335 lines
11 KiB
ReStructuredText
335 lines
11 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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kcov is also able to collect comparison operands from the instrumented code
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(this feature currently requires that the kernel is compiled with clang).
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Prerequisites
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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 6.1.0 or later.
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If the comparison operands need to be collected, set::
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CONFIG_KCOV_ENABLE_COMPARISONS=y
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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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Coverage collection
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-------------------
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The following program demonstrates coverage collection from within a test
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program using kcov:
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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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#define KCOV_TRACE_PC 0
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#define KCOV_TRACE_CMP 1
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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, KCOV_TRACE_PC))
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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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Comparison operands collection
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------------------------------
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Comparison operands collection is similar to coverage collection:
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.. code-block:: c
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/* Same includes and defines as above. */
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/* Number of 64-bit words per record. */
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#define KCOV_WORDS_PER_CMP 4
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/*
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* The format for the types of collected comparisons.
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*
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* Bit 0 shows whether one of the arguments is a compile-time constant.
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* Bits 1 & 2 contain log2 of the argument size, up to 8 bytes.
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*/
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#define KCOV_CMP_CONST (1 << 0)
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#define KCOV_CMP_SIZE(n) ((n) << 1)
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#define KCOV_CMP_MASK KCOV_CMP_SIZE(3)
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int main(int argc, char **argv)
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{
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int fd;
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uint64_t *cover, type, arg1, arg2, is_const, size;
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unsigned long n, i;
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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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if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
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perror("ioctl"), exit(1);
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/*
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* Note that the buffer pointer is of type uint64_t*, because all
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* the comparison operands are promoted to uint64_t.
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*/
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cover = (uint64_t *)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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/* Note KCOV_TRACE_CMP instead of KCOV_TRACE_PC. */
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if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_CMP))
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perror("ioctl"), exit(1);
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__atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
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read(-1, NULL, 0);
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/* Read number of comparisons 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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type = cover[i * KCOV_WORDS_PER_CMP + 1];
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/* arg1 and arg2 - operands of the comparison. */
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arg1 = cover[i * KCOV_WORDS_PER_CMP + 2];
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arg2 = cover[i * KCOV_WORDS_PER_CMP + 3];
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/* ip - caller address. */
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ip = cover[i * KCOV_WORDS_PER_CMP + 4];
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/* size of the operands. */
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size = 1 << ((type & KCOV_CMP_MASK) >> 1);
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/* is_const - true if either operand is a compile-time constant.*/
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is_const = type & KCOV_CMP_CONST;
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printf("ip: 0x%lx type: 0x%lx, arg1: 0x%lx, arg2: 0x%lx, "
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"size: %lu, %s\n",
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ip, type, arg1, arg2, size,
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is_const ? "const" : "non-const");
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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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Note that the kcov modes (coverage collection or comparison operands) are
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mutually exclusive.
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Remote coverage collection
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--------------------------
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With KCOV_ENABLE coverage is collected only for syscalls that are issued
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from the current process. With KCOV_REMOTE_ENABLE it's possible to collect
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coverage for arbitrary parts of the kernel code, provided that those parts
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are annotated with kcov_remote_start()/kcov_remote_stop().
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This allows to collect coverage from two types of kernel background
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threads: the global ones, that are spawned during kernel boot in a limited
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number of instances (e.g. one USB hub_event() worker thread is spawned per
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USB HCD); and the local ones, that are spawned when a user interacts with
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some kernel interface (e.g. vhost workers); as well as from soft
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interrupts.
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To enable collecting coverage from a global background thread or from a
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softirq, a unique global handle must be assigned and passed to the
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corresponding kcov_remote_start() call. Then a userspace process can pass
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a list of such handles to the KCOV_REMOTE_ENABLE ioctl in the handles
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array field of the kcov_remote_arg struct. This will attach the used kcov
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device to the code sections, that are referenced by those handles.
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Since there might be many local background threads spawned from different
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userspace processes, we can't use a single global handle per annotation.
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Instead, the userspace process passes a non-zero handle through the
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common_handle field of the kcov_remote_arg struct. This common handle gets
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saved to the kcov_handle field in the current task_struct and needs to be
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passed to the newly spawned threads via custom annotations. Those threads
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should in turn be annotated with kcov_remote_start()/kcov_remote_stop().
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Internally kcov stores handles as u64 integers. The top byte of a handle
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is used to denote the id of a subsystem that this handle belongs to, and
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the lower 4 bytes are used to denote the id of a thread instance within
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that subsystem. A reserved value 0 is used as a subsystem id for common
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handles as they don't belong to a particular subsystem. The bytes 4-7 are
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currently reserved and must be zero. In the future the number of bytes
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used for the subsystem or handle ids might be increased.
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When a particular userspace process collects coverage via a common
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handle, kcov will collect coverage for each code section that is annotated
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to use the common handle obtained as kcov_handle from the current
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task_struct. However non common handles allow to collect coverage
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selectively from different subsystems.
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.. code-block:: c
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struct kcov_remote_arg {
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__u32 trace_mode;
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__u32 area_size;
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__u32 num_handles;
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__aligned_u64 common_handle;
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__aligned_u64 handles[0];
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};
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#define KCOV_INIT_TRACE _IOR('c', 1, unsigned long)
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#define KCOV_DISABLE _IO('c', 101)
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#define KCOV_REMOTE_ENABLE _IOW('c', 102, struct kcov_remote_arg)
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#define COVER_SIZE (64 << 10)
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#define KCOV_TRACE_PC 0
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#define KCOV_SUBSYSTEM_COMMON (0x00ull << 56)
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#define KCOV_SUBSYSTEM_USB (0x01ull << 56)
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#define KCOV_SUBSYSTEM_MASK (0xffull << 56)
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#define KCOV_INSTANCE_MASK (0xffffffffull)
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static inline __u64 kcov_remote_handle(__u64 subsys, __u64 inst)
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{
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if (subsys & ~KCOV_SUBSYSTEM_MASK || inst & ~KCOV_INSTANCE_MASK)
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return 0;
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return subsys | inst;
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}
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#define KCOV_COMMON_ID 0x42
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#define KCOV_USB_BUS_NUM 1
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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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struct kcov_remote_arg *arg;
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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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if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
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perror("ioctl"), exit(1);
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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 via common handle and from USB bus #1. */
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arg = calloc(1, sizeof(*arg) + sizeof(uint64_t));
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if (!arg)
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perror("calloc"), exit(1);
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arg->trace_mode = KCOV_TRACE_PC;
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arg->area_size = COVER_SIZE;
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arg->num_handles = 1;
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arg->common_handle = kcov_remote_handle(KCOV_SUBSYSTEM_COMMON,
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KCOV_COMMON_ID);
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arg->handles[0] = kcov_remote_handle(KCOV_SUBSYSTEM_USB,
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KCOV_USB_BUS_NUM);
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if (ioctl(fd, KCOV_REMOTE_ENABLE, arg))
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perror("ioctl"), free(arg), exit(1);
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free(arg);
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/*
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* Here the user needs to trigger execution of a kernel code section
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* that is either annotated with the common handle, or to trigger some
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* activity on USB bus #1.
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*/
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sleep(2);
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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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if (ioctl(fd, KCOV_DISABLE, 0))
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perror("ioctl"), exit(1);
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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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