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diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst index d83c9ab49427..6611434e2dd2 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 supported with GCC 8+ or with Clang. -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 |