linux-kernel - "Failed to create /dev/root: -14" after commit e6978e4bf1 ("ARM: save and reset the address limit when entering an exception")

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Date:	Tue, 2 Aug 2016 19:51:45 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Russell King <rmk+kernel@....linux.org.uk>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Will Deacon <will.deacon@....com>
Subject: "Failed to create /dev/root: -14" after commit e6978e4bf1 ("ARM: save
 and reset the address limit when entering an exception")

Hi,

I see the following crash when running a qemu arm 'kzm' runtime test with the current mainline.

Failed to create /dev/root: -14

[ followed by panic ]

A complete log file is at [1].

Bisect points to commit e6978e4bf1 ("ARM: save and reset the address limit when entering
an exception"). Some additional debugging shows lots of similar error returns from
strncpy_from_user(). For the error case resulting in the crash, the call chain
is as follows.

sys_mknod
   sys_mknodat()
     user_path_create()
       getname()
         getname_flags()
           strncpy_from_user()

Reverting the patch fixes the problem.

I don't see the problem with any other qemu arm test.

It looks like the kernel runs in exception context. My gut feeling is that qemu may be
to blame, but I am not really sure.

Any idea how to track this down ?

Thanks,
Guenter

---
[1] http://kerneltests.org/builders/qemu-arm-master/builds/615