On Thu, May 23, 2019 at 11:24 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > > On Thu, May 23, 2019 at 10:50:24PM +0800, Kairui Song wrote: > > > > Hi Josh, this still won't fix the problem. > > > > > > > > Problem is not (or not only) with ___bpf_prog_run, what actually went > > > > wrong is with the JITed bpf code. > > > > > > There seem to be a bunch of issues. My patch at least fixes the failing > > > selftest reported by Alexei for ORC. > > > > > > How can I recreate your issue? > > > > Hmm, I used bcc's example to attach bpf to trace point, and with that > > fix stack trace is still invalid. > > > > CMD I used with bcc: > > python3 ./tools/stackcount.py t:sched:sched_fork > > I've had problems in the past getting bcc to build, so I was hoping it > was reproducible with a standalone selftest. > > > And I just had another try applying your patch, self test is also failing. > > Is it the same selftest reported by Alexei? > > test_stacktrace_map:FAIL:compare_map_keys stackid_hmap vs. stackmap err -1 errno 2 > > > I'm applying on my local master branch, a few days older than > > upstream, I can update and try again, am I missing anything? > > The above patch had some issues, so with some configs you might see an > objtool warning for ___bpf_prog_run(), in which case the patch doesn't > fix the test_stacktrace_map selftest. > > Here's the latest version which should fix it in all cases (based on > tip/master): > > https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/commit/?h=bpf-orc-fix Hmm, I still get the failure: test_stacktrace_map:FAIL:compare_map_keys stackid_hmap vs. stackmap err -1 errno 2 And I didn't see how this will fix the issue. As long as ORC need to unwind through the JITed code it will fail. And that will happen before reaching ___bpf_prog_run. > > > > > For frame pointer unwinder, it seems the JITed bpf code will have a > > > > shifted "BP" register? (arch/x86/net/bpf_jit_comp.c:217), so if we can > > > > unshift it properly then it will work. > > > > > > Yeah, that looks like a frame pointer bug in emit_prologue(). > > > > > > > I tried below code, and problem is fixed (only for frame pointer > > > > unwinder though). Need to find a better way to detect and do any > > > > similar trick for bpf part, if this is a feasible way to fix it: > > > > > > > > diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c > > > > index 9b9fd4826e7a..2c0fa2aaa7e4 100644 > > > > --- a/arch/x86/kernel/unwind_frame.c > > > > +++ b/arch/x86/kernel/unwind_frame.c > > > > @@ -330,8 +330,17 @@ bool unwind_next_frame(struct unwind_state *state) > > > > } > > > > > > > > /* Move to the next frame if it's safe: */ > > > > - if (!update_stack_state(state, next_bp)) > > > > - goto bad_address; > > > > + if (!update_stack_state(state, next_bp)) { > > > > + // Try again with shifted BP > > > > + state->bp += 5; // see AUX_STACK_SPACE > > > > + next_bp = (unsigned long > > > > *)READ_ONCE_TASK_STACK(state->task, *state->bp); > > > > + // Clean and refetch stack info, it's marked as error outed > > > > + state->stack_mask = 0; > > > > + get_stack_info(next_bp, state->task, > > > > &state->stack_info, &state->stack_mask); > > > > + if (!update_stack_state(state, next_bp)) { > > > > + goto bad_address; > > > > + } > > > > + } > > > > > > > > return true; > > > > > > Nack. > > > > > > > For ORC unwinder, I think the unwinder can't find any info about the > > > > JITed part. Maybe if can let it just skip the JITed part and go to > > > > kernel context, then should be good enough. > > > > > > If it's starting from a fake pt_regs then that's going to be a > > > challenge. > > > > > > Will the JIT code always have the same stack layout? If so then we > > > could hard code that knowledge in ORC. Or even better, create a generic > > > interface for ORC to query the creator of the generated code about the > > > stack layout. > > > > I think yes. > > > > Not sure why we have the BP shift yet, if the prolog code could be > > tweaked to work with frame pointer unwinder it will be good to have. > > But still not for ORC. > > > > Will it be a good idea to have a region reserved for the JITed code? > > Currently it shares the region with "module mapping space". If let it > > have a separate region, when the unwinder meet any code in that region > > it will know it's JITed code and then can do something special about > > it. > > > > This should make it much easier for both frame pointer and ORC unwinder to work. > > There's no need to put special cases in the FP unwinder when we can > instead just fix the frame pointer usage in the JIT code. > > 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. Right, this sounds the right way to fix it, I believe this can fix everything well. > > Once I get a reproducer I can do the patches for all that. > > -- > Josh -- Best Regards, Kairui Song
