Re: [PATCH v2 1/3] kcov: support comparison operands collection

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Oct 09, 2017 at 08:15:10PM +0200, 'Dmitry Vyukov' via syzkaller wrote:
> On Mon, Oct 9, 2017 at 5:46 PM, Mark Rutland <mark.rutland@arm.com> wrote:
> > On Mon, Oct 09, 2017 at 05:05:19PM +0200, Alexander Potapenko wrote:

> > ... I note that a few places in the kernel use a 128-bit type. Are
> > 128-bit comparisons not instrumented?
> 
> Yes, they are not instrumented.
> How many are there? Can you give some examples?

>From a quick scan, it doesn't looks like there are currently any
comparisons.

It's used as a data type in a few places under arm64:

arch/arm64/include/asm/checksum.h:      __uint128_t tmp;
arch/arm64/include/asm/checksum.h:      tmp = *(const __uint128_t *)iph;
arch/arm64/include/asm/fpsimd.h:                        __uint128_t vregs[32];
arch/arm64/include/uapi/asm/ptrace.h:   __uint128_t     vregs[32];
arch/arm64/include/uapi/asm/sigcontext.h:       __uint128_t vregs[32];
arch/arm64/kernel/signal32.c:   __uint128_t     raw;
arch/arm64/kvm/guest.c: __uint128_t tmp;

[...]

> >> +     area = t->kcov_area;
> >> +     /* The first 64-bit word is the number of subsequent PCs. */
> >> +     pos = READ_ONCE(area[0]) + 1;
> >> +     if (likely(pos < t->kcov_size)) {
> >> +             area[pos] = ip;
> >> +             WRITE_ONCE(area[0], pos);
> >
> > Not a new problem, but if the area for one thread is mmap'd, and read by
> > another thread, these two writes could be seen out-of-order, since we
> > don't have an smp_wmb() between them.
> >
> > I guess Syzkaller doesn't read the mmap'd kcov file from another thread?
> 
> 
> Yes, that's the intention. If you read coverage from another thread,
> you can't know coverage from what exactly you read. So the usage
> pattern is:
> 
> reset coverage;
> do something;
> read coverage;

Ok. I guess without a use-case for reading this from another thread it doesn't
really matter.

Thanks,
Mark.

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