Re: signal delivery, was Re: reliable reproducer

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.

.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?