mem.c

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		realp += sz;
		count -= sz;
		written += sz;
	}
#endif

	while (count > 0) {
		char *ptr;
		/*
		 * Handle first page in case it's not aligned
		 */
		if (-realp & (PAGE_SIZE - 1))
			sz = -realp & (PAGE_SIZE - 1);
		else
			sz = PAGE_SIZE;

		sz = min_t(unsigned long, sz, count);

		/*
		 * On ia64 if a page has been mapped somewhere as
		 * uncached, then it must also be accessed uncached
		 * by the kernel or data corruption may occur
		 */
		ptr = xlate_dev_kmem_ptr(p);

		copied = copy_from_user(ptr, buf, sz);
		if (copied) {
			written += sz - copied;
			if (written)
				break;
			return -EFAULT;
		}
		buf += sz;
		p += sz;
		realp += sz;
		count -= sz;
		written += sz;
	}

	*ppos += written;
	return written;
}


/*
 * This function writes to the *virtual* memory as seen by the kernel.
 */
static ssize_t write_kmem(struct file * file, const char __user * buf, 
			  size_t count, loff_t *ppos)
{
	unsigned long p = *ppos;
	ssize_t wrote = 0;
	ssize_t virtr = 0;
	ssize_t written;
	char * kbuf; /* k-addr because vwrite() takes vmlist_lock rwlock */

	if (p < (unsigned long) high_memory) {

		wrote = count;
		if (count > (unsigned long) high_memory - p)
			wrote = (unsigned long) high_memory - p;

		written = do_write_kmem((void*)p, p, buf, wrote, ppos);
		if (written != wrote)
			return written;
		wrote = written;
		p += wrote;
		buf += wrote;
		count -= wrote;
	}

	if (count > 0) {
		kbuf = (char *)__get_free_page(GFP_KERNEL);
		if (!kbuf)
			return wrote ? wrote : -ENOMEM;
		while (count > 0) {
			int len = count;

			if (len > PAGE_SIZE)
				len = PAGE_SIZE;
			if (len) {
				written = copy_from_user(kbuf, buf, len);
				if (written) {
					if (wrote + virtr)
						break;
					free_page((unsigned long)kbuf);
					return -EFAULT;
				}
			}
			len = vwrite(kbuf, (char *)p, len);
			count -= len;
			buf += len;
			virtr += len;
			p += len;
		}
		free_page((unsigned long)kbuf);
	}

 	*ppos = p;
 	return virtr + wrote;
}

#ifdef CONFIG_DEVPORT
static ssize_t read_port(struct file * file, char __user * buf,
			 size_t count, loff_t *ppos)
{
	unsigned long i = *ppos;
	char __user *tmp = buf;

	if (!access_ok(VERIFY_WRITE, buf, count))
		return -EFAULT; 
	while (count-- > 0 && i < 65536) {
		if (__put_user(inb(i),tmp) < 0) 
			return -EFAULT;  
		i++;
		tmp++;
	}
	*ppos = i;
	return tmp-buf;
}

static ssize_t write_port(struct file * file, const char __user * buf,
			  size_t count, loff_t *ppos)
{
	unsigned long i = *ppos;
	const char __user * tmp = buf;

	if (!access_ok(VERIFY_READ,buf,count))
		return -EFAULT;
	while (count-- > 0 && i < 65536) {
		char c;
		if (__get_user(c, tmp)) {
			if (tmp > buf)
				break;
			return -EFAULT; 
		}
		outb(c,i);
		i++;
		tmp++;
	}
	*ppos = i;
	return tmp-buf;
}
#endif

static ssize_t read_null(struct file * file, char __user * buf,
			 size_t count, loff_t *ppos)
{
	return 0;
}

static ssize_t write_null(struct file * file, const char __user * buf,
			  size_t count, loff_t *ppos)
{
	return count;
}

static int pipe_to_null(struct pipe_inode_info *info, struct pipe_buffer *buf,
			struct splice_desc *sd)
{
	return sd->len;
}

static ssize_t splice_write_null(struct pipe_inode_info *pipe,struct file *out,
				 loff_t *ppos, size_t len, unsigned int flags)
{
	return splice_from_pipe(pipe, out, ppos, len, flags, pipe_to_null);
}

