Re: [PATCH] ARM: fix alignement of __bug_table section entries

[Robert Jarzmik]

Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> writes:

> On Tue, Sep 08, 2015 at 07:01:00PM +0200, Robert Jarzmik wrote:
>> Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> 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
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