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

Geert Uytterhoeven
2023-03-06T13:04:12Z

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.

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds