Re: [PATCH 0/3] Remove accidental VLA usage

[Josh Poimboeuf]

On Thu, Mar 08, 2018 at 10:02:01AM -0800, Kees Cook wrote:
> On Thu, Mar 8, 2018 at 7:02 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > On Wed, Mar 07, 2018 at 07:30:44PM -0800, Kees Cook wrote:
> >> This series adds SIMPLE_MAX() to be used in places where a stack array
> >> is actually fixed, but the compiler still warns about VLA usage due to
> >> confusion caused by the safety checks in the max() macro.
> >>
> >> I'm sending these via -mm since that's where I've introduced SIMPLE_MAX(),
> >> and they should all have no operational differences.
> >
> > What if we instead simplify the max() macro's type checking so that GCC
> > can more easily fold the array size constants?  The below patch seems to
> > work:
> 
> Oooooh magic! Very nice. I couldn't figure out how to do this when I
> stared at it. Yes, let me respin. (I assume I can add your S-o-b?)

I'm going to be traveling for a few days, so I bequeath the patch to
you.  You can add my SOB.  I agree with Steve's suggestion to run it
through 0-day.

> 
> -Kees
> 
> >
> > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > index 3fd291503576..ec863726da29 100644
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -782,42 +782,32 @@ ftrace_vprintk(const char *fmt, va_list ap)
> >  static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
> >  #endif /* CONFIG_TRACING */
> >
> > -/*
> > - * min()/max()/clamp() macros that also do
> > - * strict type-checking.. See the
> > - * "unnecessary" pointer comparison.
> > - */
> > -#define __min(t1, t2, min1, min2, x, y) ({             \
> > -       t1 min1 = (x);                                  \
> > -       t2 min2 = (y);                                  \
> > -       (void) (&min1 == &min2);                        \
> > -       min1 < min2 ? min1 : min2; })
> > +extern long __error_incompatible_types_in_min_macro;
> > +extern long __error_incompatible_types_in_max_macro;
> > +
> > +#define __min(t1, t2, x, y)                                            \
> > +       __builtin_choose_expr(__builtin_types_compatible_p(t1, t2),     \
> > +                             (t1)(x) < (t2)(y) ? (t1)(x) : (t2)(y),    \
> > +                             (t1)__error_incompatible_types_in_min_macro)
> >
> >  /**
> >   * min - return minimum of two values of the same or compatible types
> >   * @x: first value
> >   * @y: second value
> >   */
> > -#define min(x, y)                                      \
> > -       __min(typeof(x), typeof(y),                     \
> > -             __UNIQUE_ID(min1_), __UNIQUE_ID(min2_),   \
> > -             x, y)
> > +#define min(x, y) __min(typeof(x), typeof(y), x, y)                    \
> >
> > -#define __max(t1, t2, max1, max2, x, y) ({             \
> > -       t1 max1 = (x);                                  \
> > -       t2 max2 = (y);                                  \
> > -       (void) (&max1 == &max2);                        \
> > -       max1 > max2 ? max1 : max2; })
> > +#define __max(t1, t2, x, y)                                            \
> > +       __builtin_choose_expr(__builtin_types_compatible_p(t1, t2),     \
> > +                             (t1)(x) > (t2)(y) ? (t1)(x) : (t2)(y),    \
> > +                             (t1)__error_incompatible_types_in_max_macro)
> >
> >  /**
> >   * max - return maximum of two values of the same or compatible types
> >   * @x: first value
> >   * @y: second value
> >   */
> > -#define max(x, y)                                      \
> > -       __max(typeof(x), typeof(y),                     \
> > -             __UNIQUE_ID(max1_), __UNIQUE_ID(max2_),   \
> > -             x, y)
> > +#define max(x, y) __max(typeof(x), typeof(y), x, y)
> >
> >  /**
> >   * min3 - return minimum of three values
> > @@ -869,10 +859,7 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
> >   * @x: first value
> >   * @y: second value
> >   */
> > -#define min_t(type, x, y)                              \
> > -       __min(type, type,                               \
> > -             __UNIQUE_ID(min1_), __UNIQUE_ID(min2_),   \
> > -             x, y)
> > +#define min_t(type, x, y) __min(type, type, x, y)
> >
> >  /**
> >   * max_t - return maximum of two values, using the specified type
> > @@ -880,10 +867,7 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
> >   * @x: first value
> >   * @y: second value
> >   */
> > -#define max_t(type, x, y)                              \
> > -       __max(type, type,                               \
> > -             __UNIQUE_ID(min1_), __UNIQUE_ID(min2_),   \
> > -             x, y)
> > +#define max_t(type, x, y) __max(type, type, x, y)                              \
> >
> >  /**
> >   * clamp_t - return a value clamped to a given range using a given type
> 
> 
> 
> -- 
> Kees Cook
> Pixel Security

-- 
Josh