Re: [Xen-devel] [PATCH 10/13] x86/alternative: Support indirect call replacement

[Juergen Gross <jgross@xxxxxxxx>]

On 16/11/17 22:19, Josh Poimboeuf wrote:
> On Wed, Oct 25, 2017 at 01:25:02PM +0200, Juergen Gross wrote:
>> On 04/10/17 17:58, Josh Poimboeuf wrote:
>>> Add alternative patching support for replacing an instruction with an
>>> indirect call.  This will be needed for the paravirt alternatives.
>>>
>>> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
>>> ---
>>>  arch/x86/kernel/alternative.c | 22 +++++++++++++++-------
>>>  1 file changed, 15 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
>>> index 3344d3382e91..81c577c7deba 100644
>>> --- a/arch/x86/kernel/alternative.c
>>> +++ b/arch/x86/kernel/alternative.c
>>> @@ -410,20 +410,28 @@ void __init_or_module noinline 
>>> apply_alternatives(struct alt_instr *start,
>>>             insnbuf_sz = a->replacementlen;
>>>  
>>>             /*
>>> -            * 0xe8 is a relative jump; fix the offset.
>>> -            *
>>> -            * Instruction length is checked before the opcode to avoid
>>> -            * accessing uninitialized bytes for zero-length replacements.
>>> +            * Fix the address offsets for call and jump instructions which
>>> +            * use PC-relative addressing.
>>>              */
>>>             if (a->replacementlen == 5 && *insnbuf == 0xe8) {
>>> +                   /* direct call */
>>>                     *(s32 *)(insnbuf + 1) += replacement - instr;
>>> -                   DPRINTK("Fix CALL offset: 0x%x, CALL 0x%lx",
>>> +                   DPRINTK("Fix direct CALL offset: 0x%x, CALL 0x%lx",
>>>                             *(s32 *)(insnbuf + 1),
>>>                             (unsigned long)instr + *(s32 *)(insnbuf + 1) + 
>>> 5);
>>> -           }
>>>  
>>> -           if (a->replacementlen && is_jmp(replacement[0]))
>>> +           } else if (a->replacementlen == 6 && *insnbuf == 0xff &&
>>> +                      *(insnbuf+1) == 0x15) {
>>> +                   /* indirect call */
>>> +                   *(s32 *)(insnbuf + 2) += replacement - instr;
>>> +                   DPRINTK("Fix indirect CALL offset: 0x%x, CALL *0x%lx",
>>> +                           *(s32 *)(insnbuf + 2),
>>> +                           (unsigned long)instr + *(s32 *)(insnbuf + 2) + 
>>> 6);
>>> +
>>> +           } else if (a->replacementlen && is_jmp(replacement[0])) {
>>
>> Is this correct? Without your patch this was:
>>
>> if (*insnbuf == 0xe8) ...
>> if (is_jmp(replacement[0])) ...
>>
>> Now you have:
>>
>> if (*insnbuf == 0xe8) ...
>> else if (*insnbuf == 0xff15) ...
>> else if (is_jmp(replacement[0])) ...
>>
>> So only one or none of the three variants will be executed. In the past
>> it could be none, one or both.
> 
> It can't be a call *and* a jump.  It's either one or the other.
> 
> Maybe it's a little confusing that the jump check uses replacement[0]
> while the other checks use *insnbuf?  They have the same value, so the

Right, I was fooled by that.

> same variable should probably be used everywhere for consistency.  I can
> make them more consistent.
> 

I'd appreciate that. :-)


Juergen

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