Messages in this thread | | | Subject | Re: [RFC PATCH v5 1/2] arm64: Introduce stack trace reliability checks in the unwinder | From | "Madhavan T. Venkataraman" <> | Date | Fri, 25 Jun 2021 12:05:18 -0500 |
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On 6/25/21 10:51 AM, Mark Brown wrote: > On Fri, Jun 25, 2021 at 10:39:57AM -0500, Madhavan T. Venkataraman wrote: >> On 6/24/21 9:40 AM, Mark Rutland wrote: > >>> At a high-level, I'm on-board with keeping track of this per unwind >>> step, but if we do that then I want to be abel to use this during >>> regular unwinds (e.g. so that we can have a backtrace idicate when a >>> step is not reliable, like x86 does with '?'), and to do that we need to >>> be a little more accurate. > >> The only consumer of frame->reliable is livepatch. So, in retrospect, my >> original per-frame reliability flag was an overkill. I was just trying to >> provide extra per-frame debug information which is not really a requirement >> for livepatch. > > It's not a requirement for livepatch but if it's there a per frame > reliability flag would have other uses - for example Mark has mentioned > the way x86 prints a ? next to unreliable entries in oops output for > example, that'd be handy for people debugging issues and would have the > added bonus of ensuring that there's more constant and widespread > exercising of the reliability stuff than if it's just used for livepatch > which is a bit niche. >
I agree. That is why I introduced the per-frame flag.
So, let us try a different approach.
First, let us get rid of the frame->reliable flag from this patch series. That flag can be implemented when all of the pieces are in place for per-frame debug and tracking.
For consumers such as livepatch that don't really care about per-frame stuff, let us solve it more cleanly via the return value of unwind_frame().
Currently, the return value from unwind_frame() is a tri-state return value which is somewhat confusing.
0 means continue unwinding -error means stop unwinding. However, -ENOENT means successful termination Other values mean an error has happened.
Instead, let unwind_frame() return one of 3 values:
enum { UNWIND_CONTINUE, UNWIND_CONTINUE_WITH_ERRORS, UNWIND_STOP, };
All consumers will stop unwinding upon seeing UNWIND_STOP.
Livepatch type consumers will stop unwinding upon seeing anything other than UNWIND_CONTINUE.
Debug type consumers can choose to continue upon seeing UNWIND_CONTINUE_WITH_ERRORS.
When we eventually implement per-frame stuff, debug consumers can examine the frame for more information when they see UNWIND_CONTINUE_WITH_ERRORS.
This way, my patch series does not have a dependency on the per-frame enhancements.
>> So, let us separate the two. I will rename frame->reliable to frame->livepatch_safe. >> This will apply to the whole stacktrace and not to every frame. > > I'd rather keep it as reliable, even with only the livepatch usage I > think it's clearer. >
See suggestion above.
>> Finally, it might be a good idea to perform reliability checks even in >> start_backtrace() so we don't assume that the starting frame is reliable even >> if the caller passes livepatch_safe=true. What do you think? > > That makes sense to me. >
Thanks.
Madhavan
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