Re: [PATCH v2 1/3] vsprintf: Remove accidental VLA usage

[Thomas Gleixner <tglx@linutronix.de>]

On Thu, 8 Mar 2018, Rasmus Villemoes wrote:
> On 2018-03-08 04:30, Kees Cook wrote:
> >  /**
> > + * SIMPLE_MAX - return maximum of two values without any type checking
> > + * @x: first value
> > + * @y: second value
> > + *
> > + * This should only be used in stack array sizes, since the type-checking
> > + * from max() confuses the compiler into thinking a VLA is being used.
> > + */
> > +#define SIMPLE_MAX(x, y)	((size_t)(x) > (size_t)(y) ? (size_t)(x) \
> > +							   : (size_t)(y))
> 
> This will be abused at some point, leading to the usual double
> evaluation etc. etc. problems. The name is also too long (and in general
> we should avoid adjectives like "simple", "safe", people reading the
> code won't know what is simple or safe about it). I think this should work
> 
> #define MAX(x, y) (__builtin_choose_expr((x) > (y), x, y))
> 
> That forces (x)>(y) to be a compile-time constant, so x and y must also
> be; hence there can be no side effects. The MIN version of this could
> replace the custom __const_min in fs/file.c, and probably other places
> as well.
> 
> I tested that this at least works in the vsprintf case, -Wvla no longer
> complains. fs/file.c also compiles with the MIN version of this.
> 
> I suppose MIN and MAX will collide with other uses in the tree. Hmm.

Make it CONST_MAX() or something like that which makes it entirely clear.

Thanks,

	tglx