Re: [PATCH v1] arm64: allow building with kcov coverage on ARM64

[Alexander Potapenko <glider@google.com>]

On Thu, Mar 31, 2016 at 6:00 PM, Mark Rutland <mark.rutland@arm.com> wrote:
> On Thu, Mar 31, 2016 at 05:09:29PM +0200, Alexander Potapenko wrote:
>> On Thu, Mar 31, 2016 at 4:29 PM, Mark Rutland <mark.rutland@arm.com> wrote:
>> > Hi,
>> >
>> > On Thu, Mar 31, 2016 at 03:54:45PM +0200, Alexander Potapenko wrote:
>> >> Add ARCH_HAS_KCOV to ARM64 config. Disable instrumentation of
>> >> arch/arm64/lib/delay.c
>> >
>> > Why do we disable instrumentation of delay.c?
>> The main purpose of kcov is collecting coverage from syscalls. As far
>> as I understand, coverage of functions from delay.c doesn't
>> deterministically depend on the syscalls being called and their
>> arguments.
>> The initial kcov implementation
>> (https://github.com/torvalds/linux/commit/5c9a8750a6409c63a0f01d51a9024861022f6593)
>> disabled instrumentation of arch/x86/lib/delay.c, so I just copied
>> that chunk.
>>
>> > What exactly does kcov instrumentation imply? Does it require certain
>> > data to be mapped or certain functions to be callable while instrumented
>> > functions are called?
>> Yes, there is __sanitizer_cov_trace_pc() that must be callable.
>
> That will definitely be a problem for the KVM code which is run at a
> different exception level with a different memory map. For GCOV, KASAN,
> and UBSAN we simply disable instrumentation of that code [1].
>
> We should be able to do similarly for KCOV.
Ok, I'll send out the updated patch.

>> At boot time |current->kcov_mode| zero, so it virtually does nothing.
>>
>> Currently kcov instrumentation is disabled for the following files:
>
>> arch/x86/boot/*
>> arch/x86/boot/compressed/*
>> arch/x86/entry/vdso/*
>> arch/x86/realmode/rm/*
>
> These are executed outside of the usual kernel context / address space,
> so excluding these makes sense to me.
>
>> arch/x86/kernel/*
>> arch/x86/kernel/apic/*
>> arch/x86/kernel/cpu/common.c
>> arch/x86/kernel/cpu/perf_event.c
>> arch/x86/lib/delay.c
>> arch/x86/mm/tlb.c
>
> For these, it's not immediately clear to me why instrumentation is
> disabled, so I don't know whether or not we can instrument the analogous
> arm64 code.
According to the comments in
https://github.com/torvalds/linux/commit/5c9a8750a6409c63a0f01d51a9024861022f6593,
instrumentation of arch/x86/kernel/apic/* and arch/x86/lib/delay.c
leads to non-deterministic coverage, instrumenting others prevent the
kernel from booting.

>> Only a handful of the above have corresponding files in arch/arm64:
>> arch/arm64/boot/*
>> arch/arm64/kernel/*
>> arch/arm64/lib/delay.c
>
> We have arch/arm64/kernel/perf_event.c, and a couple of other files that
> are directly analogous, even if the paths don't quite line up.
Ok, it makes sense to also disable arch/arm64/kernel/perf_event.c then.

>> My patch explicitly disables instrumentation for arch/arm64/lib/delay.c.
>> I never had problems with arch/arm64/boot/* and arch/arm64/kernel/* in
>> the 3.18 kernel, although instrumentation of the corresponding x86
>> code is claimed to cause boot-time hangs.
>> We can act conservatively and still disable instrumentation for these
>> two dirs just to make sure nothing breaks in the future.
>
> I'd rather that we understood why instrumentation of the above is
> disabled, such that we can make a sensible decision from the outset.
>
>> > We have some C code that is run outside of the normal kernel context
>> > (e.g. EFI stub, KVM hyp code), and I suspect it may be necessary to
>> > disable instrumentation for those also.
>> EFI stub and a number of other files is already disabled by the
>> initial kcov patch.
>> I understand there might be some code specific to ARM64 that I may
>> have overlooked, so I'd be grateful if someone could try the patch out
>> with the upstream kernel.
>
> The only such code that I'm immediately aware of is the hyp-context KVM
> code, as mentioned above.
>
> Thanks,
> Mark.
>
> [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-March/416790.html



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Alexander Potapenko
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