Re: signal delivery, was Re: reliable reproducer

[Michael Schmitz <schmitzmic@gmail.com>]

Hi Finn,

Am 26.04.2023 um 15:27 schrieb Finn Thain:
> On Wed, 26 Apr 2023, Michael Schmitz wrote:
>
>> On 26/04/23 07:46, Michael Schmitz wrote:
>>>
>>> I had thought the 030 could resume the interrupted instruction using
>>> the information from the exception frame - and that does appear to
>>> work in all other cases except where signal delivery gets in the way,
>>> and it also works if moving the exception frame a little bit further
>>> down the stack. So our treatment of the bus error exception frame
>>> during signal delivery appears to be incorrect. Wouldn't you agree?
>>
>> Inspection of the format b frame placed in the signal frame in both rt
>> and non-rt cases (at the time the signal handler runs) shows the
>> expected contents in the data output buffer, data fault address and ssw.
>> At that time, returning to user space with rte would correctly resume
>> the instruction execution.  I had previously confirmed that the register
>> contents saved in the rt signal frame is correct also.
>>
>> That is with a kernel patched similar to above patch by Finn (using an
>> offset of 128 or 64 instead of 256).
>
> That means things go awry during sys_sigreturn or sys_rt_sigreturn. I'm
> not sure what happens to the exception frame:
>
> 1:
>         | stack contents now:
>         |   [original pt_regs address] [original switch_stack address]
>         |   [unused part of the gap] [moved switch_stack] [moved pt_regs]
>         |   [replacement exception frame]
>         | return value of do_{rt_,}sigreturn() points to moved switch_stack.
>
>         movel   %d0,%sp                         | discard the leftover junk
>         RESTORE_SWITCH_STACK
>         | stack contents now is just [syscall return address] [pt_regs] [frame]
>         | return pt_regs.d0
>         movel   %sp@(PT_OFF_D0+4),%d0
>         rts
>
> ... but I noticed that the sys_rt_sigreturn entry point does
> SAVE_SWITCH_STACK while the sys_sigreturn entry point does not. Yet both
> jump to label 1 above, so both syscalls do RESTORE_SWITCH_STACK. Hmmm.

In my copy of entry.S, sys_sigreturn has SAVE_SWITCH_STACK, just as 
sys_rt_sigreturn does.

Cheers,

	Michael


> .macro SAVE_SWITCH_STACK
>         moveml  %a3-%a6/%d6-%d7,%sp@-
> .endm
>
> .macro RESTORE_SWITCH_STACK
>         moveml  %sp@+,%a3-%a6/%d6-%d7
> .endm
>
> Well, that has to corrupt %a3, right?
>