On Sun, 30 Apr 2023, Michael Schmitz wrote:
Am 29.04.2023 um 21:49 schrieb Finn Thain:
That seems over-complicated to me... I must be missing something.
If signals don't nest, a page fault on the signal stack should never
produce a new signal frame there, so this bug should never show up there.
I think that was the point I'd missed, because it means the signal stack
may be exempted here. But is the extra complexity worth it?
What I had in mind was something like this (untested):
unsigned long usp;
if (CPU_IS_020_OR_030 && tregs->format == 0xb)
/* USP is unreliable so use the worst-case value. */
usp = sigsp(rdusp() - 256, ksig);
else
usp = sigsp(rdusp(), ksig);
return (void __user *)((usp - frame_size) & -8UL);
sigsp() may return an address from the alternate signal stack. In that
case, no adjustment to the signal stack address is advised.
...
It'd be ill advised as it would reduce data locality when it expanded the
signal stack across a cache line or page boundary, and there may be a
performance penalty for that. However, the signal stack seems to be prone
to poor data locality anyway, compared to the normal user stack. So I
wonder if this penalty could be measured. It seems like a premature
optimization to me. Unfortunately, I don't think we get to measure cache
misses on m68k.
