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

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Oct 08, 2026 at 10:23:59AM +0200, Lukas Herbolt wrote:
> On 2026-10-07 19:00, Darrick J. Wong 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;
> > > +	if (apply_mask) it ^= MASK;
> > 
> > Doesn't _random suffer the same flaw?
> > 
> > Oh.  It's dead code, maybe it should go away?
> Good point.
> > 
> > Also, what about converting all the variables to unsigned?
> 
> It would break two signed checks: `it <= 0` (LFSR feedback -
> only fires at zero instead of when MSB set) and `leh < 0`
> (output folding — never fires). The `it` register degenerates
> to 33 repeating values after 64 calls. Reducing the pool of
> random numbers.

Ah.  Well in that case I'm satisfied, so
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D