Re: Runtime failure running sh:qemu in -next due to 'sh: fix copy_from_user()'

[Guenter Roeck <linux@roeck-us.net>]

On Fri, Sep 16, 2016 at 05:39:18PM -0400, Rich Felker wrote:
> 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)
> 
> It would be useful to know what compiler version was used to build the
> kernel. I wouldn't be surprised if some are buggy.
> 
4.6.3 from kernel.org.

Guenter