Re: Assignment of GDT entries

["Albert Cahalan" <acahalan@gmail.com>]

Linus Torvalds writes:
> On Wed, 13 Sep 2006, Jeremy Fitzhardinge wrote:

>> So does this mean that moving the user-visible cs/ds isn't
>> likely to break stuff, if it has been done before?
>
> Yes. I _think_ we could do it. It's been done before, and nobody noticed.
>
> That said, it may actually be that programs have since become much more
> aware of segments, for a rather perverse reason: the TLS stuff. Old
> programs are all very much coded and compiled for a totally flat model,
> and as such they really don't know _anything_ about segments. But with
> more TLS stuff, it's possible that a modern threded program is at least
> aware of _some_ of it.

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.

So a 32-bit app using set_thread_area can work on i386 or x86_64,
but not both. I guess glibc gets %gs set up free via clone() with
the right flags, and thus does not need to determine the kernel.
For anything involving set_thread_area though, it gets nasty.

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.