Re: [RFC PATCH 4/6] uprobes/x86: Emulate rip-relative call's

[Masami Hiramatsu]

(2014/04/10 22:41), Denys Vlasenko wrote:
> On 04/09/2014 05:43 PM, Oleg Nesterov wrote:
>> On 04/08, Jim Keniston wrote:
>>>
>>> On Sun, 2014-04-06 at 22:16 +0200, Oleg Nesterov wrote:
>>>> 0xe8. Anything else?
>>>
>>> No, I think e8 is the only call instruction uprobes will see.
>>
>> Good.
> 
> There is this monstrosity, "16-bit override for branches" in 64-mode:
> 
> 66 e8 nn nn       callw   <offset16>
> 
> Nobody sane uses it because it truncates instruction pointer.

No problem, insn.c can handle that too. :)

Thank you,

> 
> Or rather, *I think* it should truncate it (i.e. zero-extend to full width),
> but conceivably some CPUs can be buggy wrt that:
> they can decide to modify only lower 16 bits of IP,
> or even they can decided to use signed <offset16> but apply it
> to full-width RIP.
> 
> AMD manuals are not clear on what exactly should happen.
> 
> I am sure no one sane uses this form of branch instructions
> in 32-bit and 64-bit code.
> 
> I don't think we should be trying to support it "correctly"
> (we can just let program crash with SIGILL or something),
> we only need to make sure we don't overlook its existence
> and thus are not tricked into touching or modifying unrelated data.
> 
> 
> Imagine that   66 e8 nn nn    bytes are exactly at the end of
> a page, and we wrongly assume that offset is 32-bit, not 16-bit.
> 
> 


-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx


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