On Fri, Mar 20, 2020 at 09:27:03AM -0400, Waiman Long wrote:

On 3/19/20 10:07 PM, Jarkko Sakkinen wrote:

quoted

On Thu, Mar 19, 2020 at 08:07:55PM -0400, Waiman Long wrote:

quoted

On 3/19/20 3:46 PM, Jarkko Sakkinen wrote:

quoted

On Wed, Mar 18, 2020 at 06:14:57PM -0400, Waiman Long wrote:

quoted

+			 * It is possible, though unlikely, that the key
+			 * changes in between the up_read->down_read period.
+			 * If the key becomes longer, we will have to
+			 * allocate a larger buffer and redo the key read
+			 * again.
+			 */
+			if (!tmpbuf || unlikely(ret > tmpbuflen)) {

Shouldn't you check that tmpbuflen stays below buflen (why else
you had made copy of buflen otherwise)?

The check above this thunk:

if ((ret > 0) && (ret <= buflen)) {

will make sure that ret will not be larger than buflen. So tmpbuflen > >> will never be bigger than buflen.  > > Ah right, of course, thanks.

What would go wrong if the condition was instead
((ret > 0) && (ret <= tmpbuflen))?

That if statement is a check to see if the actual key length is longer
than the user-supplied buffer (buflen). If that is the case, it will
just return the expected length without storing anything into the user
buffer. For the case that buflen >= ret > tmpbuflen, the revised check
above will incorrectly skip the storing step causing the caller to
incorrectly think the key is there in the buffer.

Maybe I should clarify that a bit more in the comment.

OK, right because it is possible in-between tmpbuflen could be
larger. Got it.

I think that longish key_data and key_data_len would be better
names than tmpbuf and tpmbuflen.

Also the comments are somewat overkill IMHO.

I'd replace them along the lines of

/* Cap the user supplied buffer length to PAGE_SIZE. */

/* Key data can change as we don not hold key->sem. */

/Jarkko