On Tue, Feb 07, 2017 at 11:10:12AM +0000, Mark Rutland wrote: > On Tue, Feb 07, 2017 at 09:34:05AM +0100, Peter Zijlstra wrote: > > On Mon, Feb 06, 2017 at 08:54:38AM -0800, Kees Cook wrote: > > > > > > > > Like I wrote, ideally we'd end up using something like the x86 exception > > > > table with a custom handler. Just no idea how to pull that off without > > > > doing a full blown arch specific implementation, so I didn't go there > > > > quite yet. > > > > > > I haven't spent much time looking at the extable stuff. (Though > > > coincidentally, I was poking at it for x86's test_nx stuff...) I > > > thought there was a way to build arch-agnostic extables already? > > > kernel/extable.c is unconditionally built-in, for example. > > > > That doesn't seem to be of much use. It only contains section sort and > > search functions. > > > > Another problem for generic code would be to figure out what register > > the relevant variable would live in at the time of exception. Here its > > 'obviously' EAX because that's what cmpxchg requires, but in generic > > you'd need a means of querying GCC's register allocator at the exception > > point and somehow using that information for the generation of the > > exception handler. > > I think we only need two arch-specific primitives: > (a) mangle a GCC assigned register into an idx stored in the extable > (b) take said index, and grab the relevant register from pt_regs > > Then you can have a BUG_VALUE(v, ...), where we use an input "r" (val), > and mangle that into the idx in the extable. In the common case, I'd > hope GCC would leave the register in-place from the cmpxchg. > > ... or have I misundertood? :)
Right something along those lines. (a) will need GCC help, and (b) would be kernel-arch specific. So this isn't something we can quickly do.
