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Date: Fri, 22 Jan 2016 11:55:03 +0000
From: Mark Rutland <mark.rutland@arm.com>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: syzkaller <syzkaller@googlegroups.com>,
        Vegard Nossum <vegard.nossum@oracle.com>,
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Subject: Re: [PATCH v2] kernel: add kcov code coverage
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On Mon, Jan 18, 2016 at 09:09:43PM +0100, Dmitry Vyukov wrote:
> On Mon, Jan 18, 2016 at 8:44 PM, Dmitry Vyukov <dvyukov@google.com> wrote:
> > I've got several comments regarding the 4-byte compressed PCs. We've
> > also discussed this internally.
> > As the result in v4 I made it possible to export both compressed
> > 4-byte PCs and full 8-byte PCs.
> > Now init ioctl accepts the following struct and kernel can say whether
> > it will export 4- or 8-byte PCs:
> >
> > struct kcov_init_trace {
> >         unsigned long        flags; /* In: reserved, must be 0. */
> >         unsigned long        size; /* In: trace buffer size. */
> >         unsigned long        version;  /* Out: trace format, currently 1. */
> >         /*
> >          * Output.
> >          * pc_size can be 4 or 8. If pc_size = 4 on a 64-bit arch,
> >          * returned PCs are compressed by subtracting pc_base and then
> >          * truncating to 4 bytes.
> >          */
> >         unsigned long        pc_size;
> >         unsigned long        pc_base;
> > };
> >
> > So for KASLR or other archs we can just export full 8-byte PCs.
> >
> > Regarding KASLR and dynamically loaded modules. I've looked at my
> > use-case and concluded
> > that most of the time I can work with "non-stable" PCs within a single
> > VM. Whenever I need to
> > store PCs persistently or send to another machine, I think I can
> > "canonicalize" PCs using
> > /proc/modules and /proc/kallsyms to something like (module hash,
> > module offset). So kernel does
> > not need to do this during coverage collection.
> 
> On second though, maybe it's better to just always export unsigned long PCs...
> Need to measure how much memory coverage information consumes,
> and how much slower it is with uint64 PCs. Maybe I can live with large PCs,
> or maybe I can make syzkaller require !KASLR and compress PCs in user-space...
> Need to think about this more.

I can imagine we might keep the expanded module range even in the
absence of full KASLR, though I don't know how realistic that thought
is.

Thanks,
Mark.