virrandom: Avoid undefined behaviour in virRandomBits

[libvirt] [PATCH] virrandom: Avoid undefined behaviour in virRandomBits

Michal Privoznik posted 1 patch 5 years, 8 months ago

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git fetch https://github.com/patchew-project/libvirt tags/patchew/db2e3637b8aa6b5abeaf1dc9074e2e3b9ea48288.1533195349.git.mprivozn@redhat.com

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src/util/virrandom.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

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[libvirt] [PATCH] virrandom: Avoid undefined behaviour in virRandomBits

Posted by Michal Privoznik 5 years, 8 months ago

If nbits is 64 (or greater) then shifting 1ULL left is undefined.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
---
 src/util/virrandom.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/util/virrandom.c b/src/util/virrandom.c
index 3c011a8615..7915f6531e 100644
--- a/src/util/virrandom.c
+++ b/src/util/virrandom.c
@@ -68,7 +68,9 @@ uint64_t virRandomBits(int nbits)
         return 0;
     }
 
-    ret &= (1ULL << nbits) - 1;
+    if (nbits < 64)
+        ret &= (1ULL << nbits) - 1;
+
     return ret;
 }
 
-- 
2.16.4

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Re: [libvirt] [PATCH] virrandom: Avoid undefined behaviour in virRandomBits

Posted by Daniel P. Berrangé 5 years, 8 months ago

On Thu, Aug 02, 2018 at 09:35:49AM +0200, Michal Privoznik wrote:
> If nbits is 64 (or greater) then shifting 1ULL left is undefined.
> 
> Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
> ---
>  src/util/virrandom.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/src/util/virrandom.c b/src/util/virrandom.c
> index 3c011a8615..7915f6531e 100644
> --- a/src/util/virrandom.c
> +++ b/src/util/virrandom.c
> @@ -68,7 +68,9 @@ uint64_t virRandomBits(int nbits)
>          return 0;
>      }
>  
> -    ret &= (1ULL << nbits) - 1;
> +    if (nbits < 64)
> +        ret &= (1ULL << nbits) - 1;
> +
>      return ret;
>  }

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

Regards,
Daniel
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