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

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Date:	Thu, 31 Mar 2016 17:00:53 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Alexander Potapenko <glider@...gle.com>
Cc:	Dmitriy Vyukov <dvyukov@...gle.com>, catalin.marinas@....com,
	quentin.casasnovas@...cle.com, will.deacon@....com,
	Kostya Serebryany <kcc@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	syzkaller@...glegroups.com, LKML <linux-kernel@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org, ard.biesheuvel@...aro.org,
	marc.zyngier@....com, christoffer.dall@...aro.org
Subject: Re: [PATCH v1] arm64: allow building with kcov coverage on ARM64

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@....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.

> 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.

> 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.

> 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