Date: Mon, 8 Oct 2007 15:37:01 -0400 From: Jung-uk Kim <jkim@FreeBSD.org> To: freebsd-emulation@FreeBSD.org Cc: freebsd-questions@FreeBSD.org, Mihai =?utf-8?q?Don=C8=9Bu?= <mihai.dontu@gmail.com> Subject: Re: amd64_set_gsbase() Message-ID: <200710081537.03836.jkim@FreeBSD.org> In-Reply-To: <200710082135.58099.mihai.dontu@gmail.com> References: <200710082135.58099.mihai.dontu@gmail.com>
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On Monday 08 October 2007 02:35 pm, Mihai Donțu wrote: > Hi, > > I have a "small" amd64 program that makes havy use of LDT (%GS to > be more specific). The trouble is, in a multithreaded environment, > the selector value gets lost (or reset?). > > The code *always* segfaults with this stack: > 4 LWP 100126 0x0000000800dec07c in select () from /lib/libc.so.6 > * 3 Thread 0x517000 (runnable) 0x000000080055cfbc in ?? () > 2 Thread 0x517400 (LWP 100125) 0x0000000800c0d85c in > pthread_testcancel () from /lib/libpthread.so.2 1 Thread 0x517800 > (runnable) 0x0000000800d5d000 in makecontext () from > /lib/libc.so.6 > > at this instruction: > 0x000000080055cfbc: mov %gs:0x10,%r11 > > (gdb) p $gs > $1 = 0 > > I've been reading on the net something about the kernel not > preserving the GS across syscalls (or stmh). Is this true? and if > so, is there a known workaround? > > I'm on a FreeBSD 6.2-STABLE-200706 (AMD64) machine. 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. 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). However, it only changes the base address via MSR, i.e., %gs itself has no meaning. Jung-uk Kim
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