Fix 32-bit regression in block device read(2)

blkdev_read_iter() wants to cap the iov_iter by the amount of data
remaining to the end of device.  That's what iov_iter_truncate() is for
(trim iter->count if it's above the given limit).  So far, so good, but
the argument of iov_iter_truncate() is size_t, so on 32bit boxen (in
case of a large device) we end up with that upper limit truncated down
to 32 bits *before* comparing it with iter->count.

Easily fixed by making iov_iter_truncate() take 64bit argument - it does
the right thing after such change (we only reach the assignment in there
when the current value of iter->count is greater than the limit, i.e.
for anything that would get truncated we don't reach the assignment at
all) and that argument is not the new value of iter->count - it's an
upper limit for such.

The overhead of passing u64 is not an issue - the thing is inlined, so
callers passing size_t won't pay any penalty.

Reported-and-tested-by: Theodore Tso <tytso@mit.edu>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
Tested-by: Bruno Wolff III <bruno@wolff.to>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/uio.h

@@ -94,8 +94,20 @@ static inline size_t iov_iter_count(struct iov_iter *i)
 	return i->count;
 }
 
-static inline void iov_iter_truncate(struct iov_iter *i, size_t count)
+/*
+ * Cap the iov_iter by given limit; note that the second argument is
+ * *not* the new size - it's upper limit for such.  Passing it a value
+ * greater than the amount of data in iov_iter is fine - it'll just do
+ * nothing in that case.
+ */
+static inline void iov_iter_truncate(struct iov_iter *i, u64 count)
 {
+	/*
+	 * count doesn't have to fit in size_t - comparison extends both
+	 * operands to u64 here and any value that would be truncated by
+	 * conversion in assignement is by definition greater than all
+	 * values of size_t, including old i->count.
+	 */
 	if (i->count > count)
 		i->count = count;
 }