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Diffstat (limited to 'Documentation/dev-tools/kcov.rst')
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diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst index 42b612677799..36890b026e77 100644 --- a/Documentation/dev-tools/kcov.rst +++ b/Documentation/dev-tools/kcov.rst @@ -34,6 +34,7 @@ Profiling data will only become accessible once debugfs has been mounted:: Coverage collection ------------------- + The following program demonstrates coverage collection from within a test program using kcov: @@ -128,6 +129,7 @@ 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 @@ -202,3 +204,130 @@ Comparison operands collection is similar to coverage collection: Note that the kcov modes (coverage collection 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). + +To enable collecting coverage from a global background thread, 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 proccess collects coverage by 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. + +.. code-block:: c + + struct kcov_remote_arg { + unsigned trace_mode; + unsigned area_size; + unsigned num_handles; + uint64_t common_handle; + uint64_t 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; + } |