Date: Tue, 9 Oct 2007 01:00:58 +0300 From: Mihai =?utf-8?q?Don=C8=9Bu?= <mihai.dontu@gmail.com> To: Jung-uk Kim <jkim@freebsd.org> Cc: freebsd-emulation@freebsd.org, freebsd-questions@freebsd.org Subject: Re: amd64_set_gsbase() Message-ID: <200710090100.58577.mihai.dontu@gmail.com> In-Reply-To: <200710081537.03836.jkim@FreeBSD.org> References: <200710082135.58099.mihai.dontu@gmail.com> <200710081537.03836.jkim@FreeBSD.org>
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On Monday 08 October 2007, Jung-uk Kim wrote:
> Yes, you are correct. A short version is "don't do that". A long
> version goes like this. %fs and %gs are not preserved while context
> switching on amd64.
But this makes emulation software such as Wine a lost hope, doesn't it?
Because Windows apps access the Thread Information Block (TIB) via %gs
(%fs on ia32).
Anyway, my so called "small" program is actually a Win64 emulator and
I need the segment selector to "stay put" across syscalls. It works
like a charm on single threaded apps, but as soon as I spawn a thread,
all hell breaks loose :)
I've managed to come up with something that *kind of* works. It goes
like this:
void my_handler( int s )
{
if ( s == SIGSEGV ) {
if ( get_gs() == 0 ) {
amd64_set_gsbase();
} else {
signal( SIGSEGV, SIG_DFL );
}
}
}
int my_init( void )
{
/* alloc TIB memory and initialize */
amd64_set_gsbase( lpTIB );
signal( SIGSEGV, my_handler );
return 0;
}
but after a series of dlopen()-s, my_handler() is called without %gs
being zero and without a valid fault (the handler does not get recalled
after signal( SIGSEGV, SIG_DFL ). I'm still working on this aspect ...
> In fact, you should not use amd64_set_gsbase()
> directly. If you *really* have to mess up with base addresses, you
> have to use sysarch(2) syscall, i.e., sysarch(AMD64_SET_GSBASE,
> args).
I found this: /usr/src/lib/libc/amd64/sys/amd64_set_gsbase.c:32
"
int
amd64_set_gsbase(void *addr)
{
return (sysarch(AMD64_SET_GSBASE, &addr));
}
"
and this (man 2 sysarch()): "The sysarch() system call should never be
called directly by user programs. Instead, they should access its
functions using the architecture-dependent library."
Who am I suppose to believe? :)
> However, it only changes the base address via MSR, i.e., %gs
> itself has no meaning.
Maybe, but the selector loaded in %gs *does* have meaning.
Anyway, the thing is I _have_ to make this work. I'll keep you posted ;)
--
Mihai Donțu
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