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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Tue, 28 Feb 2023 10:37:40 +0100
Message-ID: <CACT4Y+Z4GvK-XCbrLp8cuH-xHYsCdh1f0948ZgkU2D0apfGG5w@mail.gmail.com>
Subject: Re: [PATCH] kcov: improve documentation
To:     andrey.konovalov@linux.dev
Cc:     Andrey Konovalov <andreyknvl@gmail.com>,
        Marco Elver <elver@google.com>,
        Alexander Potapenko <glider@google.com>,
        kasan-dev@googlegroups.com,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrey Konovalov <andreyknvl@google.com>
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Precedence: bulk

On Mon, 27 Feb 2023 at 18:17, <andrey.konovalov@linux.dev> wrote:
>
> From: Andrey Konovalov <andreyknvl@google.com>
>
> Improve KCOV documentation:
>
> - Use KCOV instead of kcov, as the former is more widely-used.
>
> - Mention Clang in compiler requirements.
>
> - Use ``annotations`` for inline code.
>
> - Rework remote coverage collection documentation for better clarity.
>
> - Various smaller changes.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
>  Documentation/dev-tools/kcov.rst | 169 +++++++++++++++++++------------
>  1 file changed, 102 insertions(+), 67 deletions(-)
>
> diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst
> index d83c9ab49427..a113a03a475f 100644
> --- a/Documentation/dev-tools/kcov.rst
> +++ b/Documentation/dev-tools/kcov.rst
> @@ -1,42 +1,50 @@
> -kcov: code coverage for fuzzing
> +KCOV: code coverage for fuzzing
>  ===============================
>
> -kcov exposes kernel code coverage information in a form suitable for coverage-
> -guided fuzzing (randomized testing). Coverage data of a running kernel is
> -exported via the "kcov" debugfs file. Coverage collection is enabled on a task
> -basis, and thus it can capture precise coverage of a single system call.
> +KCOV collects and exposes kernel code coverage information in a form suitable
> +for coverage-guided fuzzing. Coverage data of a running kernel is exported via
> +the ``kcov`` debugfs file. Coverage collection is enabled on a task basis, and
> +thus KCOV can capture precise coverage of a single system call.
>
> -Note that kcov does not aim to collect as much coverage as possible. It aims
> -to collect more or less stable coverage that is function of syscall inputs.
> -To achieve this goal it does not collect coverage in soft/hard interrupts
> -and instrumentation of some inherently non-deterministic parts of kernel is
> -disabled (e.g. scheduler, locking).
> +Note that KCOV does not aim to collect as much coverage as possible. It aims
> +to collect more or less stable coverage that is a function of syscall inputs.
> +To achieve this goal, it does not collect coverage in soft/hard interrupts
> +(unless remove coverage collection is enabled, see below) and from some
> +inherently non-deterministic parts of the kernel (e.g. scheduler, locking).
>
> -kcov is also able to collect comparison operands from the instrumented code
> -(this feature currently requires that the kernel is compiled with clang).
> +Besides collecting code coverage, KCOV can also collect comparison operands.
> +See the "Comparison operands collection" section for details.
> +
> +Besides collecting coverage data from syscall handlers, KCOV can also collect
> +coverage for annotated parts of the kernel executing in background kernel
> +tasks or soft interrupts. See the "Remote coverage collection" section for
> +details.
>
>  Prerequisites
>  -------------
>
> -Configure the kernel with::
> +KCOV relies on compiler instrumentation and requires GCC 6.1.0 or later
> +or any Clang version supported by the kernel.
>
> -        CONFIG_KCOV=y
> +Collecting comparison operands is only supported with Clang.

Are you sure?
I see -fsanitize-coverage=trace-cmp in gcc sources and man page.

Otherwise looks good to me.

> -CONFIG_KCOV requires gcc 6.1.0 or later.
