Re: [libvirt] [PATCH 1/2] util: Don't overflow in virRandomBits

On 08/01/2018 07:16 AM, Daniel P. Berrangé wrote: > On Wed, Aug 01, 2018 at 01:44:32PM +0200, Michal Privoznik wrote: >> The function is supposed to return up to 64bit long integer. In >> order to do that it calls virRandomBytes() to fill the integer >> with random bytes and then masks out everything but requested >> bits. However, when doing that it shifts 1U and not 1ULL. So >> effectively, requesting 32 random bis or more always return 0 >> which is not random enough. >> >> Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com&gt; >> --- >>   src/util/virrandom.c | 2 +- >>   1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/src/util/virrandom.c b/src/util/virrandom.c >> index 01cc82a052..3c011a8615 100644 >> --- a/src/util/virrandom.c >> +++ b/src/util/virrandom.c >> @@ -68,7 +68,7 @@ uint64_t virRandomBits(int nbits) >>           return 0; >>       } >>   -    ret &= (1U << nbits) - 1; >> +    ret &= (1ULL << nbits) - 1; 1ULL << 64 is undefined in C. We need to write this as: if (nbits < 64)     ret &= (1ULL << nbits) - 1;