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Re: New .nops directive, to aid Linux alternatives patching?
- From: Andrew Cooper <andrew dot cooper3 at citrix dot com>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>
- Cc: Binutils <binutils at sourceware dot org>
- Date: Thu, 8 Feb 2018 20:18:20 +0000
- Subject: Re: New .nops directive, to aid Linux alternatives patching?
- Authentication-results: sourceware.org; auth=none
- References: <9aef8e7e-bb7c-3e8e-ddb6-4501801d4bca@citrix.com> <CAMe9rOpd-3-rAhjDaTSSquE6AmfdgcK1=OBT8btGHjkKd=zLrQ@mail.gmail.com>
On 08/02/2018 20:10, H.J. Lu wrote:
> On Thu, Feb 8, 2018 at 11:26 AM, Andrew Cooper
> <andrew.cooper3@citrix.com> wrote:
>> Hello,
>>
>> I realise this is a little bit niche, but how feasible would it be to
>> introduce a new .nops directive which takes a size parameter, and
>> outputs long nops covering the number of specified bytes?
>>
> Sounds to me you want a pseudo NOP instruction:
>
> pseudo-NOP N
>
> which generates a long NOP with N byte. Is that correct. If yes,
> what is the range of N?
Currently 255 based on other implementation limits, and I expect that
ought to be long enough for anyone. There is one existing user for
N=43, and I expect that to grow a bit.
The real answer properly depends at what point it is more efficient to
jmp rather than wasting decode bandwidth decoding nops, and I don't know
the answer, but expect that it isn't larger than 255.
~Andrew