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

[Lukas Herbolt <lukas@herbolt.com>]

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.

-- 
-lhe