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

Geert Uytterhoeven
2023-03-07T07:44:44Z

Hi Michael,

On Tue, Mar 7, 2023 at 1:28 AM Michael Schmitz <schmitzmic@gmail.com> wrote:
> 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?

I guess it will be auto-backported to stable once it has reached
upstream, together with the two other fixes.

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