On Sat, Oct 19, 2019 at 01:01:35PM +0200, Torsten Duwe wrote: > Hi Mark! Hi Torsten! > On Fri, 18 Oct 2019 18:41:02 +0100 Mark Rutland > <mark.rutland@xxxxxxx> wrote: > > > In the process of reworking this I spotted some issues that will get > > in the way of livepatching. Notably: > > > > * When modules can be loaded far away from the kernel, we'll > > potentially need a PLT for each function within a module, if each can > > be patched to a unique function. Currently we have a fixed number, > > which is only sufficient for the two ftrace entry trampolines. > > > > IIUC, the new code being patched in is itself a module, in which > > case we'd need a PLT for each function in the main kernel image. > > When no live patching is involved, obviously all cases need to have > been handled so far. And when a live patching module comes in, there > are calls in and out of the new patch code: > > Calls going into the live patch are not aware of this. They are caught > by an active ftrace intercept, and the actual call into the LP module > is done in klp_arch_set_pc, by manipulating the intercept (call site) > return address (in case thread lives in the "new world", for > completeness' sake). This is an unsigned long write in C. I was under the impression that (at some point) the patch site would be patched to call the LP code directly. From the above I understand that's not the case, and it will always be directed via the regular ftrace entry code -- have I got that right? Assuming that is the case, that sounds fine to me, and sorry for the noise. > All calls going _out_ from the KLP module are newly generated, as part > of the KLP module building process, and are thus aware of them being > "extern" -- a PLT entry should be generated and accounted for in the > KLP module. Yup; understood. > > We have a few options here, e.g. changing which memory size model we > > use, or reserving space for a PLT before each function using > > -f patchable-function-entry=N,M. > > Nonetheless I'm happy I once added the ,M option here. You never know :) Yup; we may have other reasons to need this in future (and I see parisc uses this today). > > * There are windows where backtracing will miss the callsite's caller, > > as its address is not live in the LR or existing chain of frame > > records. Thus we cannot claim to have a reliable stacktrace. > > > > I suspect we'll have to teach the stacktrace code to handle this as > > a special-case. > > Yes, that's where I had to step back. The unwinder needs to stop where > the chain is even questionable. In _all_ cases. Missing only one race > condition means a lurking inconsistency. Sure. I'm calling this out now so that we don't miss this in future. I've added comments to the ftrace entry asm to this effect for now. > OTOH it's not a problem to report "not reliable" when in doubt; the > thread in question will then get woken up and unwind itself. > It is only an optimisation to let all kernel threads which are > guaranteed to not contain any patched functions sleep on. I just want to make it clear that some care will be needed if/when adding CONFIG_HAVE_RELIABLE_STACKTRACE so that we handle this case correctly. > > I'll try to write these up, as similar probably applies to other > > architectures with a link register. > > I thought I'd quickly give you my feedback upfront here. Thanks; it's much appreciated! Mark.
