On Oct 7, 2014 9:09 AM, "David Daney" <ddaney@xxxxxxxxxxxxxxxxxx> wrote: > > On 10/07/2014 04:11 AM, Rich Felker wrote: >> >> On Mon, Oct 06, 2014 at 09:50:47PM -0700, David Daney wrote: >>>>> >>>>> the out-of-line execution trick, but do it somewhere other than in >>>>> stack memory. >>>> >>>> How do you answer Andy Lutomirski's question about what happens when a >>>> signal handler interrupts execution while the program counter is >>>> pointing at this "out-of-line execution" trampoline? This seems like a >>>> show-stopper for using anything other than the stack. >>> >>> It would be nice to support, but not doing so would not be a >>> regression from current behavior. >> >> >> It's not just "nice" to support, it's mandatory. Otherwise you will >> execute essentially *random instructions* in this case, providing a >> very nice attack vector that can almost certainly be elevated to >> arbitrary code execution via timing of signals during floating point >> code. >> >> The current behavior in regards to this is correct: because you have a >> *stack*, each trampoline is pushed onto the stack in its own context, >> and popped when it's no longer needed. You can have arbitrarily many >> such trampolines up to the stack size. Note that each nested signal >> handler already requires sizeof(ucontext_t) in stack space, so these >> trampolines are a negligible additional cost without major effects on >> the number of signal handlers you can nest without overflowing the >> stack. > > > Yes, the stack takes care of the allocations, but the current implementation has many problems: > > 1) Signals clobber the emulation area. > 2) Signals caused by the emulation, have incorrect saved machine state. > > We have a low bar to pass, any new solution doesn't have to be perfect, it only has to be an improvement. > > Keep in mind that we are not starting from a clean slate, there are many years of legacy code that has built up here. A lesson I learned when doing the x86 vsyscall stuff: Don't waste time improving legacy crap without a really good reason. Especially don't extend the interface. Deprecate it (without breaking working user code) and move on. --Andy > > David Daney
