Message-ID: <4508F279.6010205@goop.org>
Date: Wed, 13 Sep 2006 23:11:05 -0700
From: Jeremy Fitzhardinge <jeremy@goop.org>
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To: Albert Cahalan <acahalan@gmail.com>
CC: torvalds@osdl.org, mingo@elte.hu, ak@suse.de, ebiederm@xmission.com,
       arjan@infradead.org, zach@vmware.com, linux-kernel@vger.kernel.org
Subject: Re: Assignment of GDT entries
References: <787b0d920609132023t1686525ei9c1703b044029909@mail.gmail.com>
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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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