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+KCOV: code coverage for fuzzing
+===============================
+
+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 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).
+
+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
+-------------
+
+KCOV relies on compiler instrumentation and requires GCC 6.1.0 or later
+or any Clang version supported by the kernel.
+
+Collecting comparison operands is supported with GCC 8+ or with Clang.
+
+To enable KCOV, configure the kernel with::
+
+ CONFIG_KCOV=y
+
+To enable comparison operands collection, set::
+
+ CONFIG_KCOV_ENABLE_COMPARISONS=y
+
+Coverage data only becomes accessible once debugfs has been mounted::
+
+ mount -t debugfs none /sys/kernel/debug
+
+Coverage collection
+-------------------
+
+The following program demonstrates how to use KCOV to collect coverage for a
+single syscall from within a test program:
+
+.. code-block:: c
+
+ #include <stdio.h>
+ #include <stddef.h>
+ #include <stdint.h>
+ #include <stdlib.h>
+ #include <sys/types.h>
+ #include <sys/stat.h>
+ #include <sys/ioctl.h>
+ #include <sys/mman.h>
+ #include <unistd.h>
+ #include <fcntl.h>
+ #include <linux/types.h>
+
+ #define KCOV_INIT_TRACE _IOR('c', 1, unsigned long)
+ #define KCOV_ENABLE _IO('c', 100)
+ #define KCOV_DISABLE _IO('c', 101)
+ #define COVER_SIZE (64<<10)
+
+ #define KCOV_TRACE_PC 0
+ #define KCOV_TRACE_CMP 1
+
+ int main(int argc, char **argv)
+ {
+ int fd;
+ unsigned long *cover, n, i;
+
+ /* A single fd descriptor allows coverage collection on a single
+ * thread.
+ */
+ fd = open("/sys/kernel/debug/kcov", O_RDWR);
+ if (fd == -1)
+ perror("open"), exit(1);
+ /* Setup trace mode and trace size. */
+ if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
+ perror("ioctl"), exit(1);
+ /* Mmap buffer shared between kernel- and user-space. */
+ cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
+ PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+ if ((void*)cover == MAP_FAILED)
+ perror("mmap"), exit(1);
+ /* Enable coverage collection on the current thread. */
+ if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_PC))
+ perror("ioctl"), exit(1);
+ /* Reset coverage from the tail of the ioctl() call. */
+ __atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
+ /* Call the target syscall call. */
+ read(-1, NULL, 0);
+ /* Read number of PCs collected. */
+ n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
+ for (i = 0; i < n; i++)
+ printf("0x%lx\n", cover[i + 1]);
+ /* Disable coverage collection for the current thread. After this call
+ * coverage can be enabled for a different thread.
+ */
+ if (ioctl(fd, KCOV_DISABLE, 0))
+ perror("ioctl"), exit(1);
+ /* Free resources. */
+ if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
+ perror("munmap"), exit(1);
+ if (close(fd))
+ perror("close"), exit(1);
+ return 0;
+ }
+
+After piping through ``addr2line`` the output of the program looks as follows::
+
+ SyS_read
+ fs/read_write.c:562
+ __fdget_pos
+ fs/file.c:774
+ __fget_light
+ fs/file.c:746
+ __fget_light
+ fs/file.c:750
+ __fget_light
+ fs/file.c:760
+ __fdget_pos
+ fs/file.c:784
+ SyS_read
+ 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.
+
+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. The child
+processes only need to enable coverage (it gets disabled automatically when
+a thread exits).
+
+Comparison operands collection
+------------------------------
+
+Comparison operands collection is similar to coverage collection:
+
+.. code-block:: c
+
+ /* Same includes and defines as above. */
+
+ /* Number of 64-bit words per record. */
+ #define KCOV_WORDS_PER_CMP 4
+
+ /*
+ * The format for the types of collected comparisons.
+ *
+ * Bit 0 shows whether one of the arguments is a compile-time constant.
+ * Bits 1 & 2 contain log2 of the argument size, up to 8 bytes.
+ */
+
+ #define KCOV_CMP_CONST (1 << 0)
+ #define KCOV_CMP_SIZE(n) ((n) << 1)
+ #define KCOV_CMP_MASK KCOV_CMP_SIZE(3)
+
+ int main(int argc, char **argv)
+ {
+ int fd;
+ uint64_t *cover, type, arg1, arg2, is_const, size;
+ unsigned long n, i;
+
+ fd = open("/sys/kernel/debug/kcov", O_RDWR);
+ if (fd == -1)
+ perror("open"), exit(1);
+ if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
+ perror("ioctl"), exit(1);
+ /*
+ * Note that the buffer pointer is of type uint64_t*, because all
+ * the comparison operands are promoted to uint64_t.
