Hi Josh, On Thu, 6 Sep 2018 11:15:21 -0500 Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > Ping. We are trying to determine whether s390 needs objtool, for the > livepatch consistency model, which needs reliable stack traces. > > The test for that is: do all (runtime) functions use backchain pointers > properly? > > Our tentative conclusion, based on Joe's research below, is that misuse > of backchain pointers is not a realistic concern, and thus that objtool > is _not_ needed. > > That said, we will probably need to add some error checking to the > unwinder. For example, it would need to properly detect unreliable > stacks caused by page faults or preemption. And some other error > conditions. The compiler is called with the -mbackchain option, all normal C function will store the backchain in the function prologue. All functions written in assembler code should do the same, if you find one that does not we should fix that. The end result is that a task that *voluntarily* called schedule() should have a proper backchain at all times. Dependent on the use case this may or may not be enough. Asynchronous interrupts may stop the CPU at the beginning of a function, if kernel preemption is enabled we can end up with a broken backchain. The production kernels for IBM Z are all compiled *without* kernel preemption. So yes, we might get away without the objtool support. On a side-note, we do have a line item to implement the ORC unwinder for the kernel, that includes the objtool support. Once we have that we can drop the -mbackchain option for the kernel build. That gives us a nice little performance benefit. I hope that the change from backchain to the ORC unwinder will not be too hard to implement in the livepatch tools. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin.
