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
