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;
> return __copy_user(to, from, __copy_size);
> __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 ?