kernel_optimize_test/arch
Alexander van Heukelum dc4c2a0aed i386: fix/simplify espfix stack switching, move it into assembly
The espfix code triggers if we have a protected mode userspace
application with a 16-bit stack. On returning to userspace, with iret,
the CPU doesn't restore the high word of the stack pointer. This is an
"official" bug, and the work-around used in the kernel is to temporarily
switch to a 32-bit stack segment/pointer pair where the high word of the
pointer is equal to the high word of the userspace stackpointer.

The current implementation uses THREAD_SIZE to determine the cut-off,
but there is no good reason not to use the more natural 64kb... However,
implementing this by simply substituting THREAD_SIZE with 65536 in
patch_espfix_desc crashed the test application. patch_espfix_desc tries
to do what is described above, but gets it subtly wrong if the userspace
stack pointer is just below a multiple of THREAD_SIZE: an overflow
occurs to bit 13... With a bit of luck, when the kernelspace
stackpointer is just below a 64kb-boundary, the overflow then ripples
trough to bit 16 and userspace will see its stack pointer changed by
65536.

This patch moves all espfix code into entry_32.S. Selecting a 16-bit
cut-off simplifies the code. The game with changing the limit dynamically
is removed too. It complicates matters and I see no value in it. Changing
only the top 16-bit word of ESP is one instruction and it also implies
that only two bytes of the ESPFIX GDT entry need to be changed and this
can be implemented in just a handful simple to understand instructions.
As a side effect, the operation to compute the original ESP from the
ESPFIX ESP and the GDT entry simplifies a bit too, and the remaining
three instructions have been expanded inline in entry_32.S.

impact: can now reliably run userspace with ESP=xxxxfffc on 16-bit
stack segment

Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
Acked-by: Stas Sergeev <stsp@aknet.ru>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
2009-06-17 21:35:09 -07:00
..
alpha Merge branch 'master' of master.kernel.org:/pub/scm/linux/kernel/git/torvalds/linux-2.6 2009-06-15 03:02:23 -07:00
arm Merge branch 'master' of master.kernel.org:/pub/scm/linux/kernel/git/torvalds/linux-2.6 2009-06-15 03:02:23 -07:00
avr32 Merge branch 'master' of master.kernel.org:/pub/scm/linux/kernel/git/torvalds/linux-2.6 2009-06-15 03:02:23 -07:00
blackfin Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/vapier/blackfin 2009-06-16 11:49:58 -07:00
cris Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/arnd/asm-generic 2009-06-12 18:15:51 -07:00
frv FRV: Fix interaction with new generic header stuff 2009-06-13 13:17:28 -07:00
h8300 Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/arnd/asm-generic 2009-06-12 18:15:51 -07:00
ia64 [IA64] fix compile error in arch/ia64/mm/extable.c 2009-06-15 09:17:50 -07:00
m32r Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/arnd/asm-generic 2009-06-12 18:15:51 -07:00
m68k Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/arnd/asm-generic 2009-06-12 18:15:51 -07:00
m68knommu module: cleanup FIXME comments about trimming exception table entries. 2009-06-12 21:47:05 +09:30
microblaze Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/arnd/asm-generic 2009-06-12 18:15:51 -07:00
mips Merge branch 'i2c-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jdelvare/staging 2009-06-16 11:29:17 -07:00
mn10300 headers_check fix: mn10300, setup.h 2009-06-14 11:51:12 +05:30
parisc Merge branch 'master' of master.kernel.org:/pub/scm/linux/kernel/git/torvalds/linux-2.6 2009-06-15 03:02:23 -07:00
powerpc Merge branch 'merge' of git://git.kernel.org/pub/scm/linux/kernel/git/benh/powerpc 2009-06-16 11:30:37 -07:00
s390 [S390] s390: hibernation support for s390 2009-06-16 10:31:22 +02:00
sh Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/arnd/asm-generic 2009-06-12 18:15:51 -07:00
sparc sparc64: Update defconfig. 2009-06-16 04:59:53 -07:00
um Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/arnd/asm-generic 2009-06-12 18:15:51 -07:00
x86 i386: fix/simplify espfix stack switching, move it into assembly 2009-06-17 21:35:09 -07:00
xtensa Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/arnd/asm-generic 2009-06-12 18:15:51 -07:00
.gitignore
Kconfig mutex: have non-spinning mutexes on s390 by default 2009-04-09 19:28:24 +02:00