linux-kernel - path-lookup inconsistency ?

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Message-ID: <CAPTae5+0v_Bec_=53Abuzxfq4raNZCt8eJHYMNeF=Pdbu===yA@mail.gmail.com>
Date:	Thu, 19 May 2016 19:20:26 -0700
From:	Badhri Jagan Sridharan <badhri@...gle.com>
To:	viro@...iv.linux.org.uk
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-fsdevel@...r.kernel.org
Subject: path-lookup inconsistency ?

Hi,

I mounted overlayfs at /

My cat /proc/mounts looks like the following.
# cat /proc/mounts
/dev/root / squashfs ro,seclabel,relatime 0 0
..
overlayfs / overlay
rw,relatime,lowerdir=/,upperdir=/cache/upper,workdir=/cache/working 0
0

The original blockdevice at fs root is squashfs formatted so doesnt
support write operations. I mounted overlayfs on fs root to cache the
writes made.

While in /, the filesystem does not allow me to create files/directories,
if I dont prefix it with ".." directive.

hikey:/ # mkdir test
mkdir: 'testt12': Read-only file system
hikey:/ # mkdir ../test

"mkdir ../test" command succeeds.

I traced through the fs/namei.c code to notice that __follow_mount_rcu
does not get called when I execute "mkdir test" whereas, it gets called from
the follow_dotdot_rcu when I execute "mkdir ../test"

What is the expected behavior ? Should "mkdir test" and "mkdir ../test"
both succeed ?

Similar inconsistency is found with "ls" and "ls .." as well while in fs root.

Thanks,
Badhri