linux-brain/kernel/kcov.c

// SPDX-License-Identifier: GPL-2.0
#define pr_fmt(fmt) "kcov: " fmt

#define DISABLE_BRANCH_PROFILING
#include <linux/atomic.h>
#include <linux/compiler.h>
#include <linux/errno.h>
#include <linux/export.h>
#include <linux/types.h>
#include <linux/file.h>
#include <linux/fs.h>
#include <linux/init.h>
#include <linux/mm.h>
#include <linux/preempt.h>
#include <linux/printk.h>
#include <linux/sched.h>
#include <linux/slab.h>
#include <linux/spinlock.h>
#include <linux/vmalloc.h>
#include <linux/debugfs.h>
#include <linux/uaccess.h>
#include <linux/kcov.h>
#include <asm/setup.h>

/* Number of 64-bit words written per one comparison: */
#define KCOV_WORDS_PER_CMP 4

/*
 * kcov descriptor (one per opened debugfs file).
 * State transitions of the descriptor:
 *  - 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, ioctl(KCOV_ENABLE, arg), where arg is
 *	KCOV_TRACE_PC - to trace only the PCs
 *	or
 *	KCOV_TRACE_CMP - to trace only the comparison operands
 *  - then, ioctl(KCOV_DISABLE) to disable the task.
 * Enabling/disabling ioctls can be repeated (only one task a time allowed).
 */
struct kcov {
	/*
	 * Reference counter. We keep one for:
	 *  - opened file descriptor
	 *  - task with enabled coverage (we can't unwire it from another task)
	 */
	atomic_t		refcount;
	/* The lock protects mode, size, area and t. */
	spinlock_t		lock;
	enum kcov_mode		mode;
	/* Size of arena (in long's for KCOV_MODE_TRACE). */
	unsigned		size;
	/* Coverage buffer shared with user space. */
	void			*area;
	/* Task for which we collect coverage, or NULL. */
	struct task_struct	*t;
};

static bool check_kcov_mode(enum kcov_mode needed_mode, struct task_struct *t)
{
	unsigned int mode;

	/*
	 * We are interested in code coverage as a function of a syscall inputs,
	 * so we ignore code executed in interrupts.
	 */
	if (!in_task())
		return false;
	mode = READ_ONCE(t->kcov_mode);
	/*
	 * 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();
	return mode == needed_mode;
}

static unsigned long canonicalize_ip(unsigned long ip)
{
#ifdef CONFIG_RANDOMIZE_BASE
	ip -= kaslr_offset();
#endif
	return ip;
}

/*
 * Entry point from instrumented code.
 * This is called once per basic-block/edge.
 */
void notrace __sanitizer_cov_trace_pc(void)
{
	struct task_struct *t;
	unsigned long *area;
	unsigned long ip = canonicalize_ip(_RET_IP_);
	unsigned long pos;

	t = current;
	if (!check_kcov_mode(KCOV_MODE_TRACE_PC, t))
		return;

	area = t->kcov_area;
	/* The first 64-bit word is the number of subsequent PCs. */
	pos = READ_ONCE(area[0]) + 1;
	if (likely(pos < t->kcov_size)) {
		area[pos] = ip;
		WRITE_ONCE(area[0], pos);
	}
}
EXPORT_SYMBOL(__sanitizer_cov_trace_pc);

#ifdef CONFIG_KCOV_ENABLE_COMPARISONS
static void write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip)
{
	struct task_struct *t;
	u64 *area;
	u64 count, start_index, end_pos, max_pos;

	t = current;
	if (!check_kcov_mode(KCOV_MODE_TRACE_CMP, t))
		return;

	ip = canonicalize_ip(ip);

	/*
	 * We write all comparison arguments and types as u64.
	 * The buffer was allocated for t->kcov_size unsigned longs.
	 */
	area = (u64 *)t->kcov_area;
	max_pos = t->kcov_size * sizeof(unsigned long);

	count = READ_ONCE(area[0]);

