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

Robert Jarzmik
2015-09-10T19:06:38Z

Robert Jarzmik <robert.jarzmik@free.fr> writes:

> 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()
And I have the proof of gcc optimization, which I'll add to the commit message
if you want :
00000728 <do_alignment>:
...
770: ee134f10 mrc 15, 0, r4, cr3, cr0, {0}
... no r4 or mrc/mcr usage
788: e3842030 orr r2, r4, #48 ; 0x30
... no r2/r4 or mrc/mcr usage
794: ee032f10 mcr 15, 0, r2, cr3, cr0, {0}
798: ee07cf95 mcr 15, 0, ip, cr7, cr5, {4}
... no r4 or mrc/mcr usage
7ac: e3c4300c bic r3, r4, #12
7b0: e3833004 orr r3, r3, #4
7b4: ee033f10 mcr 15, 0, r3, cr3, cr0, {0}
... no mrc/mcr usage
7cc: ebfffffe bl 0 <arm_copy_from_user>

Here, we have in probe_kernel_address() in do_alignment():
- @770 : r4 = DACR
- @794 : DACR = r4 | 0x30
- @7b4 : DACR = (r4 & 0x0c) | 0x04 => the 0x30 is lost !!!

I'll send my patch to the mailing list tomorrow, as well as the other one to
align the __bug_table session.

Cheers.

--
Robert