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@xxxxxxx> 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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