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Re: SafeStack proposal in GCC

On Mon, May 09, 2016 at 10:03:02PM +0200, Michael Matz wrote:
> Hi,
> On Mon, 9 May 2016, Rich Felker wrote:
> > > > The *context APIs are deprecated and I'm not sure they're worth 
> > > > supporting with this. It would be a good excuse to get people to 
> > > > stop using them.
> > > 
> > > How?  POSIX decided to remove the facilities without any adequate 
> > > replacement (thread aren't).
> > 
> > Threads work just as well as the ucontext api for coroutines. Due to the 
> > requirement to save/restore signal masks, the latter requires a syscall, 
> > making it no faster than a voluntary context switch via futex syscall.
> Uhm, no.  If you disregard efficiency, sure, POSIX threads are sometimes a 
> replacement on some platforms.  They still have completely different 
> activation models (being synchronous with *context, for which you need 
> even further slow synchronization in a threading model).

	while (sem_wait(self->sem));

It can actually be done more idiomatically with cond vars, but I don't
see a way to make it as efficient.

> > Most of the other hacks people used the ucontext API for were complete 
> > hacks with undefined behavior, anyway.
> Sure, that doesn't imply the facility should be removed.  I can misuse all 
> kinds of stuff.


> > BTW it's not even possible to implement makecontext on most targets due 
> > to the wacky variadic calling convention it uses -- in most ABIs, 
> > there's simply no way to shift the variadic args into the right slots 
> > for calling the start function for the new context without knowing their 
> > types, and the implementation has no way to know the types. So it's 
> > really an unusably broken API.
> Of course.  But _that_ implies that a workable replacement should have 
> been put in place, not the unrealistic stance POSIX took with the removal:
>   makecontext2(ucontext_t *ucp, void (*func)(void*), void* cookie);

It could have been done even more simply, without a new function, by
just saying the behavior is undefined unless func has type
void(*)(void), argc==1, and the first variadic arg has type void*.

> Done.  I never understood why they left in the hugely 
> unuseful {sig,}{set,long}jmp() but removed the actually useful *context()
> (amended somehow like above).

Because those are actually part of the C language (the non-sig
versions, but the sig versions are needed to work around broken unices
that made the non-sig versions save/restore signal mask and thus too
slow to ever use). They're also much more useful for actually
reasonable code (non-local exit across functions that were badly
designed with no error paths) as opposed to just nasty hacks that are
mostly/entirely UB anyway (coroutines, etc.).


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