linux-kernel - Re: Runtime failure running sh:qemu in -next due to 'sh: fix copy_from

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Date:   Sat, 17 Sep 2016 00:00:08 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     linux-kernel@...r.kernel.org,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        linux-sh@...r.kernel.org, Rich Felker <dalias@...c.org>
Subject: Re: Runtime failure running sh:qemu in -next due to 'sh: fix
 copy_from_user()'

On Fri, Sep 16, 2016 at 03:47:04PM -0700, Guenter Roeck wrote:

> Adding pr_info() just before the "if (unlikely..." fixes the problem.
> 
> Commenting out the "if (unlikely())" code fixes the problem. 
> 
> Using a new variable "unsigned long x" for the return code instead of
> re-using __copy_size fixes the problem.
> 
> Replacing "return __copy_size;" with "return __copy_size & 0xffffffff;"
> fixes the problem.
> 
> The problem only seems to be seen if the copy size length is odd (more
> specifically, the failing copy always has a length of 25 bytes).
> 
> No idea what is going on. Bug in __copy_user() ? Compiler bug ?

Definitely a compiler bug.  __kernel_size_t is 32 bits on sh; that & 0xffffffff
should've been optimized away, for crying out loud!