Re: [PATCH 6/6] selinux: improve symtab string hashing

[Stephen Smalley]

On Fri, Aug 18, 2023 at 11:12 AM Christian Göttsche
<cgzones@xxxxxxxxxxxxxx> wrote:
>
> The number of buckets is calculated by performing a binary AND against
> the mask of the hash table, which is one less than its size (which is a
> power of two).  This leads to all top bits being discarded, requiring
> for short or similar inputs a hash function with a good avalanche
> effect.
>
> Use djb2a:
>
>     # current
>     common prefixes:  7 entries and 5/8 buckets used, longest chain length 2, sum of chain length^2 11
>     classes:  134 entries and 100/256 buckets used, longest chain length 5, sum of chain length^2 234
>     roles:  15 entries and 6/16 buckets used, longest chain length 5, sum of chain length^2 57
>     types:  4448 entries and 3016/8192 buckets used, longest chain length 41, sum of chain length^2 14922
>     users:  7 entries and 3/8 buckets used, longest chain length 3, sum of chain length^2 17
>     bools:  306 entries and 221/512 buckets used, longest chain length 4, sum of chain length^2 524
>     levels:  1 entries and 1/1 buckets used, longest chain length 1, sum of chain length^2 1
>     categories:  1024 entries and 400/1024 buckets used, longest chain length 4, sum of chain length^2 2740
>
>     # patch
>     common prefixes:  7 entries and 5/8 buckets used, longest chain length 2, sum of chain length^2 11
>     classes:  134 entries and 101/256 buckets used, longest chain length 3, sum of chain length^2 210
>     roles:  15 entries and 9/16 buckets used, longest chain length 3, sum of chain length^2 31
>     types:  4448 entries and 3459/8192 buckets used, longest chain length 5, sum of chain length^2 6778
>     users:  7 entries and 5/8 buckets used, longest chain length 3, sum of chain length^2 13
>     bools:  306 entries and 236/512 buckets used, longest chain length 5, sum of chain length^2 470
>     levels:  1 entries and 1/1 buckets used, longest chain length 1, sum of chain length^2 1
>     categories:  1024 entries and 518/1024 buckets used, longest chain length 7, sum of chain length^2 2992
>
> Signed-off-by: Christian Göttsche <cgzones@xxxxxxxxxxxxxx>
> ---
>  security/selinux/ss/symtab.c | 18 ++++++++----------
>  1 file changed, 8 insertions(+), 10 deletions(-)
>
> diff --git a/security/selinux/ss/symtab.c b/security/selinux/ss/symtab.c
> index 43d7f0319ccd..b6761b96eee4 100644
> --- a/security/selinux/ss/symtab.c
> +++ b/security/selinux/ss/symtab.c
> @@ -11,16 +11,14 @@
>
>  static unsigned int symhash(const void *key)
>  {
> -       const char *p, *keyp;
> -       unsigned int size;
> -       unsigned int val;
> -
> -       val = 0;
> -       keyp = key;
> -       size = strlen(keyp);
> -       for (p = keyp; (p - keyp) < size; p++)
> -               val = (val << 4 | (val >> (8*sizeof(unsigned int)-4))) ^ (*p);
> -       return val;
> +       /* djb2a */

Do we need/want something that specifies the author/license (I assume
public domain) of this code?

> +       unsigned int hash = 5381;
> +       unsigned char c;
> +
> +       while ((c = *(const unsigned char *)key++))
> +               hash = ((hash << 5) + hash) ^ c;
> +
> +       return hash;
>  }
>
>  static int symcmp(const void *key1, const void *key2)
> --
> 2.40.1
>