> +To enable KCOV, configure the kernel with::
>
> -If the comparison operands need to be collected, set::
> +        CONFIG_KCOV=y
> +
> +To enable comparison operands collection, set::
>
>         CONFIG_KCOV_ENABLE_COMPARISONS=y
>
> -Profiling data will only become accessible once debugfs has been mounted::
> +Coverage data only becomes accessible once debugfs has been mounted::
>
>          mount -t debugfs none /sys/kernel/debug
>
>  Coverage collection
>  -------------------
>
> -The following program demonstrates coverage collection from within a test
> -program using kcov:
> +The following program demonstrates how to use KCOV to collect coverage for a
> +single syscall from within a test program:
>
>  .. code-block:: c
>
> @@ -84,7 +92,7 @@ program using kcov:
>                 perror("ioctl"), exit(1);
>         /* Reset coverage from the tail of the ioctl() call. */
>         __atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
> -       /* That's the target syscal call. */
> +       /* Call the target syscall call. */
>         read(-1, NULL, 0);
>         /* Read number of PCs collected. */
>         n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
> @@ -103,7 +111,7 @@ program using kcov:
>         return 0;
>      }
>
> -After piping through addr2line output of the program looks as follows::
> +After piping through ``addr2line`` the output of the program looks as follows::
>
>      SyS_read
>      fs/read_write.c:562
> @@ -121,12 +129,13 @@ After piping through addr2line output of the program looks as follows::
>      fs/read_write.c:562
>
>  If a program needs to collect coverage from several threads (independently),
> -it needs to open /sys/kernel/debug/kcov in each thread separately.
> +it needs to open ``/sys/kernel/debug/kcov`` in each thread separately.
>
>  The interface is fine-grained to allow efficient forking of test processes.
> -That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode,
> -mmaps coverage buffer and then forks child processes in a loop. Child processes
> -only need to enable coverage (disable happens automatically on thread end).
> +That is, a parent process opens ``/sys/kernel/debug/kcov``, enables trace mode,
> +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).
>
>  Comparison operands collection
>  ------------------------------
> @@ -205,52 +214,78 @@ Comparison operands collection is similar to coverage collection:
>         return 0;
>      }
>
> -Note that the kcov modes (coverage collection or comparison operands) are
> -mutually exclusive.
> +Note that the KCOV modes (collection of code coverage or comparison operands)
> +are mutually exclusive.
>
>  Remote coverage collection
>  --------------------------
>
> -With KCOV_ENABLE coverage is collected only for syscalls that are issued
> -from the current process. With KCOV_REMOTE_ENABLE it's possible to collect
> -coverage for arbitrary parts of the kernel code, provided that those parts
> -are annotated with kcov_remote_start()/kcov_remote_stop().
> -
> -This allows to collect coverage from two types of kernel background
> -threads: the global ones, that are spawned during kernel boot in a limited
> -number of instances (e.g. one USB hub_event() worker thread is spawned per
> -USB HCD); and the local ones, that are spawned when a user interacts with
> -some kernel interface (e.g. vhost workers); as well as from soft
> -interrupts.
> -
> -To enable collecting coverage from a global background thread or from a
> -softirq, a unique global handle must be assigned and passed to the
> -corresponding kcov_remote_start() call. Then a userspace process can pass
> -a list of such handles to the KCOV_REMOTE_ENABLE ioctl in the handles
> -array field of the kcov_remote_arg struct. This will attach the used kcov
> -device to the code sections, that are referenced by those handles.
> -
> -Since there might be many local background threads spawned from different
> -userspace processes, we can't use a single global handle per annotation.
> -Instead, the userspace process passes a non-zero handle through the
> -common_handle field of the kcov_remote_arg struct. This common handle gets
> -saved to the kcov_handle field in the current task_struct and needs to be
> -passed to the newly spawned threads via custom annotations. Those threads
> -should in turn be annotated with kcov_remote_start()/kcov_remote_stop().
