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SubjectRe: [PATCH] ARM: fix alignement of __bug_table section entries
Date
Russell King - ARM Linux <linux@arm.linux.org.uk> writes:

> On Tue, Sep 08, 2015 at 07:01:00PM +0200, Robert Jarzmik wrote:
>> Russell King - ARM Linux <linux@arm.linux.org.uk> writes:
> At the point we call into this code, the DACR should be 0x75, which
> should allow us to read the instruction at 0xbf00202c. But this is
> failing with a permission error - which it would do if it thought
> the kernel domain was in manager mode (iow, 0x55).

Okay Russell, I have a good idea what's happening now. Basically, it boils down
to compiler optimization of get_domain() which is called twice (set_fs() ->
modify_domain() -> get_domain()). See the piece in [1] for a more complete
explanation.

I still haven't finished my work, as I need to disassemble the do_alignment()
function to confirm the DACR read by get_domain() is only done once in the
probe_kernel_address() call, and yet my hopes are high this is the cause as I
traced the DACR modifications, which led me to :
[-0] 0xc0017080: set_domain(0x00000055) => dacr = 0x00000055 => second set_domain()
[-1] 0xc0017080: set_domain(0x00000071) => dacr = 0x00000071 => first set_domain()
[-2] 0xc008a124: set_domain(0x00000051) => dacr = 0x00000051

Once I have my disassembly properly analyzed (ie. the second set_fs() doesn't do
a mrc instruction), I'll have my proof. My setup is too full of traces and
attempts to stall pipeline/prefetch to conclude yet, but there is a fair chance
I'm closer to the solution now.

Cheers.

--
Robert

[1] Current patch
=================
---8<---
From 07cdda877b1b0c4fcdba3d756f6a22ce035ee672 Mon Sep 17 00:00:00 2001
From: Robert Jarzmik <robert.jarzmik@free.fr>
Date: Thu, 10 Sep 2015 00:32:26 +0200
Subject: [PATCH] ARM: fix domain access

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@free.fr>
---
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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