	/* Every record is KCOV_WORDS_PER_CMP 64-bit words. */
	start_index = 1 + count * KCOV_WORDS_PER_CMP;
	end_pos = (start_index + KCOV_WORDS_PER_CMP) * sizeof(u64);
	if (likely(end_pos <= max_pos)) {
		area[start_index] = type;
		area[start_index + 1] = arg1;
		area[start_index + 2] = arg2;
		area[start_index + 3] = ip;
		WRITE_ONCE(area[0], count + 1);
	}
}

void notrace __sanitizer_cov_trace_cmp1(u8 arg1, u8 arg2)
{
	write_comp_data(KCOV_CMP_SIZE(0), arg1, arg2, _RET_IP_);
}
EXPORT_SYMBOL(__sanitizer_cov_trace_cmp1);

void notrace __sanitizer_cov_trace_cmp2(u16 arg1, u16 arg2)
{
	write_comp_data(KCOV_CMP_SIZE(1), arg1, arg2, _RET_IP_);
}
EXPORT_SYMBOL(__sanitizer_cov_trace_cmp2);

void notrace __sanitizer_cov_trace_cmp4(u32 arg1, u32 arg2)
{
	write_comp_data(KCOV_CMP_SIZE(2), arg1, arg2, _RET_IP_);
}
EXPORT_SYMBOL(__sanitizer_cov_trace_cmp4);

void notrace __sanitizer_cov_trace_cmp8(u64 arg1, u64 arg2)
{
	write_comp_data(KCOV_CMP_SIZE(3), arg1, arg2, _RET_IP_);
}
EXPORT_SYMBOL(__sanitizer_cov_trace_cmp8);

void notrace __sanitizer_cov_trace_const_cmp1(u8 arg1, u8 arg2)
{
	write_comp_data(KCOV_CMP_SIZE(0) | KCOV_CMP_CONST, arg1, arg2,
			_RET_IP_);
}
EXPORT_SYMBOL(__sanitizer_cov_trace_const_cmp1);

void notrace __sanitizer_cov_trace_const_cmp2(u16 arg1, u16 arg2)
{
	write_comp_data(KCOV_CMP_SIZE(1) | KCOV_CMP_CONST, arg1, arg2,
			_RET_IP_);
}
EXPORT_SYMBOL(__sanitizer_cov_trace_const_cmp2);

void notrace __sanitizer_cov_trace_const_cmp4(u32 arg1, u32 arg2)
{
	write_comp_data(KCOV_CMP_SIZE(2) | KCOV_CMP_CONST, arg1, arg2,
			_RET_IP_);
}
EXPORT_SYMBOL(__sanitizer_cov_trace_const_cmp4);

void notrace __sanitizer_cov_trace_const_cmp8(u64 arg1, u64 arg2)
{
	write_comp_data(KCOV_CMP_SIZE(3) | KCOV_CMP_CONST, arg1, arg2,
			_RET_IP_);
}
EXPORT_SYMBOL(__sanitizer_cov_trace_const_cmp8);

void notrace __sanitizer_cov_trace_switch(u64 val, u64 *cases)
{
	u64 i;
	u64 count = cases[0];
	u64 size = cases[1];
	u64 type = KCOV_CMP_CONST;

	switch (size) {
	case 8:
		type |= KCOV_CMP_SIZE(0);
		break;
	case 16:
		type |= KCOV_CMP_SIZE(1);
		break;
	case 32:
		type |= KCOV_CMP_SIZE(2);
		break;
	case 64:
		type |= KCOV_CMP_SIZE(3);
		break;
	default:
		return;
	}
	for (i = 0; i < count; i++)
		write_comp_data(type, cases[i + 2], val, _RET_IP_);
}
EXPORT_SYMBOL(__sanitizer_cov_trace_switch);
#endif /* ifdef CONFIG_KCOV_ENABLE_COMPARISONS */

static void kcov_get(struct kcov *kcov)
{
	atomic_inc(&kcov->refcount);
}

static void kcov_put(struct kcov *kcov)
{
	if (atomic_dec_and_test(&kcov->refcount)) {
		vfree(kcov->area);
		kfree(kcov);
	}
}

void kcov_task_init(struct task_struct *t)
{
	WRITE_ONCE(t->kcov_mode, KCOV_MODE_DISABLED);
	barrier();
	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);
	if (WARN_ON(kcov->t != t)) {
		spin_unlock(&kcov->lock);
		return;
	}
	/* Just to not leave dangling references behind. */
	kcov_task_init(t);
	kcov->t = NULL;
	kcov->mode = KCOV_MODE_INIT;
	spin_unlock(&kcov->lock);
	kcov_put(kcov);
}

