mihai.dontu at gmail.com
Mon Oct 8 12:03:38 PDT 2007
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.
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