On Wed, Jun 12, 2019 at 10:54:23AM +0200, Peter Zijlstra wrote: > On Tue, Jun 11, 2019 at 10:05:01PM -0500, Josh Poimboeuf wrote: > > On Fri, May 24, 2019 at 10:53:19AM +0200, Peter Zijlstra wrote: > > > > For ORC, I'm thinking we may be able to just require that all generated > > > > code (BPF and others) always use frame pointers. Then when ORC doesn't > > > > recognize a code address, it could try using the frame pointer as a > > > > fallback. > > > > > > Yes, this seems like a sensible approach. We'd also have to audit the > > > ftrace and kprobe trampolines, IIRC they only do framepointer setup for > > > CONFIG_FRAME_POINTER currently, which should be easy to fix (after the > > > patches I have to fix the FP generation in the first place: > > > > > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/log/?h=x86/wip > > > > Right now, ftrace has a special hook in the ORC unwinder > > (orc_ftrace_find). It would be great if we could get rid of that in > > favor of the "always use frame pointers" approach. I'll hold off on > > doing the kpatch/kprobe trampoline conversions in my patches since it > > would conflict with yours. > > > > Though, hm, because of pt_regs I guess ORC would need to be able to > > decode an encoded frame pointer? I was hoping we could leave those > > encoded frame pointers behind in CONFIG_FRAME_POINTER-land forever... > > Ah, I see.. could a similar approach work for the kprobe trampolines > perhaps?
If you mean requiring that kprobes trampolines always use frame pointers, I think it should work. > > Here are my latest BPF unwinder patches in case anybody wants a sneak > > peek: > > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/log/?h=bpf-orc-fix > > On a quick read-through, that looks good to me. A minor nit: > > /* mov dst_reg, %r11 */ > EMIT_mov(dst_reg, AUX_REG); > > The disparity between %r11 and AUX_REG is jarring. I understand the > whole bpf register mapping thing, but it is just weird when reading > this. True, but there are several cases where the r11 is hard-coded in the instruction encoding itself, like: /* mov imm32, %r11 */ EMIT3_off32(0x49, 0xC7, 0xC3, imm32); If the code were more decoupled, like if it had helpers where you could always just pass AUX_REG, and the code never had to know what the value of AUX_REG is, then using "AUX_REG" in the comments would make sense. But since there are inconsistencies, with hard-coded register mapping knowledge in many places, I find it easier to follow what's going on when the specific register name is always shown in the comments. > Other than that, the same note as before, the 32bit JIT still seems > buggered, but I'm not sure you (or anybody else) cares enough about that > to fix it though. It seems to use ebp as its own frame pointer, which > completely defeats an unwinder. I'm still trying to decide if I care about 32-bit. It does indeed use ebp everywhere. But I'm not sure if I want to poke the beehive... Also factoring into the equation is the fact that I'll be on PTO next week :-) If I have time in the next couple days then I may take a look. -- Josh
