On 07/31/2018 05:00 PM, Kees Cook wrote: > From: Jason Gunthorpe <j...@mellanox.com> > > Add shift_overflow() helper to assist driver authors in ensuring that > shift operations don't cause overflows or other odd conditions. > > Signed-off-by: Jason Gunthorpe <j...@mellanox.com> > Signed-off-by: Leon Romanovsky <leo...@mellanox.com> > [kees: tweaked comments and commit log, dropped unneeded assignment] > Signed-off-by: Kees Cook <keesc...@chromium.org> > --- > include/linux/overflow.h | 31 +++++++++++++++++++++++++++++++ > 1 file changed, 31 insertions(+) > > diff --git a/include/linux/overflow.h b/include/linux/overflow.h > index 8712ff70995f..69fc366ce865 100644 > --- a/include/linux/overflow.h > +++ b/include/linux/overflow.h > @@ -202,6 +202,37 @@ > > #endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */ > > +/** check_shift_overflow() - Calculate a left-shifted value and check > overflow > + * > + * @a: Value to be shifted > + * @b: How many bits left to shift > + * @d: Pointer to where to store the result > + * > + * Computes *@d = (@a << @s)
@b) > + * > + * Returns true if '*d' cannot hold the result or when 'a << s' doesn't > + * make sense. Example conditions: > + * - 'a << s' causes bits to be lost when stored in *d. > + * - 's' is garbage (e.g. negative) or so large that the result of > + * 'a << s' is guaranteed to be 0. > + * - 'a' is negative. > + * - 'a << s' sets the sign bit, if any, in '*d'. > + * > + * '*d' will hold the results of the attempted shift, but is not > + * considered "safe for use" if false is returned. > + */ > +#define check_shift_overflow(a, s, d) ({ \ > + typeof(a) _a = a; \ > + typeof(s) _s = s; \ > + typeof(d) _d = d; \ > + u64 _a_full = _a; \ > + unsigned int _to_shift = \ > + _s >= 0 && _s < 8 * sizeof(*d) ? _s : 0; \ > + *_d = (_a_full << _to_shift); \ > + (_to_shift != _s || *_d < 0 || _a < 0 || \ > + (*_d >> _to_shift) != _a); \ > +}) > + > /** > * array_size() - Calculate size of 2-dimensional array. > * > -- ~Randy