Panic in quirk_usb_early_handoff

[Mason]

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.