linux-kernel - Re: Assignment of GDT entries

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Date:	Wed, 13 Sep 2006 23:11:05 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Albert Cahalan <acahalan@...il.com>
CC:	torvalds@...l.org, mingo@...e.hu, ak@...e.de,
	ebiederm@...ssion.com, arjan@...radead.org, zach@...are.com,
	linux-kernel@...r.kernel.org
Subject: Re: Assignment of GDT entries

Albert Cahalan wrote:
> We actually have an ABI problem right now because of this.
> Note that i386 and x86_64 use different GDT slots.
>
> As far as I can tell, users need to hard-code the mapping
> from TLS slot to segment number. They use 0,1,2 to ask the
> kernel to set things up (via set_thread_area), but can't
> just pop that into %fs or %gs.

That's not true at all.  The program I posted earlier in this thread 
uses set_thread_area() to allocate a GDT slot, and it works on both 
native 32 bit and 32-under-64.  The entry_number field in the struct 
user_desc is an actual entry number, so you can easily construct a 
selector from it.

> Typical hacks that result from this:
>
> call uname() and look for "x86_64"
> see of the addresses of local variables exceed 0xbfffffff
> examine /proc/1/maps
> check for a /lib64 directory
> change SSE register 8 in a signal handler frame and see if it sticks
> checksum the vdso code
> ...
>
> Please save us from these foul hacks.

Er, that all looks completely unnecessary.

    J
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