Fix invalid access errors in blk_lookup_devt

Commit 30f2f0eb4bd2c43d10a8b0d872c6e5ad8f31c9a0 ("block: do_mounts -
accept root=<non-existant partition>") extended blk_lookup_devt() to be
able to look up partitions that had not yet been registered, but in the
process made the assumption that the '&block_class.devices' list only
contains disk devices and that you can do 'dev_to_disk(dev)' on them.

That isn't actually true.  The block_class device list also contains the
partitions we've discovered so far, and you can't just do a
'dev_to_disk()' on those.

So make sure to only work on devices that block/genhd.c has registered
itself, something we can test by checking the 'dev->type' member.  This
makes the loop in blk_lookup_devt() match the other such loops in this
file.

[ We may want to do an alternate version that knows to handle _either_
  whole-disk devices or partitions, but for now this is the minimal fix
  for a series of crashes reported by Mariusz Kozlowski in

	http://lkml.org/lkml/2008/5/25/25

  and Ingo in

	http://lkml.org/lkml/2008/6/9/39 ]

Reported-by: Mariusz Kozlowski <m.kozlowski@tuxland.pl>
Reported-by: Ingo Molnar <mingo@elte.hu>
Cc: Neil Brown <neilb@suse.de>
Cc: Joao Luis Meloni Assirati <assirati@nonada.if.usp.br>
Acked-by: Kay Sievers <kay.sievers@vrfy.org>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/block/genhd.c b/block/genhd.c
index 129ad939f9dd..b922d4801c87 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -660,6 +660,8 @@ dev_t blk_lookup_devt(const char *name, int part)
 
         mutex_lock(&block_class_lock);
         list_for_each_entry(dev, &block_class.devices, node) {
+                if (dev->type != &disk_type)
+                        continue;
                 if (strcmp(dev->bus_id, name) == 0) {
                         struct gendisk *disk = dev_to_disk(dev);