> -
> -Internally kcov stores handles as u64 integers. The top byte of a handle
> -is used to denote the id of a subsystem that this handle belongs to, and
> -the lower 4 bytes are used to denote the id of a thread instance within
> -that subsystem. A reserved value 0 is used as a subsystem id for common
> -handles as they don't belong to a particular subsystem. The bytes 4-7 are
> -currently reserved and must be zero. In the future the number of bytes
> -used for the subsystem or handle ids might be increased.
> -
> -When a particular userspace process collects coverage via a common
> -handle, kcov will collect coverage for each code section that is annotated
> -to use the common handle obtained as kcov_handle from the current
> -task_struct. However non common handles allow to collect coverage
> -selectively from different subsystems.
> +Besides collecting coverage data from handlers of syscalls issued from a
> +userspace process, KCOV can also collect coverage for parts of the kernel
> +executing in other contexts - so-called "remote" coverage.
> +
> +Using KCOV to collect remote coverage requires:
> +
> +1. Modifying kernel code to annotate the code section from where coverage
> +   should be collected with ``kcov_remote_start`` and ``kcov_remote_stop``.
> +
> +2. Using `KCOV_REMOTE_ENABLE`` instead of ``KCOV_ENABLE`` in the userspace
> +   process that collects coverage.
> +
> +Both ``kcov_remote_start`` and ``kcov_remote_stop`` annotations and the
> +``KCOV_REMOTE_ENABLE`` ioctl accept handles that identify particular coverage
> +collection sections. The way a handle is used depends on the context where the
> +matching code section executes.
> +
> +KCOV supports collecting remote coverage from the following contexts:
> +
> +1. Global kernel background tasks. These are the tasks that are spawned during
> +   kernel boot in a limited number of instances (e.g. one USB ``hub_event``
> +   worker is spawned per one USB HCD).
> +
> +2. Local kernel background tasks. These are spawned when a userspace process
> +   interacts with some kernel interface and are usually killed when the process
> +   exits (e.g. vhost workers).
> +
> +3. Soft interrupts.
> +
> +For #1 and #3, a unique global handle must be chosen and passed to the
> +corresponding ``kcov_remote_start`` call. Then a userspace process must pass
> +this handle to ``KCOV_REMOTE_ENABLE`` in the ``handles`` array field of the
> +``kcov_remote_arg`` struct. This will attach the used KCOV device to the code
> +section referenced by this handle. Multiple global handles identifying
> +different code sections can be passed at once.
> +
> +For #2, the userspace process instead must pass a non-zero handle through the
> +``common_handle`` field of the ``kcov_remote_arg`` struct. This common handle
> +gets saved to the ``kcov_handle`` field in the current ``task_struct`` and
> +needs to be passed to the newly spawned local tasks via custom kernel code
> +modifications. Those tasks should in turn use the passed handle in their
> +``kcov_remote_start`` and ``kcov_remote_stop`` annotations.
> +
> +KCOV follows a predefined format for both global and common handles. Each
> +handle is a ``u64`` integer. Currently, only the one top and the lower 4 bytes
> +are used. Bytes 4-7 are reserved and must be zero.
> +
> +For global handles, the top byte of the handle denotes the id of a subsystem
> +this handle belongs to. For example, KCOV uses ``1`` as the USB subsystem id.
> +The lower 4 bytes of a global handle denote the id of a task instance within
> +that subsystem. For example, each ``hub_event`` worker uses the USB bus number
> +as the task instance id.
> +
> +For common handles, a reserved value ``0`` is used as a subsystem id, as such
> +handles don't belong to a particular subsystem. The lower 4 bytes of a common
> +handle identify a collective instance of all local tasks spawned by the
> +userspace process that passed a common handle to ``KCOV_REMOTE_ENABLE``.
> +
> +In practice, any value can be used for common handle instance id if coverage
> +is only collected from a single userspace process on the system. However, if
> +common handles are used by multiple processes, unique instance ids must be
> +used for each process. One option is to use the process id as the common
> +handle instance id.
> +
> +The following program demonstrates using KCOV to collect coverage from both
> +local tasks spawned by the process and the global task that handles USB bus #1:
>
>  .. code-block:: c
>
> --
> 2.25.1
>