llvm dev - Oct 2010 - [LLVMdev] Implementing the hotpatch attribute for X86

Hi,

According to
http://blogs.msdn.com/b/freik/archive/2006/03/07/x64-hotpatchability.aspx,
'hotpatchable' functions on x86 (and by extension, x86-64) are preceded
by six bytes of padding and start with a two byte instruction. The
problem is that, still being relatively unfamiliar with the x86 backend,
I have no idea how to implement this for the 'hotpatch' attribute I just
added. I do remember the emitPrologue() method from when I was working
on the alignstack() attribute, but I still don't know how to implement
the first part (the six bytes of padding). Does anyone have any pointers?

Chip

On Tue, Oct 26, 2010 at 7:02 PM, Charles Davis <cdavis at
mymail.mines.edu> wrote:
> Hi,
>
> According to
> http://blogs.msdn.com/b/freik/archive/2006/03/07/x64-hotpatchability.aspx,
> 'hotpatchable' functions on x86 (and by extension, x86-64) are
preceded
> by six bytes of padding and start with a two byte instruction. The
> problem is that, still being relatively unfamiliar with the x86 backend,
> I have no idea how to implement this for the 'hotpatch' attribute I
just
> added. I do remember the emitPrologue() method from when I was working
> on the alignstack() attribute, but I still don't know how to implement
> the first part (the six bytes of padding). Does anyone have any pointers?
>
> Chip

The linker adds the padding. Also, the first instruction just has to
be two bytes or longer. Not exactly two bytes.

- Michael Spencer

On 10/26/10 5:24 PM, Michael Spencer wrote:
> The linker adds the padding. Also, the first instruction just has to
> be two bytes or longer. Not exactly two bytes.
How then does the linker know to add the padding? I assume there's a
PE-COFF attribute that will do that, but what about other file formats,
like ELF or Mach-O? Bear in mind that I'm doing this for the Wine
project, so I'm very concerned about those two formats.

Chip