Date:	Thu, 14 Jan 2016 13:50:15 +0300
From:	Andrey Ryabinin <ryabinin.a.a@...il.com>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, drysdale@...gle.com,
	Kees Cook <keescook@...gle.com>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>, taviso@...gle.com,
	bhelgaas@...gle.com, syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH v2] kernel: add kcov code coverage

2016-01-13 15:48 GMT+03:00 Dmitry Vyukov <dvyukov@...gle.com>:

> +       /* 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. */
> +       for (i = 0; i < n; i++)
> +               printf("0xffffffff%0lx\n", (unsigned long)cover[i + 1]);

This works only for x86-64.
Probably there is no simple way to make this arch-independent with
32-bit values.


> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 61aa9bb..9e9e9f6 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1807,6 +1807,16 @@ struct task_struct {
>         /* bitmask and counter of trace recursion */
>         unsigned long trace_recursion;
>  #endif /* CONFIG_TRACING */
> +#ifdef CONFIG_KCOV
> +       /* Coverage collection mode enabled for this task (0 if disabled). */
> +       int             kcov_mode;
> +       /* Size of the kcov_area. */
> +       unsigned long   kcov_size;

Could be just 'unsigned'

> +       /* Buffer for coverage collection. */
> +       void            *kcov_area;

So, these fields above are duplicates the same fields from kcov struct.
Consider embedding kcov struct (since it's relatively small) into task_struct.


> +       /* kcov desciptor wired with this task or NULL. */
> +       void            *kcov;

struct kcov *kcov;

> +#endif
>  #ifdef CONFIG_MEMCG
>         struct mem_cgroup *memcg_in_oom;
>         gfp_t memcg_oom_gfp_mask;



> diff --git a/kernel/kcov/Makefile b/kernel/kcov/Makefile
> new file mode 100644
> index 0000000..88892b7
> --- /dev/null
> +++ b/kernel/kcov/Makefile
> @@ -0,0 +1,5 @@
> +KCOV_INSTRUMENT := n
> +KASAN_SANITIZE := n
> +
> +obj-y := kcov.o
> +

New directory for just one file seems like overkill.

> diff --git a/kernel/kcov/kcov.c b/kernel/kcov/kcov.c
> new file mode 100644
> index 0000000..05ec361
> --- /dev/null
> +++ b/kernel/kcov/kcov.c

> +
> +enum kcov_mode {
> +       /* Tracing coverage collection mode.
> +        * Covered PCs are collected in a per-task buffer.
> +        */

Multi-line comment format is following:
/*
 * <text>
 * <text>
 */


> +       kcov_mode_trace = 1,
> +};
> +
> +/* kcov descriptor (one per opened debugfs file). */
> +struct kcov {
> +       /* Reference counter. We keep one for:
> +        *  - opened file descriptor
> +        *  - mmapped region (including copies after fork)
> +        *  - task with enabled coverage (we can't unwire it from another task)
> +        */
> +       atomic_t                rc;
> +       /* The lock protects state transitions of the descriptor:

Just say what fields are protected by this lock. Comment bellow looks
unrelated to the lock itself,
I think it should be somewhere else.