+ */
+ cover = (uint64_t *)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
+ PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+ if ((void*)cover == MAP_FAILED)
+ perror("mmap"), exit(1);
+ /* Note KCOV_TRACE_CMP instead of KCOV_TRACE_PC. */
+ if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_CMP))
+ perror("ioctl"), exit(1);
+ __atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
+ read(-1, NULL, 0);
+ /* Read number of comparisons collected. */
+ n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
+ for (i = 0; i < n; i++) {
+ uint64_t ip;
+
+ type = cover[i * KCOV_WORDS_PER_CMP + 1];
+ /* arg1 and arg2 - operands of the comparison. */
+ arg1 = cover[i * KCOV_WORDS_PER_CMP + 2];
+ arg2 = cover[i * KCOV_WORDS_PER_CMP + 3];
+ /* ip - caller address. */
+ ip = cover[i * KCOV_WORDS_PER_CMP + 4];
+ /* size of the operands. */
+ size = 1 << ((type & KCOV_CMP_MASK) >> 1);
+ /* is_const - true if either operand is a compile-time constant.*/
+ is_const = type & KCOV_CMP_CONST;
+ printf("ip: 0x%lx type: 0x%lx, arg1: 0x%lx, arg2: 0x%lx, "
+ "size: %lu, %s\n",
+ ip, type, arg1, arg2, size,
+ is_const ? "const" : "non-const");
+ }
+ if (ioctl(fd, KCOV_DISABLE, 0))
+ perror("ioctl"), exit(1);
+ /* Free resources. */
+ if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
+ perror("munmap"), exit(1);
+ if (close(fd))
+ perror("close"), exit(1);
+ return 0;
+ }
+
+Note that the KCOV modes (collection of code coverage or comparison operands)
+are mutually exclusive.
+
+Remote coverage collection
+--------------------------
+
+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
+
+ /* Same includes and defines as above. */
+
+ struct kcov_remote_arg {
+ __u32 trace_mode;
+ __u32 area_size;
+ __u32 num_handles;
+ __aligned_u64 common_handle;
+ __aligned_u64 handles[0];
+ };
+
+ #define KCOV_INIT_TRACE _IOR('c', 1, unsigned long)
+ #define KCOV_DISABLE _IO('c', 101)
+ #define KCOV_REMOTE_ENABLE _IOW('c', 102, struct kcov_remote_arg)
+
+ #define COVER_SIZE (64 << 10)
+
+ #define KCOV_TRACE_PC 0
+
+ #define KCOV_SUBSYSTEM_COMMON (0x00ull << 56)
+ #define KCOV_SUBSYSTEM_USB (0x01ull << 56)
+
+ #define KCOV_SUBSYSTEM_MASK (0xffull << 56)
+ #define KCOV_INSTANCE_MASK (0xffffffffull)
+
+ static inline __u64 kcov_remote_handle(__u64 subsys, __u64 inst)
+ {
+ if (subsys & ~KCOV_SUBSYSTEM_MASK || inst & ~KCOV_INSTANCE_MASK)
+ return 0;
+ return subsys | inst;
+ }
+
+ #define KCOV_COMMON_ID 0x42
+ #define KCOV_USB_BUS_NUM 1
+
+ int main(int argc, char **argv)
+ {
+ int fd;
+ unsigned long *cover, n, i;
+ struct kcov_remote_arg *arg;
+
+ fd = open("/sys/kernel/debug/kcov", O_RDWR);
+ if (fd == -1)
+ perror("open"), exit(1);
+ if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
+ perror("ioctl"), exit(1);
+ cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
+ PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+ if ((void*)cover == MAP_FAILED)
+ perror("mmap"), exit(1);
+
+ /* Enable coverage collection via common handle and from USB bus #1. */
+ arg = calloc(1, sizeof(*arg) + sizeof(uint64_t));
+ if (!arg)
+ perror("calloc"), exit(1);
+ arg->trace_mode = KCOV_TRACE_PC;
+ arg->area_size = COVER_SIZE;
+ arg->num_handles = 1;
+ arg->common_handle = kcov_remote_handle(KCOV_SUBSYSTEM_COMMON,
+ KCOV_COMMON_ID);
+ arg->handles[0] = kcov_remote_handle(KCOV_SUBSYSTEM_USB,
+ KCOV_USB_BUS_NUM);
+ if (ioctl(fd, KCOV_REMOTE_ENABLE, arg))
+ perror("ioctl"), free(arg), exit(1);
+ free(arg);
+
+ /*
+ * Here the user needs to trigger execution of a kernel code section
+ * that is either annotated with the common handle, or to trigger some
+ * activity on USB bus #1.
+ */
+ sleep(2);
+
+ n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
+ for (i = 0; i < n; i++)
+ printf("0x%lx\n", cover[i + 1]);
+ if (ioctl(fd, KCOV_DISABLE, 0))
+ perror("ioctl"), exit(1);
+ if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
+ perror("munmap"), exit(1);
+ if (close(fd))
+ perror("close"), exit(1);
+ return 0;
+ }