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

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Hi Michael,

On Tue, Mar 7, 2023 at 1:28 AM Michael Schmitz <schmitzmic@xxxxxxxxx> wrote:
On 7/03/23 02:03, Geert Uytterhoeven wrote:
On Wed, Mar 1, 2023 at 3:13 AM Michael Schmitz <schmitzmic@xxxxxxxxx> 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@xxxxxxxxx>
Reported-by: Eero Tamminen <oak@xxxxxxxxxxxxxx>
CC: Eero Tamminen <oak@xxxxxxxxxxxxxx>
CC: Finn Thain <fthain@xxxxxxxxxxxxxx>
CC: Andreas Schwab <schwab@xxxxxxxxxxxxxx>
CC: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
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@xxxxxxxxx
--

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@xxxxxxxxxxxxxx>
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@xxxxxxxxxxxxxx

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



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