #1) for the scratch register.
>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0*/
>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 uprobe->arch_info.fixups =3D UPROBES_FIX_R=
IP_CX;
>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 /* Change modrm from 00 000 101 to 00 000 =
001. */
>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 *cursor =3D 0x1;

Hmm. I think we have a bug here.

What if this instruction has REX.B =3D 1? Granted, REX.B =3D 1 has no effec=
t 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 con=
vert
0x12345678(%rip) reference to (%rcx), but if REX.B =3D 1, then you in fact
converted it to (%r9)!

--=20
vda

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