32-bit hotpatching question

[Alexandre Ganea]

That is quite interesting, thanks for the explanation. Normally, 66 90 was introduced with i386, so it would be a valid opcode for /arch:IA32. But I suppose reverting to 8F FF was done for compatibility reasons.
We should maybe mimic the same behavior in Clang. My intent was to support MSVC /hotpath in Clang, not GCC's ms_hook_prologue attribute. That could perhaps land later.

-----Message d'origine-----
De : Thomas Faber <thomas.faber at reactos.org> 
Envoyé : June 7, 2020 11:24 AM
À : Alexandre Ganea <alexandre.ganea at ubisoft.com>
Cc : Stefan Dösinger <stefandoesinger at gmail.com>; wine-devel at winehq.org
Objet : Re: 32-bit hotpatching question

On 2020-06-07 11:56, Stefan Dösinger wrote:
> Am 07.06.20 um 11:47 schrieb Stefan Dösinger:
>> Check the nop opcode in Windows DLLs. If Microsoft's kernel32.dll 
>> switched to 66 90 we're safe to switch too. If they still use 8b ff 
>> we probably should stick to that.
> 
> I just checked my Win10 19.09, it still uses 8b ff. So either 
> Microsoft compiles Windows with an old Visual Studio or they kept the 
> old style nops for Windows itself.


Looks like the /arch:IA32 option changes the hotpatch instruction from
66 90 back to 8b ff. This is true in a simple test and also my observation from looking at ReactOS binaries, which use that option.

It makes sense to assume that 32-bit Windows binaries would be built with that option for compatibility; the compiler default is to assume
SSE2 these days.