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;
	unsigned long size, off;
	struct page *page;

	area = vmalloc_user(vma->vm_end - vma->vm_start);
	if (!area)
		return -ENOMEM;

	spin_lock(&kcov->lock);
	size = kcov->size * sizeof(unsigned long);
	if (kcov->mode != KCOV_MODE_INIT || vma->vm_pgoff != 0 ||
	    vma->vm_end - vma->vm_start != size) {
		res = -EINVAL;
		goto exit;
	}
	if (!kcov->area) {
		kcov->area = area;
		vma->vm_flags |= VM_DONTEXPAND;
		spin_unlock(&kcov->lock);
		for (off = 0; off < size; off += PAGE_SIZE) {
			page = vmalloc_to_page(kcov->area + off);
			if (vm_insert_page(vma, vma->vm_start + off, page))
				WARN_ONCE(1, "vm_insert_page() failed");
		}
		return 0;
	}
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;
	kcov->mode = KCOV_MODE_DISABLED;
	atomic_set(&kcov->refcount, 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;
}

/*
 * Fault in a lazily-faulted vmalloc area before it can be used by
 * __santizer_cov_trace_pc(), to avoid recursion issues if any code on the
 * vmalloc fault handling path is instrumented.
 */
static void kcov_fault_in_area(struct kcov *kcov)
{
	unsigned long stride = PAGE_SIZE / sizeof(unsigned long);
	unsigned long *area = kcov->area;
	unsigned long offset;

	for (offset = 0; offset < kcov->size; offset += stride)
		READ_ONCE(area[offset]);
}

static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
			     unsigned long arg)
{
	struct task_struct *t;
	unsigned long size, unused;

	switch (cmd) {
	case KCOV_INIT_TRACE:
		/*
		 * Enable kcov in trace mode and setup buffer size.
		 * Must happen before anything else.
		 */
		if (kcov->mode != KCOV_MODE_DISABLED)
			return -EBUSY;
		/*
		 * Size must be at least 2 to hold current position and one PC.
		 * Later we allocate size * sizeof(unsigned long) memory,
		 * that must not overflow.
		 */
		size = arg;
		if (size < 2 || size > INT_MAX / sizeof(unsigned long))
			return -EINVAL;
		kcov->size = size;
		kcov->mode = KCOV_MODE_INIT;
		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 != KCOV_MODE_INIT || !kcov->area)
			return -EINVAL;
		t = current;
		if (kcov->t != NULL || t->kcov != NULL)
			return -EBUSY;
		if (arg == KCOV_TRACE_PC)
			kcov->mode = KCOV_MODE_TRACE_PC;
		else if (arg == KCOV_TRACE_CMP)
#ifdef CONFIG_KCOV_ENABLE_COMPARISONS
			kcov->mode = KCOV_MODE_TRACE_CMP;
#else
		return -ENOTSUPP;
#endif
		else
			return -EINVAL;
		kcov_fault_in_area(kcov);
		/* Cache in task struct for performance. */
		t->kcov_size = kcov->size;
		t->kcov_area = kcov->area;
		/* See comment in check_kcov_mode(). */
		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. */
		unused = arg;
		if (unused != 0 || current->kcov != kcov)
			return -EINVAL;
		t = current;
		if (WARN_ON(kcov->t != t))
			return -EINVAL;
		kcov_task_init(t);
		kcov->t = NULL;
		kcov->mode = KCOV_MODE_INIT;
		kcov_put(kcov);
		return 0;
	default:
		return -ENOTTY;
	}
}

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,
	.compat_ioctl	= kcov_ioctl,
	.mmap		= kcov_mmap,
	.release        = kcov_close,
};

static int __init kcov_init(void)
{
	/*
	 * The kcov debugfs file won't ever get removed and thus,
	 * there is no need to protect it against removal races. The
	 * use of debugfs_create_file_unsafe() is actually safe here.
	 */
	if (!debugfs_create_file_unsafe("kcov", 0600, NULL, NULL, &kcov_fops)) {
		pr_err("failed to create kcov in debugfs\n");
		return -ENOMEM;
	}
	return 0;
}

device_initcall(kcov_init);