[PATCH] ARM: fix domain access

[Robert Jarzmik]

On the pxa310 platform, unaligned accesses are not fixed anymore with
the SW_DOMAIN_PAN configuration activated.

The trouble is that probe_kernel_address() cannot read the faulting
instruction, because of a permission error. The permission error comes
from the DACR register, whose value is incorrectly set to 0x55 instead
of 0x75.

I appears that at least gcc 4.8.2 optmizes the do_alignment() function,
and inside probe_kernel_address(), where set_fs() is called twice,
ie. modify_domain() is called twice, the get_domain() is called only
once, and the first call's value is reused. As a consequence, instead of
changing DACR as 0x51 -> 0x71 -> 0x75, it instead does 0x51 -> 0x71 ->
0x55. This is because instead of doing the last transition as :
 - (as (0x71 & ~0x03) | 0x04 = 0x75)
it does
 - (as (0x51 & ~0x03) | 0x04 = 0x55)

As a consequence, the alignment fault cannot be fixed, and triggers an
Oops:
Unable to handle kernel paging request at virtual address e1c20ec3
pgd = e1ce4000
[e1c20ec3] *pgd=c1c0044e(bad)
Internal error: Oops: 823 [#1] ARM
Modules linked in: unalign(+)
CPU: 0 PID: 610 Comm: insmod Not tainted 4.2.0-rc8-next-20150828-cm-x300+ #946
Hardware name: CM-X300 module
task: e1c69500 ti: e1cc0000 task.ti: e1cc0000
PC is at u_init+0x2c/0x40 [unalign]
LR is at u_init+0x14/0x40 [unalign]
pc : [<bf00202c>]    lr : [<bf002014>]    psr: a0000013
sp : e1cc1df0  ip : e1c20640  fp : 1e3dffdc
r10: 00000001  r9 : e1c20040  r8 : 00000000
r7 : bf002000  r6 : e1c5ee80  r5 : c0be5b80  r4 : c0be5b80
r3 : e1c20ec0  r2 : 00000004  r1 : a0000013  r0 : 00000000
Flags: NzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
Control: 0000397f  Table: c1ce4018  DAC: 00000051
Process insmod (pid: 610, stack limit = 0xe1cc0198)
Stack: (0xe1cc1df0 to 0xe1cc2000)

Signed-off-by: Robert Jarzmik <robert.jarzmik@xxxxxxx>
---
 arch/arm/include/asm/domain.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm/include/asm/domain.h b/arch/arm/include/asm/domain.h
index 10c9a38636ac..47f3b8445237 100644
--- a/arch/arm/include/asm/domain.h
+++ b/arch/arm/include/asm/domain.h
@@ -87,7 +87,7 @@ static inline unsigned int get_domain(void)
 {
 	unsigned int domain;
 
-	asm(
+	asm volatile(
 	"mrc	p15, 0, %0, c3, c0	@ get domain"
 	 : "=r" (domain));
 
-- 
2.1.4

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