v4.19.13 snapshot.
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+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.
+
+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).
+
+kcov is also able to collect comparison operands from the instrumented code
+(this feature currently requires that the kernel is compiled with clang).
+
+Prerequisites
+-------------
+
+Configure the kernel with::
+
+ CONFIG_KCOV=y
+
+CONFIG_KCOV requires gcc built on revision 231296 or later.
+
+If the comparison operands need to be collected, set::
+
+ CONFIG_KCOV_ENABLE_COMPARISONS=y
+
+Profiling data will only become 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:
+
+.. 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>
+
+ #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);
+ /* That's the target syscal 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 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. Child processes
+only need to enable coverage (disable happens automatically on thread end).
+
+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++) {
+ 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 (coverage collection or comparison operands) are
+mutually exclusive.