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

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Date:   Fri, 16 Sep 2016 15:47:04 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Al Viro <viro@...IV.linux.org.uk>
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 10:31:41PM +0100, Al Viro wrote:
> On Fri, Sep 16, 2016 at 01:59:38PM -0700, Guenter Roeck wrote:
> > Yes, reverting 6e050503a150 fixes the problem.
> > 
> > I added a BUG() into the "if (unlikely())" below, but it doesn't catch,
> > and I still get the ip: OVERRUN errors. Which leaves me a bit puzzled.
> > 
> > Guenter
> > 
> > > The change in question is
> > >         if (__copy_size && __access_ok(__copy_from, __copy_size))
> > > -               return __copy_user(to, from, __copy_size);
> > > +               __copy_size = __copy_user(to, from, __copy_size);
> > > +
> > > +       if (unlikely(__copy_size))
> > > +               memset(to + (n - __copy_size), 0, __copy_size);
> > >  
> > >         return __copy_size;
> 
> So we don't even hit that memset()?  What the hell?  __copy_user() is
> declared as
> __kernel_size_t __copy_user(void *to, const void *from, __kernel_size_t n);
> 
> and __copy_size copy_from_user() is
> 
>         __kernel_size_t __copy_size = (__kernel_size_t) n;
> 
> So
> 	return __copy_user(to, from, __copy_size);
> and
> 	__copy_size = __copy_user(to, from, __copy_size);
> 	return __copy_size;
> ought to be doing exactly the same thing.  At that point it's starting to
> smell like a compiler bug somewhere in there.
> 
> Try to remove that (not triggered) if (unlikely(__copy_size)) memset(...)
> and see if that's enough to recover.  And it would be nice to see what
> all three variants (as it is, with commit reverted and with just that if
> removed) generate in e.g. sys_utimensat() (fs/utimes.s)

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 ?

Guenter