Re: [PATCH v10 24/40] arm64/signal: Expose GCS state in signal frames

[Dave Martin]

On Thu, Aug 15, 2024 at 04:46:04PM +0100, Mark Brown wrote:
> On Thu, Aug 15, 2024 at 04:33:25PM +0100, Dave Martin wrote:
> > On Thu, Aug 15, 2024 at 04:05:32PM +0100, Mark Brown wrote:
> 
> > > The expectation (at least for arm64) is that the main program will only
> > > have shadow stacks if everything says it can support them.  If the
> > > dynamic linker turns them on during startup prior to parsing the main
> > > executables this means that it should turn them off before actually
> > > starting the executable, taking care to consider any locking of features.
> 
> > Hmm, so we really do get a clear "enable shadow stack" call to the
> > kernel, which we can reasonaly expect won't happen for ancient software?
> 
> Yes, userspace always has to explicitly enable the GCS.
> 
> > If so, I think dumping the GCS state in the sigframe could be made
> > conditional on that without problems (?)
> 
> It is - we only allocate the sigframe if the task has GCS enabled.

OK, makes sense.

> > > > Related question: does shadow stack work with ucontext-based coroutines?
> > > > Per-context stacks need to be allocated by the program for that.
> 
> > > Yes, ucontext based coroutines are the sort of thing I meant when I was
> > > talking about returning to a different context?  
> 
> > Ah, right.  Doing this asynchronously on the back of a signal (instead
> > of doing a sigreturn) is the bad thing.  setcontext() officially
> > doesn't work for this any more, and doing it by hacking or rebuilding
> > the sigframe is extremely hairy and probably a terrible idea for the
> > reasons I gave.
> 
> I see.  I tend to view this as more adventurous than I personally would
> be when writing userspace code but equally I don't see a need to
> actively break things.  There's no *requirement* to use libc...
> 
> > So, overall I think making ucontext coroutines with with GCS is purely
> > a libc matter that is "interesting" here, but we don't need to worry
> > about.
> 
> Yes, it's not our problem so long as we don't get in the way somehow.

Sure.  "Hairy and probably a terrible idea" is not the same as
"impossible", but you need to know what you're doing and you get
exposed to all sorts of portability challenges.

There's a limit to how much we should attempt to smooth over all that.

Anyway, I think what the GCS patches are doing looks reasonable.

Cheers
---Dave