On 2024-07-16 12:13, Florian Weimer wrote:
* Soni L.:
quoted
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