From: Peter Zijlstra <peterz@infradead.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>,
xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
Miroslav Benes <mbenes@suse.cz>,
Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Subject: Re: [PATCH 3/3] objtool: Support stack layout changes in alternatives
Date: Mon, 4 Jan 2021 16:48:18 +0100 [thread overview]
Message-ID: <20210104154818.GB3040@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20210104151633.ojv3wggzpxzn2alx@treble>
On Mon, Jan 04, 2021 at 09:16:33AM -0600, Josh Poimboeuf wrote:
> > 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
Aaah, I was thinking the (LHS) NOP2 lookup would find the (RHS) PUSHF
and fail, but yes, it will emit it's own +8 and find that ofcourse!
So then yes, we only need to concern outselves with same offset
conflicts, and that does indeed simplify things considerably.
next prev parent reply other threads:[~2021-01-04 15:49 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-23 5:18 [PATCH 0/3] Alternatives vs ORC, a slightly easier way Josh Poimboeuf
2020-12-23 5:18 ` [PATCH 1/3] objtool: Refactor ORC section generation Josh Poimboeuf
2020-12-23 5:18 ` [PATCH 2/3] objtool: Add 'alt_group' struct Josh Poimboeuf
2020-12-23 5:18 ` [PATCH 3/3] objtool: Support stack layout changes in alternatives Josh Poimboeuf
2021-01-04 14:09 ` Peter Zijlstra
2021-01-04 15:16 ` Josh Poimboeuf
2021-01-04 15:48 ` Peter Zijlstra [this message]
2021-01-07 13:22 ` Miroslav Benes
2021-01-07 18:18 ` Josh Poimboeuf
2021-01-08 8:15 ` Miroslav Benes
2021-01-14 0:21 ` Josh Poimboeuf
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