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Re: SafeStack proposal in GCC


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).

> 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);
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).


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