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Date:   Mon, 4 Jan 2021 16:48:18 +0100
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
Message-ID: <20210104154818.GB3040@hirez.programming.kicks-ass.net>
References: <cover.1608700338.git.jpoimboe@redhat.com>
 <9f78604e49b400eb3b2ca613591f8c357474ed4e.1608700338.git.jpoimboe@redhat.com>
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 <20210104151633.ojv3wggzpxzn2alx@treble>
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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.