On Thu, Jun 23, 2016 at 1:40 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > On Thu, Jun 23, 2016 at 01:31:32PM -0500, Josh Poimboeuf wrote: >> On Thu, Jun 23, 2016 at 09:35:29AM -0700, Andy Lutomirski wrote: >> > > So which is the least-bad option? To summarize: >> > > >> > > 1) task flag(s) for preemption and page faults >> > > >> > > 2) turn pt_regs into a stack frame >> > > >> > > 3) annotate all calls from entry code in a table >> > > >> > > 4) encode rbp on entry >> > > >> > > They all have their issues, though I'm partial to #2. >> > > >> > > Any more hare-brained ideas? :-) >> > >> > I'll try to take a closer look at #2 and see just how much I dislike >> > all the stack frame munging. >> >> Ok. >> >> > Also, in principle, it's only the >> > sleeping calls and the calls that make it into real (non-entry) kernel >> > code that really want to be unwindable through this mechanism. >> >> Yeah, that's true. We could modify options 2 or 3 to be less absolute. >> Though I think that makes them more prone to future breakage. >> >> > FWIW, I don't care that much about preserving gdb's partial ability to >> > unwind through pt_regs, especially because gdb really ought to be able >> > to use DWARF, too. >> >> Hm, that's a good point. I really don't know if there are any other >> external tools out there that would care. Maybe we could try option 4 >> and then see if anybody complains. > > I'm starting to think hare-brained option 4 is the way to go. Any > external tooling should really be relying on DWARF anyway. > > Here's a sneak preview. If this general approach looks ok to you, I'll > go ahead and port all the in-tree unwinders and post a proper patch. > > Instead of using xor -1 on the pt_regs pointer, I just cleared the > high-order bit. That makes the unwinding experience much more pleasant > for a human stack walker, and also ensures that anybody trying to > dereference it gets slapped with an oops, at least in the 48-bit address > space era. > > diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h > index 9a9e588..bf397426 100644 > --- a/arch/x86/entry/calling.h > +++ b/arch/x86/entry/calling.h > @@ -201,6 +201,23 @@ For 32-bit we have the following conventions - kernel is built with > .byte 0xf1 > .endm > > + /* > + * This is a sneaky trick to help the unwinder find pt_regs on the > + * stack. The frame pointer is replaced with an encoded pointer to > + * pt_regs. The encoding is just a clearing of the highest-order bit, > + * which makes it an invalid address and is also a signal to the > + * unwinder that it's a pt_regs pointer in disguise. > + * > + * NOTE: This must be called *after* SAVE_EXTRA_REGS because it > + * corrupts rbp. > + */ > + .macro ENCODE_FRAME_POINTER ptregs_offset=0 > +#ifdef CONFIG_FRAME_POINTER > + leaq \ptregs_offset(%rsp), %rbp > + btr $63, %rbp > +#endif > + .endm > + Maybe optimize slightly: .ifeq \ptregs_offset mov %rsp, %rbp .else leaq \ptregs_offset(%rsp), %rbp .endif -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
