linux-kernel - Re: [PATCH 4.4 035/108] dont put symlink bodies in pagecache into highmem

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Date:   Mon, 5 Mar 2018 12:33:12 -0800
From:   Eric Biggers <ebiggers3@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Ben Hutchings <ben.hutchings@...ethink.co.uk>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        Jin Qian <jinqian@...gle.com>, Jin Qian <jinqian@...roid.com>
Subject: Re: [PATCH 4.4 035/108] dont put symlink bodies in pagecache into
 highmem

On Mon, Mar 05, 2018 at 07:02:45AM +0100, Greg Kroah-Hartman wrote:
> On Mon, Mar 05, 2018 at 12:37:59AM +0000, Ben Hutchings wrote:
> > On Thu, 2018-02-15 at 16:16 +0100, Greg Kroah-Hartman wrote:
> > > 4.4-stable review patch.  If anyone has any objections, please let me know.
> > > 
> > > ------------------
> > > 
> > > From: Al Viro <viro@...iv.linux.org.uk>
> > > 
> > > commit 21fc61c73c3903c4c312d0802da01ec2b323d174 upstream.
> > > 
> > > kmap() in page_follow_link_light() needed to go - allowing to hold
> > > an arbitrary number of kmaps for long is a great way to deadlocking
> > > the system.
> > > 
> > > new helper (inode_nohighmem(inode)) needs to be used for pagecache
> > > symlinks inodes; done for all in-tree cases.  page_follow_link_light()
> > > instrumented to yell about anything missed.
> > 
> > Except that this backport only updates ext4 and f2fs (and doesn't add
> > the warning).  I suppose that's all matters for Android, but it would
> > be nice to get a more complete backport into 4.4-stable...
> 
> That should be all the in-tree users of this, for 4.4, right?  If not,
> ick.  Jin, can you look into this?
> 
> thanks,
> 
> greg k-h

This was discussed already; see https://patchwork.kernel.org/patch/10203819/.
It turns out that Al's original commit incidentally fixed symlink decryption for
ext4 and f2fs on 32-bit systems, so the backport is wanted for that reason.  And
pulling in the rest of the commit, while it could be done, would have broken
things and required additional follow-on patches.  Jin and Greg, it would have
been helpful if the discussion had been captured in the commit message, so that
people like Ben who are watching the stable commits aren't left wondering.

Eric