114 lines
3.6 KiB
ReStructuredText
114 lines
3.6 KiB
ReStructuredText
kcov: code coverage for fuzzing
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===============================
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kcov exposes kernel code coverage information in a form suitable for coverage-
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guided fuzzing (randomized testing). Coverage data of a running kernel is
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exported via the "kcov" debugfs file. Coverage collection is enabled on a task
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basis, and thus it can capture precise coverage of a single system call.
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Note that kcov does not aim to collect as much coverage as possible. It aims
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to collect more or less stable coverage that is function of syscall inputs.
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To achieve this goal it does not collect coverage in soft/hard interrupts
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and instrumentation of some inherently non-deterministic parts of kernel is
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disabled (e.g. scheduler, locking).
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Usage
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-----
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Configure the kernel with::
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CONFIG_KCOV=y
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CONFIG_KCOV requires gcc built on revision 231296 or later.
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Profiling data will only become accessible once debugfs has been mounted::
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mount -t debugfs none /sys/kernel/debug
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The following program demonstrates kcov usage from within a test program:
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.. code-block:: c
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#include <stdio.h>
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#include <stddef.h>
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#include <stdint.h>
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#include <stdlib.h>
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#include <sys/types.h>
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#include <sys/stat.h>
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#include <sys/ioctl.h>
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#include <sys/mman.h>
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#include <unistd.h>
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#include <fcntl.h>
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#define KCOV_INIT_TRACE _IOR('c', 1, unsigned long)
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#define KCOV_ENABLE _IO('c', 100)
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#define KCOV_DISABLE _IO('c', 101)
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#define COVER_SIZE (64<<10)
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int main(int argc, char **argv)
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{
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int fd;
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unsigned long *cover, n, i;
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/* A single fd descriptor allows coverage collection on a single
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* thread.
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*/
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fd = open("/sys/kernel/debug/kcov", O_RDWR);
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if (fd == -1)
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perror("open"), exit(1);
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/* Setup trace mode and trace size. */
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if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
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perror("ioctl"), exit(1);
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/* Mmap buffer shared between kernel- and user-space. */
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cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
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PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
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if ((void*)cover == MAP_FAILED)
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perror("mmap"), exit(1);
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/* Enable coverage collection on the current thread. */
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if (ioctl(fd, KCOV_ENABLE, 0))
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perror("ioctl"), exit(1);
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/* Reset coverage from the tail of the ioctl() call. */
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__atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
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/* That's the target syscal call. */
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read(-1, NULL, 0);
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/* Read number of PCs collected. */
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n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
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for (i = 0; i < n; i++)
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printf("0x%lx\n", cover[i + 1]);
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/* Disable coverage collection for the current thread. After this call
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* coverage can be enabled for a different thread.
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*/
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if (ioctl(fd, KCOV_DISABLE, 0))
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perror("ioctl"), exit(1);
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/* Free resources. */
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if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
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perror("munmap"), exit(1);
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if (close(fd))
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perror("close"), exit(1);
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return 0;
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}
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After piping through addr2line output of the program looks as follows::
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SyS_read
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fs/read_write.c:562
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__fdget_pos
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fs/file.c:774
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__fget_light
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fs/file.c:746
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__fget_light
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fs/file.c:750
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__fget_light
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fs/file.c:760
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__fdget_pos
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fs/file.c:784
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SyS_read
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fs/read_write.c:562
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If a program needs to collect coverage from several threads (independently),
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it needs to open /sys/kernel/debug/kcov in each thread separately.
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The interface is fine-grained to allow efficient forking of test processes.
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That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode,
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mmaps coverage buffer and then forks child processes in a loop. Child processes
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only need to enable coverage (disable happens automatically on thread end).
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