Re: [PATCH v3] kernel: add kcov code coverage

Kirill A. Shutemov
2016-01-15T12:42:11Z

On Thu, Jan 14, 2016 at 03:22:21PM +0100, Dmitry Vyukov wrote:
> +#include <stdio.h>
> +#include <stddef.h>
> +#include <stdint.h>
> +#include <sys/types.h>
> +#include <sys/stat.h>
> +#include <sys/ioctl.h>
> +#include <sys/mman.h>
> +#include <fcntl.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)
> +
> +int main(int argc, char **argv)
> +{
> + int fd;
> + uint32_t *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");
> + /* Setup trace mode and trace size. */
> + if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
> + perror("ioctl");
> + /* Mmap buffer shared between kernel- and user-space. */
> + cover = (uint32_t*)mmap(NULL, COVER_SIZE * sizeof(uint32_t),
> + PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> + if ((void*)cover == MAP_FAILED)
> + perror("mmap");
> + /* Enable coverage collection on the current thread. */
> + if (ioctl(fd, KCOV_ENABLE, 0))
> + perror("ioctl");
> + /* 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);
> + /* PCs are shorten to uint32_t, so we need to restore the upper part. */

Why do we do this in the first place? I don't think it's very portable.

I would rather trancate upper bits on 32-bit system.

> + for (i = 0; i < n; i++)
> + printf("0xffffffff%0lx\n", (unsigned long)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");
> + /* Free resources. */
> + if (munmap(cover, COVER_SIZE * sizeof(uint32_t)))
> + perror("munmap");
> + if (close(fd))
> + perror("close");
> + return 0;
> +}

....

> +static void kcov_unmap(struct vm_area_struct *vma)
> +{
> + kcov_put(vma->vm_file->private_data);

Can be dropped, if I'm right about mmap counterpart.

> +}
> +
> +static void kcov_map_copied(struct vm_area_struct *vma)
> +{
> + struct kcov *kcov;
> +
> + kcov = vma->vm_file->private_data;
> + kcov_get(kcov);

As with kcov_mmap(), I think this is redundant.

> +}
> +
> +static const struct vm_operations_struct kcov_vm_ops = {
> + .fault = kcov_vm_fault,
> + .close = kcov_unmap,
> + /* Called on fork()/clone() when the mapping is copied. */
> + .open = kcov_map_copied,
> +};
> +
> +static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
> +{
> + int res = 0;
> + void *area;
> + struct kcov *kcov = vma->vm_file->private_data;
> +
> + area = vmalloc_user(vma->vm_end - vma->vm_start);
> + if (!area)
> + return -ENOMEM;
> +
> + spin_lock(&kcov->lock);
> + if (kcov->mode == 0 || vma->vm_pgoff != 0 ||
> + vma->vm_end - vma->vm_start != kcov->size * sizeof(u32)) {
> + res = -EINVAL;
> + goto exit;
> + }
> + if (!kcov->area) {
> + kcov->area = area;
> + area = NULL;
> + }
> + /*
> + * The file drops a reference on close, but the file
> + * descriptor can be closed with the mmaping still alive so we keep
> + * a reference for those. This is put in kcov_unmap().

No, this is other way around: mmap takes file reference (see get_file() in
mmap_region()). So kcov_close() cannot happen, until last mmap gone.
I think this kcov_get() is redundant.

> + */
> + kcov_get(kcov);
> + vma->vm_ops = &kcov_vm_ops;
> +exit:
> + spin_unlock(&kcov->lock);
> + vfree(area);
> + return res;
> +}
> +
> +static int kcov_open(struct inode *inode, struct file *filep)
> +{
> + struct kcov *kcov;
> +
> + kcov = kzalloc(sizeof(*kcov), GFP_KERNEL);
> + if (!kcov)
> + return -ENOMEM;
> + atomic_set(&kcov->rc, 1);
> + spin_lock_init(&kcov->lock);
> + filep->private_data = kcov;
> + return nonseekable_open(inode, filep);
> +}
> +
> +static int kcov_close(struct inode *inode, struct file *filep)
> +{
> + kcov_put(filep->private_data);
> + return 0;
> +}
> +
> +static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
> + unsigned long arg)
> +{
> + struct task_struct *t;
> +
> + switch (cmd) {
> + case KCOV_INIT_TRACE:
> + /*
> + * Enable kcov in trace mode and setup buffer size.
> + * Must happen before anything else.
> + * Size must be at least 2 to hold current position and one PC.
> + */
> + if (arg < 2 || arg > INT_MAX)
> + return -EINVAL;
> + if (kcov->mode != 0)
> + return -EBUSY;
> + kcov->mode = kcov_mode_trace;
> + kcov->size = arg;
> + return 0;
> + case KCOV_ENABLE:
> + /*
> + * Enable coverage for the current task.
> + * At this point user must have been enabled trace mode,
> + * and mmapped the file. Coverage collection is disabled only
> + * at task exit or voluntary by KCOV_DISABLE. After that it can
> + * be enabled for another task.
> + */
> + if (kcov->mode == 0 || kcov->area == NULL)
> + return -EINVAL;
> + if (kcov->t != NULL)
> + return -EBUSY;
> + t = current;
> + /* Cache in task struct for performance. */
> + t->kcov_size = kcov->size;
> + t->kcov_area = kcov->area;
> + /* See comment in __sanitizer_cov_trace_pc(). */
> + barrier();
> + WRITE_ONCE(t->kcov_mode, kcov->mode);
> + t->kcov = kcov;
> + kcov->t = t;
> + /* This is put either in kcov_task_exit() or in KCOV_DISABLE. */
> + kcov_get(kcov);
> + return 0;
> + case KCOV_DISABLE:
> + /* Disable coverage for the current task. */
> + if (current->kcov != kcov)
> + return -EINVAL;
> + t = current;
> + if (WARN_ON(kcov->t != t))
> + return -EINVAL;
> + kcov_task_init(t);
> + kcov->t = NULL;
> + kcov_put(kcov);
> + return 0;

You probably want to verify that arg is 0 for enable/disable just in case
if you would want to pass addtional information in the future.

> + default:
> + return -EINVAL;
> + }
> +}
> +
> +static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
> +{
> + struct kcov *kcov;
> + int res;
> +
> + kcov = filep->private_data;
> + spin_lock(&kcov->lock);
> + res = kcov_ioctl_locked(kcov, cmd, arg);
> + spin_unlock(&kcov->lock);
> + return res;
> +}
> +
> +static const struct file_operations kcov_fops = {
> + .open = kcov_open,
> + .unlocked_ioctl = kcov_ioctl,
> + .mmap = kcov_mmap,
> + .release = kcov_close,
> +};
> +
> +static int __init kcov_init(void)
> +{
> + if (!debugfs_create_file("kcov", 0666, NULL, NULL, &kcov_fops)) {
> + pr_err("init failed\n");

I guess you should be more specific about which init failed.

> + return 1;
> + }
> + return 0;
> +}
> +
> +device_initcall(kcov_init);
--
Kirill A. Shutemov