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!

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