Re: [PATCH v2] m68k: kernel/traps.c - only force 030 bus error if PC not in exception table

Michael Schmitz
2023-03-07T00:28:21Z

Hi Geert,

On 7/03/23 02:03, Geert Uytterhoeven wrote:
> On Wed, Mar 1, 2023 at 3:13 AM Michael Schmitz <schmitzmic@gmail.com> wrote:
>> __get_kernel_nofault() does copy data in supervisor mode when
>> forcing a task backtrace log through /proc/sysrq_trigger.
>> This is expected cause a bus error exception on e.g. NULL
>> pointer dereferencing when logging a kernel task has no
>> workqueue associated. This bus error ought to be ignored.
>>
>> Our 030 bus error handler is ill equipped to deal with this:
>>
>> Whenever ssw indicates a kernel mode access on a data fault,
>> we don't even attempt to handle the fault and instead always
>> send a SEGV signal (or panic). As a result, the check
>> for exception handling at the fault PC (buried in
>> send_sig_fault() which gets called from do_page_fault()
>> eventually) is never used.
>>
>> In contrast, both 040 and 060 access error handlers do not
>> care whether a fault happened on supervisor mode access,
>> and will call do_page_fault() on those, ultimately honoring
>> the exception table.
>>
>> Add a check in bus_error030 to call do_page_fault() in case
>> we do have an entry for the fault PC in our exception table.
>>
>> I had attempted a fix for this earlier in 2019 that did rely
>> on testing pagefault_disabled() (see link below) to achieve
>> the same thing, but this patch should be more generic.
>>
>> Tested on 030 Atari Falcon.
>>
>> Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
>> Reported-by: Eero Tamminen <oak@helsinkinet.fi>
>> CC: Eero Tamminen <oak@helsinkinet.fi>
>> CC: Finn Thain <fthain@linux-m68k.org>
>> CC: Andreas Schwab <schwab@linux-m68k.org>
>> CC: Geert Uytterhoeven <geert@linux-m68k.org>
>> Link: https://lore.kernel.org/r/alpine.LNX.2.21.1904091023540.25@nippy.intranet
>> Link: https://lore.kernel.org/r/63130691-1984-c423-c1f2-73bfd8d3dcd3@gmail.com
>> --
>>
>> Changes from v1:
>>
>> - add comment
>> - reword commit message
>> - add link to old patch as well (stick to lore.kernel.org
>> even though these links are currently not functional)
> Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
> i.e. will queue as a fix in the m68k for-v6.3 branch.

Thanks - I neglected to search far enough back to find where the context
of this diff originates, but upon a closer look, it appears to be the
well-known commit 1da177e4c3f4 (start of git history). Do you think this
is worth a backport to stable?

Cheers,

Michael

> Gr{oetje,eeting}s,
>
> Geert
>