340 lines
11 KiB
ReStructuredText
340 lines
11 KiB
ReStructuredText
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 6.1.0 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>
|
|
#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);
|
|
/* 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++) {
|
|
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 (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); as well as from soft
|
|
interrupts.
|
|
|
|
To enable collecting coverage from a global background thread or from a
|
|
softirq, 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 process collects coverage 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
|
|
|
|
/* 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;
|
|
}
|