static ssize_t read_zero(struct file * file, char __user * buf, 
			 size_t count, loff_t *ppos)
{
	size_t written;

	if (!count)
		return 0;

	if (!access_ok(VERIFY_WRITE, buf, count))
		return -EFAULT;

	written = 0;
	while (count) {
		unsigned long unwritten;
		size_t chunk = count;

		if (chunk > PAGE_SIZE)
			chunk = PAGE_SIZE;	/* Just for latency reasons */
		unwritten = clear_user(buf, chunk);
		written += chunk - unwritten;
		if (unwritten)
			break;
		buf += chunk;
		count -= chunk;
		cond_resched();
	}
	return written ? written : -EFAULT;
}

static int mmap_zero(struct file * file, struct vm_area_struct * vma)
{
#ifndef CONFIG_MMU
	return -ENOSYS;
#endif
	if (vma->vm_flags & VM_SHARED)
		return shmem_zero_setup(vma);
	return 0;
}

static ssize_t write_full(struct file * file, const char __user * buf,
			  size_t count, loff_t *ppos)
{
	return -ENOSPC;
}

/*
 * Special lseek() function for /dev/null and /dev/zero.  Most notably, you
 * can fopen() both devices with "a" now.  This was previously impossible.
 * -- SRB.
 */

static loff_t null_lseek(struct file * file, loff_t offset, int orig)
{
	return file->f_pos = 0;
}

/*
 * The memory devices use the full 32/64 bits of the offset, and so we cannot
 * check against negative addresses: they are ok. The return value is weird,
 * though, in that case (0).
 *
 * also note that seeking relative to the "end of file" isn't supported:
 * it has no meaning, so it returns -EINVAL.
 */
static loff_t memory_lseek(struct file * file, loff_t offset, int orig)
{
	loff_t ret;

	mutex_lock(&file->f_path.dentry->d_inode->i_mutex);
	switch (orig) {
		case 0:
			file->f_pos = offset;
			ret = file->f_pos;
			force_successful_syscall_return();
			break;
		case 1:
			file->f_pos += offset;
			ret = file->f_pos;
			force_successful_syscall_return();
			break;
		default:
			ret = -EINVAL;
	}
	mutex_unlock(&file->f_path.dentry->d_inode->i_mutex);
	return ret;
}

static int open_port(struct inode * inode, struct file * filp)
{
	return capable(CAP_SYS_RAWIO) ? 0 : -EPERM;
}

#define zero_lseek	null_lseek
#define full_lseek      null_lseek
#define write_zero	write_null
#define read_full       read_zero
#define open_mem	open_port
#define open_kmem	open_mem
#define open_oldmem	open_mem

static const struct file_operations mem_fops = {
	.llseek		= memory_lseek,
	.read		= read_mem,
	.write		= write_mem,
	.mmap		= mmap_mem,
	.open		= open_mem,
	.get_unmapped_area = get_unmapped_area_mem,
};

static const struct file_operations kmem_fops = {
	.llseek		= memory_lseek,
	.read		= read_kmem,
	.write		= write_kmem,
	.mmap		= mmap_kmem,
	.open		= open_kmem,
	.get_unmapped_area = get_unmapped_area_mem,
};

static const struct file_operations null_fops = {
	.llseek		= null_lseek,
	.read		= read_null,
	.write		= write_null,
	.splice_write	= splice_write_null,
};

#ifdef CONFIG_DEVPORT
static const struct file_operations port_fops = {
	.llseek		= memory_lseek,
	.read		= read_port,
	.write		= write_port,
	.open		= open_port,
};
#endif

static const struct file_operations zero_fops = {
	.llseek		= zero_lseek,
	.read		= read_zero,
	.write		= write_zero,
	.mmap		= mmap_zero,
};

/*
 * capabilities for /dev/zero
 * - permits private mappings, "copies" are taken of the source of zeros
 */
static struct backing_dev_info zero_bdi = {
	.capabilities	= BDI_CAP_MAP_COPY,
};

static const struct file_operations full_fops = {
	.llseek		= full_lseek,
	.read		= read_full,
	.write		= write_full,
};

