On 04/03/2017 09:07, Ard Biesheuvel wrote:
> On 4 March 2017 at 00:24, Mason wrote:
>> On 03/03/2017 20:02, Robin Murphy wrote:
>>
>>> On 03/03/17 17:15, Mason wrote:
>>>
>>>> [ 1.261813] Unable to handle kernel paging request at virtual address d08611e4
>>>> [ 1.269167] pgd = c0004000
>>>> [ 1.271979] [d08611e4] *pgd=8f804811, *pte=00000000, *ppte=00000000
>>>> [ 1.278394] Internal error: Oops: 7 [#1] PREEMPT SMP ARM
>>>> [ 1.283815] Modules linked in:
>>>> [ 1.286970] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.9.7-1-rc2 #157
>>>> [ 1.293614] Hardware name: Sigma Tango DT
>>>> [ 1.297726] task: cf82c9c0 task.stack: cf838000
>>>> [ 1.302364] PC is at quirk_usb_early_handoff+0x3e8/0x790
>>>> [ 1.307790] LR is at ioremap_page_range+0xf8/0x1a8
>>>> [ 1.312688] pc : [<c039fe44>] lr : [<c02d0a10>] psr: 000e0013
>>>> [ 1.312688] sp : cf839d78 ip : 00000000 fp : cf839e38
>>>> [ 1.324399] r10: c10248a0 r9 : 00000000 r8 : d08611e4
>>>> [ 1.329733] r7 : d084e000 r6 : 00002000 r5 : 000c0300 r4 : cfb4e800
>>>> [ 1.336377] r3 : 000131e4 r2 : 00000000 r1 : 91001e13 r0 : d084e000
>>>
>>> ...and again. And always at the same PC, too.
>>
>> By the way, isn't LR supposed to point to the caller of the
>> current function? ("LR is at ioremap_page_range")
>>
>> If so, why does it not appear in the back trace?
>
> lr is supposed to point to the return address at function entry. After
> that, all bets are off, really, since ARM usually pops the return
> address from the stack straight into the pc register. So in this case,
> it looks like it still contains the address that the most recent leaf
> function returned to (or another function that actually restores the
> return address into lr before branching to it). But it could easily
> contain garbage as well.
If there is only a tiny chance that LR contains genuinely useful
information, then what is the rationale for providing the info
at all in the panic message?
I would argue that no info is better than info that is wrong
most of the time.
Regards.
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