linux-kernel - Re: [PATCH v2] arm64: allow building with kcov coverage on ARM64

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Date:	Wed, 15 Jun 2016 15:25:51 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Alexander Potapenko <glider@...gle.com>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	marc.zyngier@....com, Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	Kostya Serebryany <kcc@...gle.com>,
	syzkaller <syzkaller@...glegroups.com>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitriy Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v2] arm64: allow building with kcov coverage on ARM64

On Wed, Jun 15, 2016 at 01:53:03PM +0200, Alexander Potapenko wrote:
> On Wed, Jun 15, 2016 at 1:44 PM, Mark Rutland <mark.rutland@....com> wrote:
> > On Wed, Jun 15, 2016 at 10:25:10AM +0100, Mark Rutland wrote:
> >> On Tue, Jun 14, 2016 at 08:16:08PM +0200, Alexander Potapenko wrote:
> >> > On Tue, Jun 14, 2016 at 7:55 PM, Mark Rutland <mark.rutland@....com> wrote:
> >> > > I built and booted (via EFI) a kernel with this feature enabled (also
> >> > > with the boot/Makefile change removed). I haven't tested the feature
> >> > > itself as such, as I'm not sure how to do that.
> >> > You can test it by running the test program from Documentation/kcov.txt.
> >>
> >> Ah, I hadn't spotted that. If I get the chance I'll try to give that a
> >> go.
> >
> > I just had a go (with this applied atop of v4.7-rc3), and I get:
> >
> > root@...bensteg:/home/nanook# ./kcov
> > mmap: No such device
> >
> > The device exists (it was able to open the fd, evidently):
> >
> > root@...bensteg:/home/nanook# ls -al /sys/kernel/debug/kcov
> > -rw------- 1 root root 0 Jan  1  1970 /sys/kernel/debug/kcov
> >
> > Strace show me:
> >
> > openat(AT_FDCWD, "/sys/kernel/debug/kcov", O_RDWR) = 3
> > ioctl(3, CHIOMOVE or CM_IOCGATR, 0x10000) = 0
> > mmap(NULL, 524288, PROT_READ|PROT_WRITE, MAP_SHARED, 3, 0) = -1 ENODEV (No such device)
> >
> > It doesn't look like the error paths in kcov_mmap are hitting.
> >
> > Any ideas?
> According to Dmitry (thanks, Dmitry!) this has regressed recently, but
> there's a pending patch that should probably fix the problem:
> http://lkml.iu.edu/hypermail/linux/kernel/1605.2/04379.html

Thanks for the pointer! With that applied, the program runs.

However, it looks like I missed a warning from the kernel build system, 
and my toolchain doesn't actually support -fsanitize-coverage=trace-pc,
so I'm not going to be able to test that further.

It would be great if we could deliberately not register the debugfs file
when there was no compiler support for the feature, for those like me
who miss the build time warning. We do something like that for the LSE
atomics on arm64.

Thanks,
Mark.