Re: [PATCH 1/1] lib/random.c fix undefined behavior on signed integer overflow.

[Zorro Lang <zlang@kernel.org>]

On Mon, Oct 12, 2026 at 04:38:26PM +0200, Lukas Herbolt wrote:
> On 2026-10-12 15:44, Zorro Lang wrote:
> > On Tue, Oct 06, 2026 at 03:51:52PM +0200, Lukas Herbolt wrote:
> > > The random.c now expects that signed integer to overflow and
> > > triggers the  ^MASK branch. But signed int overflow is undefined
> > > behavior and GCC can optimize this branch out with certain
> > > CFLAGS/LDFLAGS.
> > > 
> > > Signed-off-by: Lukas Herbolt <lukas@herbolt.com>
> > > ---
> > >  lib/random.c | 13 +++++++------
> > >  1 file changed, 7 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/lib/random.c b/lib/random.c
> > > index d5c81be817c8..f767ed1d1337 100644
> > > --- a/lib/random.c
> > > +++ b/lib/random.c
> > > @@ -186,14 +186,15 @@ _random (int32_t is [2])
> > >  int32_t
> > >  _irandm (int32_t is [2])
> > >  {
> > > -	int32_t it, leh, nit;
> > > -
> > > +	int32_t it, leh, nit, apply_mask;
> > >  	it = is [0];
> > >  	leh = is [1];
> > > -	if (it <= 0)
> > > -		it = (it + it) ^ MASK;
> > > -	else
> > > -		it = it + it;
> > > +
> > > +/* 	evaluate on original value — no UB here */
> > > +	apply_mask = (it <= 0);
> > > +/*	double via unsigned shift — well-defined */
> > > +	it = (unsigned)it << 1;
> > 
> > I'll explicitly change it to unsigned int when applying the patch.
> > 
> > Reviewed-by: Zorro Lang <zlang@kernel.org>
> > 
> > > +	if (apply_mask) it ^= MASK;
> > 
> > Ahaha, this .c program is quite legacy. I'm not sure it's worth pulling
> > in
> > <stdint.h> and <stdbool.h> just to use *bool apply_mask* and
> > *(uint32_t)*.
> > 
> Yeah, I did't want to pull anything else into it.

Makes sense :)

> I will send v2 cleaning up the dead code.

OK, If you plan to send a v2, I'll hold off on merging this one and wait for
your next version.

> 
> -- 
> -lhe
>