Re: RFC: userspace exception fixups

[Florian Weimer <fweimer@redhat.com>]

* Andy Lutomirski:

> The basic idea would be to allow libc, or maybe even any library, to
> register a handler that gets a chance to act on an exception caused by
> a user instruction before a signal is delivered.  As a straw-man
> example for how this could work, there could be a new syscall:
>
> long register_exception_handler(void (*handler)(int, siginfo_t *, void *));
>
> If a handler is registered, then, if a synchronous exception happens
> (page fault, etc), the kernel would set up an exception frame as usual
> but, rather than checking for signal handlers, it would just call the
> registered handler.  That handler is expected to either handle the
> exception entirely on its own or to call one of two new syscalls to
> ask for normal signal delivery or to ask to retry the faulting
> instruction.

Would the exception handler be a per-thread resource?

If it is: Would the setup and teardown overhead be prohibitive for many
use cases (at least those do not expect a fault)?

Something peripherally related to this interface: Wrappers for signal
handlers (and not just CPU exceptions).  Ideally, we want to maintain a
flag that indicates whether we are in a signal handler, and save and
restore errno around the installed handler.

> Alternatively, we could do something a lot more like the kernel's
> internal fixups where there's a table in user memory that maps
> potentially faulting instructions to landing pads that handle
> exceptions.

GCC already supports that on most Linux targets.  You can unwind from
synchronously invoked signal handlers if you compile with
-fnon-call-exceptions.

However, it's tough to set up a temporary signal handler to trigger such
unwinding because those aren't per-thread.

> On Windows, you can use SEH to do crazy things like running
> known-buggy code and eating the page faults.  I don't think we want to
> go there.

The original SEH was also a rich target for exploiting vulnerabilities.
That's something we really should avoid as well.

I wonder if it would be possible to tack this function onto rseq.

Thanks,
Florian