Re: proc_file_read() question

[Mike Galbraith]

On Mon, 25 Jun 2001, Martin Wilck wrote:

> Hi,
>
> the "hack" below in proc_file_read() fs/proc/generic.c (2.4.5)
> irritates me:
>
> If I do use "start" for a pointer into a memory area
> allocated in read_proc, will it be always guaranteed
> that (start > page)?
>
> If no, this will IMO lead to spuriously wrong output.
> If yes, I'd like to understand why.

Shhh ;-)  Last time that hack was mentioned, someone wanted to _remove_
it.  It's a very nice little hack to have around, and IKD uses it.

	-Mike

diff -urN linux-2.4.4.virgin/fs/proc/generic.c linux-2.4.4.ikd.mike/fs/proc/generic.c
--- linux-2.4.4.virgin/fs/proc/generic.c	Mon Dec 11 22:45:42 2000
+++ linux-2.4.4.ikd/fs/proc/generic.c	Sun Dec 17 07:58:56 2000
@@ -104,6 +104,11 @@
  		 * return the bytes, and set `start' to the desired offset
  		 * as an unsigned int. - [e-mail]
 		 */
+		/* Ensure that the data will fit when using the ppos hack,
+		 * otherwise userland receives truncated data.
+		 */
+		if (n > count-1 && start && start < page)
+			break;
  		n -= copy_to_user(buf, start < page ? page : start, n);
 		if (n == 0) {
 			if (retval == 0)