[fpc-devel] OO rewrite - technical questions

[Daniël Mantione]

Op Mon, 19 Jul 2010, schreef Sven Barth:

> Hi!
>
>> Yes, though I am still puzzled how it works on x86_64; it seems regvars
>> there are also accessed using fs, but x86_64 prevents you from writing
>> to segment registers.
>
> As mentioned here http://en.wikipedia.org/wiki/X86_64#Windows in the 10th 
> point Windows uses the GS segment register to store pointers to the TEB (see 
> http://en.wikipedia.org/wiki/Thread_Environment_Block ) of the current 
> running thread (x86 uses FS). So they should still be writable as the TEB is 
> written to by the Win32 subsystem (of which a part runs in usermode) as part 
> of its bookkeeping and maybe also by user applications to implement the 64 
> bit equivalent of SEH.

Well, the following works on i386 but doesn't on x86_64, even though 
modify_ldt succeeds:

program ldt_test;

{$asmmode intel}

uses baseunix,linux;

var p:pointer;
     ud:user_desc;

begin
   p:=pointer(fpmmap(nil,4096,3,MAP_PRIVATE+MAP_ANONYMOUS,-1,0));
   ud.entry_number:=1;
   pointer(ud.base_addr):=p;
   ud.limit:=4096;
   ud.flags:=UD_SEG_32BIT or UD_CONTENTS_DATA or UD_USEABLE;
   modify_ldt(1, at ud,sizeof(ud));
   asm
     mov ax,15
     mov fs,ax
     mov fs:[0].dword,29
   end;
   writeln(Pcardinal(p)^);
end.

If you know how to make it work, I keep myself recommended :)

Daniël