From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f44.google.com (mail-wm0-f44.google.com [74.125.82.44]) by kanga.kvack.org (Postfix) with ESMTP id 111BE6B0006 for ; Thu, 7 Jan 2016 04:33:58 -0500 (EST) Received: by mail-wm0-f44.google.com with SMTP id f206so90050430wmf.0 for ; Thu, 07 Jan 2016 01:33:58 -0800 (PST) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id 76si18610110wms.44.2016.01.07.01.33.56 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Thu, 07 Jan 2016 01:33:56 -0800 (PST) Date: Thu, 7 Jan 2016 10:34:02 +0100 From: Jan Kara Subject: Re: [PATCH v7 1/9] dax: fix NULL pointer dereference in __dax_dbg() Message-ID: <20160107093402.GA8380@quack.suse.cz> References: <1452103263-1592-1-git-send-email-ross.zwisler@linux.intel.com> <1452103263-1592-2-git-send-email-ross.zwisler@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Dan Williams Cc: Ross Zwisler , "linux-kernel@vger.kernel.org" , "H. Peter Anvin" , "J. Bruce Fields" , Theodore Ts'o , Alexander Viro , Andreas Dilger , Andrew Morton , Dave Chinner , Dave Hansen , Ingo Molnar , Jan Kara , Jeff Layton , Matthew Wilcox , Matthew Wilcox , Thomas Gleixner , linux-ext4 , linux-fsdevel , Linux MM , "linux-nvdimm@lists.01.org" , X86 ML , XFS Developers On Wed 06-01-16 11:14:09, Dan Williams wrote: > On Wed, Jan 6, 2016 at 10:00 AM, Ross Zwisler > wrote: > > __dax_dbg() currently assumes that bh->b_bdev is non-NULL, passing it into > > bdevname() where is is dereferenced. This assumption isn't always true - > > when called for reads of holes, ext4_dax_mmap_get_block() returns a buffer > > head where bh->b_bdev is never set. I hit this BUG while testing the DAX > > PMD fault path. > > > > Instead, verify that we have a valid bh->b_bdev, else just say "unknown" > > for the block device. > > > > Signed-off-by: Ross Zwisler > > Cc: Dan Williams > > --- > > fs/dax.c | 7 ++++++- > > 1 file changed, 6 insertions(+), 1 deletion(-) > > > > diff --git a/fs/dax.c b/fs/dax.c > > index 7af8797..03cc4a3 100644 > > --- a/fs/dax.c > > +++ b/fs/dax.c > > @@ -563,7 +563,12 @@ static void __dax_dbg(struct buffer_head *bh, unsigned long address, > > { > > if (bh) { > > char bname[BDEVNAME_SIZE]; > > - bdevname(bh->b_bdev, bname); > > + > > + if (bh->b_bdev) > > + bdevname(bh->b_bdev, bname); > > + else > > + snprintf(bname, BDEVNAME_SIZE, "unknown"); > > + > > pr_debug("%s: %s addr: %lx dev %s state %lx start %lld " > > "length %zd fallback: %s\n", fn, current->comm, > > address, bname, bh->b_state, (u64)bh->b_blocknr, > > I'm assuming there's no danger of a such a buffer_head ever being used > for the bdev parameter to dax_map_atomic()? Shouldn't we also/instead > go fix ext4 to not send partially filled buffer_heads? No. The real problem is a long-standing abuse of struct buffer_head to be used for passing block mapping information (it's on my todo list to remove that at least from DAX code and use cleaner block mapping interface but first I want basic DAX functionality to settle down to avoid unnecessary conflicts). Filesystem is not supposed to touch bh->b_bdev. If you need that filled in, set it yourself in before passing bh to the block mapping function. Honza -- Jan Kara SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org