Re: mprotect PROT_SIGNAL

["Soni \"They/Them\" L." <fakedme@gmail.com>]

On 2024-07-16 12:13, Florian Weimer wrote:
> * Soni L.:
>
> > we'd like an mprotect PROT_SIGNAL flag for a compiler we're making
> >
> > PROT_SIGNAL - marks the pages as unmapped while running signal handlers
> >
> > this would be very useful, do you think you could provide it?
> >
> > (not much more to say about it, it's supposed to do what it says on
> > the tin, we want to unmap pages in signal handlers so as to catch bugs
> > without causing memory corruption.)
>
> The challenge is going to be to detect abnormal exit from the signal
> handler.
>
> You can already get some of this behavior to today with memory
> protection keys, on x86-64 at least.  (I consider this a glitch in the
> implementation, it makes it less useful.)  Access is revoked
> automatically when the hander is invoked.  However, access is not
> restored if you jump out of the handler using longjmp or by throwing an
> exception.

As it should be - POSIX specifies that longjmping out of a handler 
remains in handler context, so you still can't call signal-unsafe functions.

Presumably, an API to manually map them back in would be necessary in 
the case of e.g. the JVM, which uses signal handlers for 
hardware-accelerated null checks.

> Thanks,
> Florian
>