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This change extends kcov remote coverage support to allow collecting coverage from soft interrupts in addition to kernel background threads. To collect coverage from code that is executed in softirq context, a part of that code has to be annotated with kcov_remote_start/stop() in a similar way as how it is done for global kernel background threads. Then the handle used for the annotations has to be passed to the KCOV_REMOTE_ENABLE ioctl. Internally this patch adjusts the __sanitizer_cov_trace_pc() compiler inserted callback to not bail out when called from softirq context. kcov_remote_start/stop() are updated to save/restore the current per task kcov state in a per-cpu area (in case the softirq came when the kernel was already collecting coverage in task context). Coverage from softirqs is collected into pre-allocated per-cpu areas, whose size is controlled by the new CONFIG_KCOV_IRQ_AREA_SIZE. [andreyknvl@google.com: turn current->kcov_softirq into unsigned int to fix objtool warning] Link: http://lkml.kernel.org/r/841c778aa3849c5cb8c3761f56b87ce653a88671.1585233617.git.andreyknvl@google.com Signed-off-by: Andrey Konovalov <andreyknvl@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Reviewed-by: Dmitry Vyukov <dvyukov@google.com> Cc: Alan Stern <stern@rowland.harvard.edu> Cc: Alexander Potapenko <glider@google.com> Cc: Andrey Konovalov <andreyknvl@gmail.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Marco Elver <elver@google.com> Link: http://lkml.kernel.org/r/469bd385c431d050bc38a593296eff4baae50666.1584655448.git.andreyknvl@google.com Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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 proccess 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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