Thanks for all the input - Mark Rutland and Mark Brown. I will send out the stack termination patch next. Since I am splitting the original series into 3 separate series, I will change the titles and start with version 1 in each case, if there is no objection.
Again, Thanks. Madhavan On 3/23/21 3:24 PM, Madhavan T. Venkataraman wrote: > > > On 3/23/21 1:30 PM, Mark Rutland wrote: >> On Tue, Mar 23, 2021 at 12:23:34PM -0500, Madhavan T. Venkataraman wrote: >>> On 3/23/21 12:02 PM, Mark Rutland wrote: >> >> [...] >> >>> I think that I did a bad job of explaining what I wanted to do. It is not >>> for any additional protection at all. >>> >>> So, let us say we create a field in the task structure: >>> >>> u64 unreliable_stack; >>> >>> Whenever an EL1 exception is entered or FTRACE is entered and pt_regs get >>> set up and pt_regs->stackframe gets chained, increment unreliable_stack. >>> On exiting the above, decrement unreliable_stack. >>> >>> In arch_stack_walk_reliable(), simply do this check upfront: >>> >>> if (task->unreliable_stack) >>> return -EINVAL; >>> >>> This way, the function does not even bother unwinding the stack to find >>> exception frames or checking for different return addresses or anything. >>> We also don't have to worry about code being reorganized, functions >>> being renamed, etc. It also may help in debugging to know if a task is >>> experiencing an exception and the level of nesting, etc. >> >> As in my other reply, since this is an optimization that is not >> necessary for functional correctness, I would prefer to avoid this for >> now. We can reconsider that in future if we encounter performance >> problems. >> >> Even with this there will be cases where we have to identify >> non-unwindable functions explicitly (e.g. the patchable-function-entry >> trampolines, where the real return address is in x9), and I'd prefer >> that we use one mechanism consistently. >> >> I suspect that in the future we'll need to unwind across exception >> boundaries using metadata, and we can treat the non-unwindable metadata >> in the same way. >> >> [...] >> >>>> 3. Figure out exception boundary handling. I'm currently working to >>>> simplify the entry assembly down to a uniform set of stubs, and I'd >>>> prefer to get that sorted before we teach the unwinder about >>>> exception boundaries, as it'll be significantly simpler to reason >>>> about and won't end up clashing with the rework. >>> >>> So, here is where I still have a question. Is it necessary for the unwinder >>> to know the exception boundaries? Is it not enough if it knows if there are >>> exceptions present? For instance, using something like num_special_frames >>> I suggested above? >> >> I agree that it would be legitimate to bail out early if we knew there >> was going to be an exception somewhere in the trace. Regardless, I think >> it's simpler overall to identify non-unwindability during the trace, and >> doing that during the trace aligns more closely with the structure that >> we'll need to permit unwinding across these boundaries in future, so I'd >> prefer we do that rather than trying to optimize for early returns >> today. >> > > OK. Fair enough. > > Thanks. > > Madhavan >
