Re: RFC: userspace exception fixups

[Rich Felker <dalias@libc.org>]

On Thu, Nov 01, 2018 at 07:09:17PM +0100, Florian Weimer wrote:
> * 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.

I think the right way to make it per-thread AND low-cost would be to
register not the handler, but the (per-thread) address of a
function-pointer object pointing to the handler. Then switching the
handler just requires a single volatile store to thread-local memory,
no syscall.

Rich