[Crash-utility] kernel module parsing failure - mips

[Sagar Borikar]

On Sat, Dec 3, 2016 at 1:59 AM, Rabin Vincent <rabin at rab.in> wrote:
> On Fri, Dec 02, 2016 at 04:06:20PM -0800, Sagar Borikar wrote:
>> With 7.1.7, crash is working for MIPS when all drivers are embedded
>> inside kernel.
>> When I make the driver loadable and panic the kernel, crash doesn't
>> locate some symbols correctly.
>>
>> please wait... (gathering module symbol data)
>> crash: invalid size request: 0  type: "pgd page"
>>
>> debugged further and find  that PGD_ORDER provides incorrect number
>> due to which the PGD_SIZE macro results in 0.
>>
>> Just for fun, I replaced PGD_ORDER with 0(I know its incorrect) and it
>> went ahead but couldn't run "mod" command successfully as it threw
>> following error
>>
>> crash> mod
>> mod: cannot access vmalloc'd module memory
>
> In order to access vmalloc'd memory we need interpret the page tables
> correctly.  This isn't needed when the modules are built in since then
> the memory will be in the direct-mapped kseg0 segment.  So the "mod"
> failure is just a consequence of replacing PGD_ORDER with 0.

Yes, mod will not work as I said earlier. Crash was exiting hence I
wanted to have quick workaround.

> So the first error should be fixed properly before attempting "mod".  What
> kernel version is this and what page size do to use?

4.4.20 kernel and page size is 16K

>
> Try running the "help -m" and "mach" commands (you can skip module loading with
> --no_modules to get to the crash> prompt) and check if the values for the
> various page table sizes and bits match what your kernel is using.
>
>  crash> help -m
>  ...
>            pagesize: 4096
>           pageshift: 12
>            pagemask: fffffffffffff000
>          pageoffset: fff
>         pgdir_shift: 22
>        ptrs_per_pgd: 1024
>        ptrs_per_pte: 1024
>  ...
>
>
>  crash-mips> mach
>            PAGE SIZE: 4096
>
>        _PAGE_PRESENT: 00000001
>           _PAGE_READ: 00000002
>          _PAGE_WRITE: 00000004
>       _PAGE_ACCESSED: 00000008
>       _PAGE_MODIFIED: 00000010
>         _PAGE_GLOBAL: 00000020
>          _PAGE_VALID: 00000040
>          _PAGE_DIRTY: 00000080
This appears to be correct info for my platform.
Thanks
Sagar

>
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