event_buffer.c

/**
 * @file event_buffer.c
 *
 * @remark Copyright 2002 OProfile authors
 * @remark Read the file COPYING
 *
 * @author John Levon <levon@movementarian.org>
 *
 * This is the global event buffer that the user-space
 * daemon reads from. The event buffer is an untyped array
 * of unsigned longs. Entries are prefixed by the
 * escape value ESCAPE_CODE followed by an identifying code.
 */

#include <linux/vmalloc.h>
#include <linux/oprofile.h>
#include <linux/sched/signal.h>
#include <linux/capability.h>
#include <linux/dcookies.h>
#include <linux/fs.h>
#include <linux/uaccess.h>

#include "oprof.h"
#include "event_buffer.h"
#include "oprofile_stats.h"

DEFINE_MUTEX(buffer_mutex);

static unsigned long buffer_opened;
static DECLARE_WAIT_QUEUE_HEAD(buffer_wait);
static unsigned long *event_buffer;
static unsigned long buffer_size;
static unsigned long buffer_watershed;
static size_t buffer_pos;
/* atomic_t because wait_event checks it outside of buffer_mutex */
static atomic_t buffer_ready = ATOMIC_INIT(0);

/*
 * Add an entry to the event buffer. When we get near to the end we
 * wake up the process sleeping on the read() of the file. To protect
 * the event_buffer this function may only be called when buffer_mutex
 * is set.
 */
void add_event_entry(unsigned long value)
{
	/*
	 * This shouldn't happen since all workqueues or handlers are
	 * canceled or flushed before the event buffer is freed.
	 */
	if (!event_buffer) {
		WARN_ON_ONCE(1);
		return;
	}

	if (buffer_pos == buffer_size) {
		atomic_inc(&oprofile_stats.event_lost_overflow);
		return;
	}

	event_buffer[buffer_pos] = value;
	if (++buffer_pos == buffer_size - buffer_watershed) {
		atomic_set(&buffer_ready, 1);
		wake_up(&buffer_wait);
	}
}


/* Wake up the waiting process if any. This happens
 * on "echo 0 >/dev/oprofile/enable" so the daemon
 * processes the data remaining in the event buffer.
 */
void wake_up_buffer_waiter(void)
{
	mutex_lock(&buffer_mutex);
	atomic_set(&buffer_ready, 1);
	wake_up(&buffer_wait);
	mutex_unlock(&buffer_mutex);
}


int alloc_event_buffer(void)
{
	unsigned long flags;

	raw_spin_lock_irqsave(&oprofilefs_lock, flags);
	buffer_size = oprofile_buffer_size;
	buffer_watershed = oprofile_buffer_watershed;
	raw_spin_unlock_irqrestore(&oprofilefs_lock, flags);

	if (buffer_watershed >= buffer_size)
		return -EINVAL;

	buffer_pos = 0;
	event_buffer = vmalloc(array_size(buffer_size, sizeof(unsigned long)));
	if (!event_buffer)
		return -ENOMEM;

	return 0;
}


void free_event_buffer(void)
{
	mutex_lock(&buffer_mutex);
	vfree(event_buffer);
	buffer_pos = 0;
	event_buffer = NULL;
	mutex_unlock(&buffer_mutex);
}


static int event_buffer_open(struct inode *inode, struct file *file)
{
	int err = -EPERM;

	if (!capable(CAP_SYS_ADMIN))
		return -EPERM;

	if (test_and_set_bit_lock(0, &buffer_opened))
		return -EBUSY;

	/* Register as a user of dcookies
	 * to ensure they persist for the lifetime of
	 * the open event file
	 */
	err = -EINVAL;
	file->private_data = dcookie_register();
	if (!file->private_data)
		goto out;

	if ((err = oprofile_setup()))
		goto fail;

	/* NB: the actual start happens from userspace
	 * echo 1 >/dev/oprofile/enable
	 */

	return nonseekable_open(inode, file);

fail:
	dcookie_unregister(file->private_data);
out:
	__clear_bit_unlock(0, &buffer_opened);
	return err;
}


static int event_buffer_release(struct inode *inode, struct file *file)
{
	oprofile_stop();
	oprofile_shutdown();
	dcookie_unregister(file->private_data);
	buffer_pos = 0;
	atomic_set(&buffer_ready, 0);
	__clear_bit_unlock(0, &buffer_opened);
	return 0;
}


static ssize_t event_buffer_read(struct file *file, char __user *buf,
				 size_t count, loff_t *offset)
{
	int retval = -EINVAL;
	size_t const max = buffer_size * sizeof(unsigned long);

	/* handling partial reads is more trouble than it's worth */
	if (count != max || *offset)
		return -EINVAL;

	wait_event_interruptible(buffer_wait, atomic_read(&buffer_ready));

	if (signal_pending(current))
		return -EINTR;

	/* can't currently happen */
	if (!atomic_read(&buffer_ready))
		return -EAGAIN;

	mutex_lock(&buffer_mutex);

	/* May happen if the buffer is freed during pending reads. */
	if (!event_buffer) {
		retval = -EINTR;
		goto out;
	}

	atomic_set(&buffer_ready, 0);

	retval = -EFAULT;

	count = buffer_pos * sizeof(unsigned long);

	if (copy_to_user(buf, event_buffer, count))
		goto out;

	retval = count;
	buffer_pos = 0;

out:
	mutex_unlock(&buffer_mutex);
	return retval;
}

const struct file_operations event_buffer_fops = {
	.open		= event_buffer_open,
	.release	= event_buffer_release,
	.read		= event_buffer_read,
	.llseek		= no_llseek,
};