linux-kernel - Re: [PATCH] fs: Add additional checks for block devices during mount

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Message-ID: <20250724072859.GA29432@lst.de>
Date: Thu, 24 Jul 2025 09:28:59 +0200
From: Christoph Hellwig <hch@....de>
To: Christian Brauner <brauner@...nel.org>
Cc: Christoph Hellwig <hch@....de>, Zizhi Wo <wozizhi@...weicloud.com>,
	kernel test robot <lkp@...el.com>, viro@...iv.linux.org.uk,
	jack@...e.com, axboe@...nel.dk, llvm@...ts.linux.dev,
	oe-kbuild-all@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, yukuai3@...wei.com,
	yangerkun@...wei.com
Subject: Re: [PATCH] fs: Add additional checks for block devices during
 mount

On Wed, Jul 23, 2025 at 02:51:27PM +0200, Christian Brauner wrote:
> > You can just add a if (IS_ENABLED(CONFIG_BLOCK)) check around it.
> > 
> > 
> > But the layering here feels wrong.  sget_dev and it's helper operate
> > purely on the dev_t.  Anything actually dealing with a block device /
> > gendisk should be in the helpers that otherwise use it.
> 
> Either we add a lookup_bdev_alive() variant that checks whether the
> inode is still hashed when looking up the dev_t and we accept that this
> deals with the obvious cases and accept that this is racy...

I don't think racyness matters here.  The block device can die any
time, and the addition here is just to catch the cases where it might
have already been dead for a nicer interface.

> Or we add lookup_bdev_no_open() that returns the block device with the
> reference bumped, paired with lookup_bdev_put_no_open(). Afaiu, that
> would prevent deletion. We could even put this behind a
> guard(bdev_no_open)(fc->source). The reference count bump shouldn't
> matter there. Christoph?

Nothing prevents deletion, it will only get delayed until after the
open_mutex critical section.  I still think GD_DEAD is the best check
here, as it potentially gets set earlier than unshashing the inode,
but in the end both of the racy checks should be perfectly fine.