#ifdef CONFIG_CRASH_DUMP
static const struct file_operations oldmem_fops = {
	.read	= read_oldmem,
	.open	= open_oldmem,
};
#endif

static ssize_t kmsg_write(struct file * file, const char __user * buf,
			  size_t count, loff_t *ppos)
{
	char *tmp;
	ssize_t ret;

	tmp = kmalloc(count + 1, GFP_KERNEL);
	if (tmp == NULL)
		return -ENOMEM;
	ret = -EFAULT;
	if (!copy_from_user(tmp, buf, count)) {
		tmp[count] = 0;
		ret = printk("%s", tmp);
		if (ret > count)
			/* printk can add a prefix */
			ret = count;
	}
	kfree(tmp);
	return ret;
}

static const struct file_operations kmsg_fops = {
	.write =	kmsg_write,
};

static int memory_open(struct inode * inode, struct file * filp)
{
	switch (iminor(inode)) {
		case 1:
			filp->f_op = &mem_fops;
			filp->f_mapping->backing_dev_info =
				&directly_mappable_cdev_bdi;
			break;
		case 2:
			filp->f_op = &kmem_fops;
			filp->f_mapping->backing_dev_info =
				&directly_mappable_cdev_bdi;
			break;
		case 3:
			filp->f_op = &null_fops;
			break;
#ifdef CONFIG_DEVPORT
		case 4:
			filp->f_op = &port_fops;
			break;
#endif
		case 5:
			filp->f_mapping->backing_dev_info = &zero_bdi;
			filp->f_op = &zero_fops;
			break;
		case 7:
			filp->f_op = &full_fops;
			break;
		case 8:
			filp->f_op = &random_fops;
			break;
		case 9:
			filp->f_op = &urandom_fops;
			break;
		case 11:
			filp->f_op = &kmsg_fops;
			break;
#ifdef CONFIG_CRASH_DUMP
		case 12:
			filp->f_op = &oldmem_fops;
			break;
#endif
		default:
			return -ENXIO;
	}
	if (filp->f_op && filp->f_op->open)
		return filp->f_op->open(inode,filp);
	return 0;
}

static const struct file_operations memory_fops = {
	.open		= memory_open,	/* just a selector for the real open */
};

static const struct {
	unsigned int		minor;
	char			*name;
	umode_t			mode;
	const struct file_operations	*fops;
} devlist[] = { /* list of minor devices */
	{1, "mem",     S_IRUSR | S_IWUSR | S_IRGRP, &mem_fops},
	{2, "kmem",    S_IRUSR | S_IWUSR | S_IRGRP, &kmem_fops},
	{3, "null",    S_IRUGO | S_IWUGO,           &null_fops},
#ifdef CONFIG_DEVPORT
	{4, "port",    S_IRUSR | S_IWUSR | S_IRGRP, &port_fops},
#endif
	{5, "zero",    S_IRUGO | S_IWUGO,           &zero_fops},
	{7, "full",    S_IRUGO | S_IWUGO,           &full_fops},
	{8, "random",  S_IRUGO | S_IWUSR,           &random_fops},
	{9, "urandom", S_IRUGO | S_IWUSR,           &urandom_fops},
	{11,"kmsg",    S_IRUGO | S_IWUSR,           &kmsg_fops},
#ifdef CONFIG_CRASH_DUMP
	{12,"oldmem",    S_IRUSR | S_IWUSR | S_IRGRP, &oldmem_fops},
#endif
};

static struct class *mem_class;

static int __init chr_dev_init(void)
{
	int i;
	int err;

	err = bdi_init(&zero_bdi);
	if (err)
		return err;

	if (register_chrdev(MEM_MAJOR,"mem",&memory_fops))
		printk("unable to get major %d for memory devs\n", MEM_MAJOR);

	mem_class = class_create(THIS_MODULE, "mem");
	for (i = 0; i < ARRAY_SIZE(devlist); i++)
		device_create(mem_class, NULL,
			      MKDEV(MEM_MAJOR, devlist[i].minor),
			      devlist[i].name);

	return 0;
}

fs_initcall(chr_dev_init);