[RFC PATCH] ceph: fix directories inode i_blkbits initialization

[Luis Henriques]

When filling an inode with info from the MDS, i_blkbits is being
initialized using fl_stripe_unit, which contains the stripe unit in
bytes.  Unfortunately, this doesn't make sense for directories as they
have fl_stripe_unit set to '0'.  This means that i_blkbits will be set
to 0xff, causing an UBSAN undefined behaviour in i_blocksize():

  UBSAN: Undefined behaviour in ./include/linux/fs.h:731:12
  shift exponent 255 is too large for 32-bit type 'int'

Fix this by initializing i_blkbits to CEPH_BLOCK_SHIFT if fl_stripe_unit
is zero.

Signed-off-by: Luis Henriques <lhenriques@xxxxxxxx>
---
 fs/ceph/inode.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

Hi Jeff,

To be honest, I'm not sure CEPH_BLOCK_SHIFT is the right value to use
here, but for sure the one currently being used isn't correct if the
inode is a directory.  Using stripe units seems to be a bug that has
been there since the beginning, but it definitely became bigger problem
after commit 69448867abcb ("fs: shave 8 bytes off of struct inode").

This fix could also be moved into the 'switch' statement later in that
function, in the S_IFDIR case, similar to commit 5ba72e607cdb ("ceph:
set special inode's blocksize to page size").  Let me know which version
you would prefer.

Cheers,
--
Luis

diff --git a/fs/ceph/inode.c b/fs/ceph/inode.c
index 791f84a13bb8..0e6d6db848b7 100644
--- a/fs/ceph/inode.c
+++ b/fs/ceph/inode.c
@@ -800,7 +800,12 @@ static int fill_inode(struct inode *inode, struct page *locked_page,
 
 	/* update inode */
 	inode->i_rdev = le32_to_cpu(info->rdev);
-	inode->i_blkbits = fls(le32_to_cpu(info->layout.fl_stripe_unit)) - 1;
+	/* directories have fl_stripe_unit set to zero */
+	if (le32_to_cpu(info->layout.fl_stripe_unit))
+		inode->i_blkbits =
+			fls(le32_to_cpu(info->layout.fl_stripe_unit)) - 1;
+	else
+		inode->i_blkbits = CEPH_BLOCK_SHIFT;
 
 	__ceph_update_quota(ci, iinfo->max_bytes, iinfo->max_files);