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