> +        *  - initial state after open()
> +        *  - then there must be a single ioctl(KCOV_INIT_TRACE) call
> +        *  - then, mmap() call (several calls are allowed but not useful)
> +        *  - then, repeated enable/disable for a task (only one task a time
> +        *    allowed
> +        */
> +       spinlock_t              lock;
> +       enum kcov_mode          mode;
> +       unsigned long           size;
> +       void                    *area;
> +       struct task_struct      *t;
> +};
> +
> +/* Entry point from instrumented code.
> + * This is called once per basic-block/edge.
> + */
> +void __sanitizer_cov_trace_pc(void)
> +{
> +       struct task_struct *t;
> +       enum kcov_mode mode;
> +
> +       t = current;
> +       /* We are interested in code coverage as a function of a syscall inputs,
> +        * so we ignore code executed in interrupts.
> +        */
> +       if (!t || in_interrupt())
> +               return;
> +       mode = READ_ONCE(t->kcov_mode);
> +       if (mode == kcov_mode_trace) {
> +               u32 *area;
> +               u32 pos;
> +
> +               /* There is some code that runs in interrupts but for which
> +                * in_interrupt() returns false (e.g. preempt_schedule_irq()).
> +                * READ_ONCE()/barrier() effectively provides load-acquire wrt
> +                * interrupts, there are paired barrier()/WRITE_ONCE() in
> +                * kcov_ioctl_locked().
> +                */
> +               barrier();
> +               area = t->kcov_area;
> +               /* The first u32 is number of subsequent PCs. */
> +               pos = READ_ONCE(area[0]) + 1;
> +               if (likely(pos < t->kcov_size)) {
> +                       area[pos] = (u32)_RET_IP_;
> +                       WRITE_ONCE(area[0], pos);

Note that this works only for cache-coherent architectures.
For incoherent arches you'll need to flush_dcache_page() somewhere.
Perhaps it could be done on exit to userspace, since flushing here is
certainly an overkill.

> +               }
> +       }
> +}
> +EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
> +
> +static void kcov_put(struct kcov *kcov)
> +{
> +       if (atomic_dec_and_test(&kcov->rc)) {
> +               vfree(kcov->area);
> +               kfree(kcov);
> +       }
> +}
> +
> +void kcov_task_init(struct task_struct *t)
> +{
> +       t->kcov_mode = 0;
> +       t->kcov_size = 0;
> +       t->kcov_area = NULL;
> +       t->kcov = NULL;
> +}
> +
> +void kcov_task_exit(struct task_struct *t)
> +{
> +       struct kcov *kcov;
> +
> +       kcov = t->kcov;
> +       if (kcov == NULL)
> +               return;
> +       spin_lock(&kcov->lock);
> +       BUG_ON(kcov->t != t);

WARN_ON please. It's not a fatal condition, so you should not kill the
machine here..

And please and reconsider other BUG_ONs too.
Use BUG/BUG_ON iff you can't do anything but killing machine.

> +       /* Just to not leave dangling references behind. */
> +       kcov_task_init(t);
> +       kcov->t = NULL;
> +       spin_unlock(&kcov->lock);
> +       kcov_put(kcov);
> +}
> +
> +static int kcov_vm_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
> +{
> +       struct kcov *kcov;
> +       unsigned long off;
> +       struct page *page;
> +
> +       /* Map the preallocated kcov->area. */
> +       kcov = vma->vm_file->private_data;
> +       off = vmf->pgoff << PAGE_SHIFT;
> +       if (off >= kcov->size * sizeof(u32))
> +               return -1;

return VM_FAULT_SIGSEGV;

> +       page = vmalloc_to_page(kcov->area + off);
> +       get_page(page);
> +       vmf->page = page;
> +       return 0;
> +}
> +
> +static void kcov_unmap(struct vm_area_struct *vma)
> +{
> +       kcov_put(vma->vm_file->private_data);
> +}
> +
> +static void kcov_map_copied(struct vm_area_struct *vma)
> +{
> +       struct kcov *kcov;
> +
> +       kcov = vma->vm_file->private_data;
> +       atomic_inc(&kcov->rc);

Add kcov_get(struct kcov *) helper. It much better pairs with kcov_put().

> +}
> +
> +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;
> +
> +       /* Can't call vmalloc_user() under a spinlock. */

This comment doesn't bring any valuable information.

> +       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().
> +        */
> +       atomic_inc(&kcov->rc);
> +       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.
> +                */
> +               if (arg < 256 || arg > (128<<20) || arg & (arg - 1))

Use constants/defines with some meaningful name.
And for the last condition: !is_power_of_2(arg).
BTW why it has to be power of 2?

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