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From: Ondrej Mosnacek <omosnace@redhat.com>
To: Jeff Vander Stoep <jeffv@google.com>
Cc: SElinux list <selinux@vger.kernel.org>,
	Paul Moore <paul@paul-moore.com>,
	Stephen Smalley <sds@tycho.nsa.gov>,
	will@kernel.org, Jovana Knezevic <jovanak@google.com>
Subject: Re: [PATCH v5] selinux: sidtab: reverse lookup hash table
Date: Thu, 7 Nov 2019 17:12:53 +0100
Message-ID: <CAFqZXNvn4tTgNcQ3-fFE5QN7dS93h2VNznZ-D=3m2gZ26di9hA@mail.gmail.com> (raw)
In-Reply-To: <20191107101743.203699-1-jeffv@google.com>

On Thu, Nov 7, 2019 at 11:17 AM Jeff Vander Stoep <jeffv@google.com> wrote:
> This replaces the reverse table lookup and reverse cache with a
> hashtable which improves cache-miss reverse-lookup times from
> O(n) to O(1)* and maintains the same performance as a reverse
> cache hit.
>
> This reduces the time needed to add a new sidtab entry from ~500us
> to 5us on a Pixel 3 when there are ~10,000 sidtab entries.
>
> The implementation uses the kernel's generic hashtable API,
> It uses the context's string represtation as the hash source,
> and the kernels generic string hashing algorithm full_name_hash()
> to reduce the string to a 32 bit value.
>
> This change also maintains the improvement introduced in
> commit ee1a84fdfeed ("selinux: overhaul sidtab to fix bug and improve
> performance") which removed the need to keep the current sidtab
> locked during policy reload. It does however introduce periodic
> locking of the target sidtab while converting the hashtable. Sidtab
> entries are never modified or removed, so the context struct stored
> in the sid_to_context tree can also be used for the context_to_sid
> hashtable to reduce memory usage.
>
> This bug was reported by:
> - On the selinux bug tracker.
>   BUG: kernel softlockup due to too many SIDs/contexts #37
>   https://github.com/SELinuxProject/selinux-kernel/issues/37
> - Jovana Knezevic on Android's bugtracker.
>   Bug: 140252993
>   "During multi-user performance testing, we create and remove users
>   many times. selinux_android_restorecon_pkgdir goes from 1ms to over
>   20ms after about 200 user creations and removals. Accumulated over
>   ~280 packages, that adds a significant time to user creation,
>   making perf benchmarks unreliable."
>
> * Hashtable lookup is only O(1) when n < the number of buckets.
>
> Changes in V2:
> -The hashtable uses sidtab_entry_leaf objects directly so these
> objects are shared between the sid_to_context lookup tree and the
> context_to_sid hashtable. This simplifies memory allocation and
> was suggested by Ondrej Mosnacek.
> -The new sidtab hash stats file in selinuxfs has been moved out of
> the avc dir and into a new "ss" dir.
>
> V3:
> -Add lock nesting notation.
>
> V4:
> -Moved to *_rcu variants of the various hashtable functions
> as suggested by Ondrej Mosnacek and Will Deacon.

I may be misunderstanding the purpose of RCU, but isn't all this RCU
stuff a big overkill when these entries are never removed? (Well, they
are removed when the sidtab is being destroyed, at which point however
no other threads are accessing them any more.) In my understanding,
RCU basically makes sure that objects that you dereference in an RCU
critical section are not destroyed until you leave it. Yes, it also
helps you to dereference/update them safely, but that can be achieved
on its own in a simpler way. Instead of using RCU here, I would
instead suggest looking into adding an equivalent of
list_for_each_entry_lockless()* for hlist and maybe introduce some
suitable hlist_add_something() function that ensures consistency
w.r.t. the lockless traversal (perhaps the WRITE_ONCE() in hlist_add()
is already sufficient, but I'm not sure...).

> -Naming/spelling fixups.
>
> Signed-off-by: Jeff Vander Stoep <jeffv@google.com>
> Reported-by: Stephen Smalley <sds@tycho.nsa.gov>
> Reported-by: Jovana Knezevic <jovanak@google.com>
> ---
>  security/selinux/Kconfig            |  12 ++
>  security/selinux/include/security.h |   1 +
>  security/selinux/selinuxfs.c        |  65 +++++++
>  security/selinux/ss/context.h       |  11 +-
>  security/selinux/ss/policydb.c      |   5 +
>  security/selinux/ss/services.c      |  83 +++++++--
>  security/selinux/ss/services.h      |   4 +-
>  security/selinux/ss/sidtab.c        | 264 ++++++++++++++--------------
>  security/selinux/ss/sidtab.h        |  17 +-
>  9 files changed, 302 insertions(+), 160 deletions(-)

<snip>

-- 
Ondrej Mosnacek <omosnace at redhat dot com>
Software Engineer, Security Technologies
Red Hat, Inc.


  parent reply index

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-07 10:17 Jeff Vander Stoep
2019-11-07 12:01 ` Jeffrey Vander Stoep
2019-11-14 23:35   ` Paul Moore
2019-11-07 14:54 ` Stephen Smalley
2019-11-07 15:42   ` Ondrej Mosnacek
2019-11-07 15:59     ` Jeffrey Vander Stoep
2019-11-07 15:49   ` Jeffrey Vander Stoep
2019-11-07 16:12 ` Ondrej Mosnacek [this message]
2019-11-07 16:44   ` Will Deacon
2019-11-07 18:41     ` Ondrej Mosnacek
2019-11-07 20:06       ` Jeffrey Vander Stoep

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