On Tue, Aug 08, 2017 at 12:03:51PM -0700, Linus Torvalds wrote: > On Tue, Aug 8, 2017 at 11:58 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > > > > Take for example the lock_is_held_type() function. In vmlinux, it has > > the following instruction: > > > > callq *0xffffffff85a94880 (pv_irq_ops.save_fl) > > > > At runtime, that instruction is patched and replaced with a fast inline > > version of arch_local_save_flags() which eliminates the call: > > > > pushfq > > pop %rax > > > > The problem is when an interrupt hits after the push: > > > > pushfq > > --- irq --- > > pop %rax > > That should actually be something easily fixable, for an odd reason: > the instruction boundaries are different. > > > I'm not sure what the solution should be. It will probably need to be > > one of the following: > > > > a) either don't allow runtime "alternative" patches to mess with the > > stack pointer (objtool could enforce this); or > > > > b) come up with some way to register such patches with the ORC > > unwinder at runtime. > > c) just add ORC data for the alternative statically and _unconditionally_. > > No runtime registration. Just an unconditional entry for the > particular IP that comes after the "pushfq". It cannot match the > "callq" instruction, since it would be in the middle of that > instruction. > > Basically, just do a "union" of the ORC data for all the alternatives. > > Now, objtool should still verify that the instruction pointers for > alternatives are unique - or that they share the same ORC unwinder > information if they are not. > > But in cases like this, when the instruction boundaires are different, > things should "just work", with no need for any special cases. > > Hmm? Yeah, that might work. Objtool already knows about alternatives, so it might not be too hard. I'll try it. And it can spit out a warning if we get two different ORC states for the same address after doing the "union". Then I guess we'd have to rearrange things or sprinkle some nops to work around it. -- Josh -- 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
