x86: fix savesegment() bug causing crashes on 64-bit
i spent a fair amount of time chasing a 64-bit bootup crash that manifested
itself as bootup segfaults:
S10network[1825]: segfault at 7f3e2b5d16b8 ip 00000031108748c9 sp 00007fffb9c14c70 error 4 in libc-2.7.so[3110800000+14d000]
eventually causing init to die and panic the system:
Kernel panic - not syncing: Attempted to kill init!
Pid: 1, comm: init Not tainted 2.6.26-rc9-tip #13878
after a maratonic bisection session, the bad commit turned out to be:
| b7675791859075418199c7af86a116ea34eaf5bd is first bad commit
| commit b7675791859075418199c7af86a116ea34eaf5bd
| Author: Jeremy Fitzhardinge <jeremy@goop.org>
| Date: Wed Jun 25 00:19:00 2008 -0400
|
| x86: remove open-coded save/load segment operations
|
| This removes a pile of buggy open-coded implementations of savesegment
| and loadsegment.
after some more bisection of this patch itself, it turns out that what
makes the difference are the savesegment() changes to __switch_to().
Taking a look at this portion of arch/x86/kernel/process_64.o revealed
this crutial difference:
| good: 99c: 8c e0 mov %fs,%eax
| 99e: 89 45 cc mov %eax,-0x34(%rbp)
|
| bad: 99c: 8c 65 cc mov %fs,-0x34(%rbp)
which is due to:
| unsigned fsindex;
| - asm volatile("movl %%fs,%0" : "=r" (fsindex));
| + savesegment(fs, fsindex);
savesegment() is implemented as:
#define savesegment(seg, value) \
asm("mov %%" #seg ",%0":"=rm" (value) : : "memory")
note the "m" modifier - it allows GCC to generate the segment move
into a memory operand as well.
But regarding segment operands there's a subtle detail in the x86
instruction set: the above 16-bit moves are zero-extend, but only
if it goes to a register.
If it goes to a memory operand, -0x34(%rbp) in the above case, there's
no zero-extend to 32-bit and the instruction will only save 16 bits
instead of the intended 32-bit.
The other 16 bits is random data - which can cause problems when that
value is used later on.
The solution is to only allow segment operands to go to registers.
This fix allows my test-system to boot up without crashing.
Signed-off-by: NIngo Molnar <mingo@elte.hu>
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