On Mon, Apr 24, 2023 at 02:55:11PM -0400, Joel Fernandes wrote:
> This is amazing debugging Boqun, like a boss! One comment below:
>
> > > > Or something simple I haven't thought of? :)
> > >
> > > At what points can r13 change? Only when some particular functions are
> > > called?
> > >
> >
> > r13 is the local paca:
> >
> > register struct paca_struct *local_paca asm("r13");
> >
> > , which is a pointer to percpu data.
> >
> > So if a task schedule from one CPU to anotehr CPU, the value gets
> > changed.
>
> It appears the whole issue, per your analysis, is that the stack
> checking code in gcc should not cache or alias r13, and must read its
> most up-to-date value during stack checking, as its value may have
> changed during a migration to a new CPU.
>
> Did I get that right?
>
> IMO, even without a reproducer, gcc on PPC should just not do that,
> that feels terribly broken for the kernel. I wonder what clang does,
> I'll go poke around with compilerexplorer after lunch.
>
> Adding +Peter Zijlstra as well to join the party as I have a feeling
> he'll be interested. ;-)
I'm a little confused; the way I understand the whole stack protector
thing to work is that we push a canary on the stack at call and on
return check it is still valid. Since in general tasks randomly migrate,
the per-cpu validation canary should be the same on all CPUs.
Additionally, the 'new' __srcu_read_{,un}lock_nmisafe() functions use
raw_cpu_ptr() to get 'a' percpu sdp, preferably that of the local cpu,
but no guarantees.
Both cases use r13 (paca) in a racy manner, and in both cases it should
be safe.
