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Message-ID: <20210104151633.ojv3wggzpxzn2alx@treble>
Date: Mon, 4 Jan 2021 09:16:33 -0600
From: Josh Poimboeuf <jpoimboe@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org,
linux-kernel@...r.kernel.org, Miroslav Benes <mbenes@...e.cz>,
Shinichiro Kawasaki <shinichiro.kawasaki@....com>
Subject: Re: [PATCH 3/3] objtool: Support stack layout changes in alternatives
On Mon, Jan 04, 2021 at 03:09:52PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 22, 2020 at 11:18:10PM -0600, Josh Poimboeuf wrote:
>
> > For example, this scenario is allowed:
> >
> > Alt1 Alt2 Alt3
> >
> > 0x00 CALL *pv_ops.save_fl CALL xen_save_fl PUSHF
> > 0x01 POP %RAX
> > 0x02 NOP
> > ...
> > 0x05 NOP
> > ...
> > 0x07 <insn>
> >
>
> > This scenario is NOT allowed:
> >
> > Alt1 Alt2
> >
> > 0x00 CALL *pv_ops.save_fl PUSHF
> > 0x01 NOP6
> > ...
> > 0x07 NOP POP %RAX
> >
>
> > The problem here is that offset-0x7, which is an instruction boundary in
> > both possible instruction patch streams, has two conflicting stack
> > layouts.
>
> There's another fun scenario:
>
> 0x00 CALL *pv_ops.save_fl PUSHF
> 0x01 NOP2
> ..
> 0x03 NOP5
> ..
> 0x07 NOP2
> 0x08 POP %RAX
> 0x09 <insn>
>
> No conflicting boundary at 0x07, but still buggered.
>
> Let me go read the actual patch to see if this is handled.
That scenario looks good, see ORC below:
.diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index cad08703c4ad..4079a430ab3f 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1483,3 +1483,8 @@ SYM_CODE_START(rewind_stack_do_exit)
call do_exit
SYM_CODE_END(rewind_stack_do_exit)
.popsection
+
+SYM_FUNC_START(peter)
+ ALTERNATIVE "call *pv_ops+288(%rip); .byte 0x66,0x90", "pushf; .byte 0x66,0x90; .byte 0x66,0x66,0x66,0x90; popq %rax", X86_FEATURE_ALWAYS
+ ret
+SYM_FUNC_END(peter)
00000000000014e0 <peter>:
14e0: ff 15 00 00 00 00 callq *0x0(%rip) # 14e6 <peter+0x6>
14e2: R_X86_64_PC32 pv_ops+0x11c
14e6: 66 90 xchg %ax,%ax
14e8: c3 retq
alt replacement:
cf: 9c pushfq
d0: 66 90 xchg %ax,%ax
d2: 66 66 66 90 data16 data16 xchg %ax,%ax
d6: 58 pop %rax
ORC:
.entry.text+14e0: sp:sp+8 bp:(und) type:call end:0
.entry.text+14e1: sp:sp+16 bp:(und) type:call end:0
.entry.text+14e6: sp:sp+8 bp:(und) type:call end:0
.entry.text+14e7: sp:sp+16 bp:(und) type:call end:0
.entry.text+14e8: sp:sp+8 bp:(und) type:call end:0
.entry.text+14e9: sp:(und) bp:(und) type:call end:0
--
Josh
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