On Thu, Jan 31, 2019 at 06:53:06PM +0000, James Morse wrote: > Hey Christoffer, > > On 31/01/2019 08:08, Christoffer Dall wrote: > > On Thu, Jan 24, 2019 at 04:32:54PM +0000, James Morse wrote: > >> On systems with VHE the kernel and KVM's world-switch code run at the > >> same exception level. Code that is only used on a VHE system does not > >> need to be annotated as __hyp_text as it can reside anywhere in the > >> kernel text. > >> > >> __hyp_text was also used to prevent kprobes from patching breakpoint > >> instructions into this region, as this code runs at a different > >> exception level. While this is no longer true with VHE, KVM still > >> switches VBAR_EL1, meaning a kprobe's breakpoint executed in the > >> world-switch code will cause a hyp-panic. > > > > Forgive potentially very stupid questions here, but: > > > > (1) Would it make sense to move the save/restore VBAR_EL1 to the last > > possible moment, and would that actually allow kprobes to work for > > the world-switch code, or does that just result in other weird > > problems? > > This would work for taking the debug exception. But next kprobes wants to > single-step the probed instruction in an out-of-line slot. I don't think we > can > do this if we've already configured the debug hardware for the guest. > (If could at least turn single-step off when we return to guest-EL0, which > guest-EL1 was single-stepping) > >
I suspected something like that, let's not go there. > > (2) Are we sure that this catches every call path of every non-inlined > > function called after switchign VBAR_EL1? Can kprobes only be > > called on exported symbols, or can you (if you know the address > > somehow) put a kprobe on a static function as well. If there are > > any concerns in this area, we might want to consider (1) more > > closely. > > Hmmm, good question. The blacklisting applies to whole symbols as seen by > kallsyms, the compiler has no idea what is going on. > > If it chose not to inline something, it would be kprobe'able yes. > > __kprobes uses a section function-attribute instead. The gcc manual[0] doesn't > say what happens when inline and the section attributes are used together. (or > at least I couldn't find it) > > A quick experiment with gcc 8.2.0 shows adding __kprobes on the inlines gets > discarded when they are inlined. I'm not sure how to trick the compiler into > not-inlining it to see what happens, but adding 'noinline' to the header file > causes it to duplicate the function everywhere, but puts it in the __kprobes > section. > > (For KVM we could use the 'flatten' attribute, but that does say 'if > possible'. > Alternatively we can decorate all the inline helpers we know we use with > __kprobes as a safety net.) > > I think this is a wider problem with kprobes. > Sounds like it. Probably in the "you did something crazy, and your kernel is going to suffer from it" category. Let's stick to your approach. Thanks for the explanation. Christoffer _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm