proc_file_read bug?

From: Thomas Hood (jdthood@mail.com)
Date: Fri Jan 25 2002 - 08:36:32 EST

I don't understand this part of proc_file_read() in
fs/proc/generic.c:

                /* This is a hack to allow mangling of file pos independent
                 * of actual bytes read. Simply place the data at page,
                 * return the bytes, and set `start' to the desired offset
                 * as an unsigned int. - Paul.Russell@rustcorp.com.au
                 */
                n -= copy_to_user(buf, start < page ? page : start, n);
                if (n == 0) {
                        if (retval == 0)
                                retval = -EFAULT;
                        break;
                }

                *ppos += start < page ? (long)start : n; /* Move down the file */
                nbytes -= n;
                buf += n;
                retval += n;

When start >= page, we copy n bytes beginning at start and
increase *ppos by n. Makes sense. But what happens when
start < page? We will copy n bytes starting at page, then
increase *ppos by start!! What sense does that make? If
there's cleverness happening here, someone please document it.

For your convenience, I append the whole existing proc_file_read
function below. 

--
Thomas

static ssize_t
proc_file_read(struct file * file, char * buf, size_t nbytes, loff_t *ppos)
{
	struct inode * inode = file->f_dentry->d_inode;
	char 	*page;
	ssize_t	retval=0;
	int	eof=0;
	ssize_t	n, count;
	char	*start;
	struct proc_dir_entry * dp;

	dp = (struct proc_dir_entry *) inode->u.generic_ip;
	if (!(page = (char*) __get_free_page(GFP_KERNEL)))
		return -ENOMEM;

	while ((nbytes > 0) && !eof)
	{
		count = MIN(PROC_BLOCK_SIZE, nbytes);

		start = NULL;
		if (dp->get_info) {
			/*
			 * Handle backwards compatibility with the old net
			 * routines.
			 */
			n = dp->get_info(page, &start, *ppos, count);
			if (n < count)
				eof = 1;
		} else if (dp->read_proc) {
			n = dp->read_proc(page, &start, *ppos,
					  count, &eof, dp->data);
		} else
			break;

		if (!start) {
			/*
			 * For proc files that are less than 4k
			 */
			start = page + *ppos;
			n -= *ppos;
			if (n <= 0)
				break;
			if (n > count)
				n = count;
		}
		if (n == 0)
			break;	/* End of file */
		if (n < 0) {
			if (retval == 0)
				retval = n;
			break;
		}
		
		/* This is a hack to allow mangling of file pos independent
 		 * of actual bytes read.  Simply place the data at page,
 		 * return the bytes, and set `start' to the desired offset
 		 * as an unsigned int. - Paul.Russell@rustcorp.com.au
		 */
 		n -= copy_to_user(buf, start < page ? page : start, n);
		if (n == 0) {
			if (retval == 0)
				retval = -EFAULT;
			break;
		}

		*ppos += start < page ? (long)start : n; /* Move down the file */
		nbytes -= n;
		buf += n;
		retval += n;
	}
	free_page((unsigned long) page);
	return retval;
}

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This archive was generated by hypermail 2b29 : Thu Jan 31 2002 - 21:00:32 EST