linux-kernel - Re: [PATCH 5/7] kcov: add ioctl(KCOV_UNIQUE

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Message-ID: <CAG_fn=X2wBJAhvwMHesQMH9kpnZFjqRL5RLNBvFT7j9ZC0+GCA@mail.gmail.com>
Date: Wed, 30 Apr 2025 08:32:07 +0200
From: Alexander Potapenko <glider@...gle.com>
To: Joey Jiao <quic_jiangenj@...cinc.com>
Cc: linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com, 
	Aleksandr Nogikh <nogikh@...gle.com>, Andrey Konovalov <andreyknvl@...il.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Ingo Molnar <mingo@...hat.com>, Josh Poimboeuf <jpoimboe@...nel.org>, Marco Elver <elver@...gle.com>, 
	Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 5/7] kcov: add ioctl(KCOV_UNIQUE_ENABLE)

On Wed, Apr 30, 2025 at 4:22 AM Joey Jiao <quic_jiangenj@...cinc.com> wrote:
>
> On Wed, Apr 16, 2025 at 10:54:43AM +0200, Alexander Potapenko wrote:
> > ioctl(KCOV_UNIQUE_ENABLE) enables collection of deduplicated coverage
> > in the presence of CONFIG_KCOV_ENABLE_GUARDS.
> >
> > The buffer shared with the userspace is divided in two parts, one holding
> > a bitmap, and the other one being the trace. The single parameter of
> > ioctl(KCOV_UNIQUE_ENABLE) determines the number of words used for the
> > bitmap.
> >
> > Each __sanitizer_cov_trace_pc_guard() instrumentation hook receives a
> > pointer to a unique guard variable. Upon the first call of each hook,
> > the guard variable is initialized with a unique integer, which is used to
> > map those hooks to bits in the bitmap. In the new coverage collection mode,
> > the kernel first checks whether the bit corresponding to a particular hook
> > is set, and then, if it is not, the PC is written into the trace buffer,
> > and the bit is set.
> >
> > Note: when CONFIG_KCOV_ENABLE_GUARDS is disabled, ioctl(KCOV_UNIQUE_ENABLE)
> > returns -ENOTSUPP, which is consistent with the existing kcov code.
> >
> > Also update the documentation.
> >
> > Signed-off-by: Alexander Potapenko <glider@...gle.com>
> > ---
> >  Documentation/dev-tools/kcov.rst |  43 +++++++++++
> >  include/linux/kcov-state.h       |   8 ++
> >  include/linux/kcov.h             |   2 +
> >  include/uapi/linux/kcov.h        |   1 +
> >  kernel/kcov.c                    | 129 +++++++++++++++++++++++++++----
> >  5 files changed, 170 insertions(+), 13 deletions(-)
> >
> > diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst
> > index 6611434e2dd24..271260642d1a6 100644
> > --- a/Documentation/dev-tools/kcov.rst
> > +++ b/Documentation/dev-tools/kcov.rst
> > @@ -137,6 +137,49 @@ mmaps coverage buffer, and then forks child processes in a loop. The child
> >  processes only need to enable coverage (it gets disabled automatically when
> >  a thread exits).
> >
> > +Unique coverage collection
> > +---------------------------
> > +
> > +Instead of collecting raw PCs, KCOV can deduplicate them on the fly.
> > +This mode is enabled by the ``KCOV_UNIQUE_ENABLE`` ioctl (only available if
> > +``CONFIG_KCOV_ENABLE_GUARDS`` is on).
> > +
> > +.. code-block:: c
> > +
> > +     /* Same includes and defines as above. */
> > +     #define KCOV_UNIQUE_ENABLE              _IOW('c', 103, unsigned long)
> in kcov.h it was defined was _IOR, but _IOW here,

Yeah, Marco spotted this on another patch, I'll fix kcov.h.