Re: [PATCH 2/2] selinux: optimize storage of filename transitions

[Stephen Smalley <sds@tycho.nsa.gov>]

On 2/11/20 11:39 AM, Ondrej Mosnacek wrote:
> In these rules, each rule with the same (target type, target class,
> filename) values is (in practice) always mapped to the same result type.
> Therefore, it is much more efficient to group the rules by (ttype,
> tclass, filename).
> 
> Thus, this patch drops the stype field from the key and changes the
> datum to be a linked list of one or more structures that contain a
> result type and an ebitmap of source types that map the given target to
> the given result type under the given filename. The size of the hash
> table is also incremented to 2048 to be more optimal for Fedora policy
> (which currently has ~2500 unique (ttype, tclass, filename) tuples,
> regardless of whether the 'unconfined' module is enabled).
> 
> Not only does this dramtically reduce memory usage when the policy
> contains a lot of unconfined domains (ergo a lot of filename based
> transitions), but it also slightly reduces memory usage of strongly
> confined policies (modeled on Fedora policy with 'unconfined' module
> disabled) and significantly reduces lookup times of these rules on
> Fedora (roughly matches the performance of the rhashtable conversion
> patch [1] posted recently to selinux@vger.kernel.org).
> 
> An obvious next step is to change binary policy format to match this
> layout, so that disk space is also saved. However, since that requires
> more work (including matching userspace changes) and this patch is
> already beneficial on its own, I'm posting it separately.
> 
> Performance/memory usage comparison:
> 
> Kernel           | Policy load | Policy load   | Mem usage | Mem usage     | openbench
>                   |             | (-unconfined) |           | (-unconfined) | (createfiles)
> -----------------|-------------|---------------|-----------|---------------|--------------
> reference        |       1,30s |         0,91s |      90MB |          77MB | 55 us/file
> rhashtable patch |       0.98s |         0,85s |      85MB |          75MB | 38 us/file
> this patch       |       0,95s |         0,87s |      75MB |          75MB | 40 us/file
> 
> (Memory usage is measured after boot. With SELinux disabled the memory
> usage was ~60MB on the same system.)
> 
> [1] https://lore.kernel.org/selinux/20200116213937.77795-1-dev@lynxeye.de/T/
> 
> Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
> ---
>   security/selinux/ss/policydb.c | 175 ++++++++++++++++++++-------------
>   security/selinux/ss/policydb.h |   8 +-
>   security/selinux/ss/services.c |  16 +--
>   3 files changed, 120 insertions(+), 79 deletions(-)
> 
> diff --git a/security/selinux/ss/policydb.c b/security/selinux/ss/policydb.c
> index 981797bfc547..62283033bb7d 100644
> --- a/security/selinux/ss/policydb.c
> +++ b/security/selinux/ss/policydb.c
> @@ -1882,64 +1884,93 @@ out:
>   
>   static int filename_trans_read_one(struct policydb *p, void *fp)
>   {
<snip>
> +	exists = false;
> +	last = NULL;
> +	datum = hashtab_search(p->filename_trans, &key);
> +	while (datum) {
> +		if (ebitmap_get_bit(&datum->stypes, stype - 1)) {
> +			exists = true;
> +			break;
> +		}
> +		if (datum->otype == otype) {
> +			last = NULL;

Why set last to NULL here?  Seemingly unused afterward if datum is non-NULL?