linux-kernel - Re: [GIT PULL] selinux/selinux-pr-20231030

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Message-ID: <CAHk-=whOtZK1g_dzEnb5NEB8ssNqotmHy3CkUwbnab_4phFRdw@mail.gmail.com>
Date:   Mon, 30 Oct 2023 20:12:51 -1000
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Paul Moore <paul@...l-moore.com>,
        Christian Göttsche <cgzones@...glemail.com>
Cc:     selinux@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL] selinux/selinux-pr-20231030

On Mon, 30 Oct 2023 at 16:16, Paul Moore <paul@...l-moore.com> wrote:
>
> * Use a better hashing function for the SELinux role tansition hash
>   table.

Bah.

While the old hash function was garbage, the new one is quite expensive.

Maybe it's worth it.

But generally, if you find that "oh, just doing a modulus with a power
of two drops all high bits", the first thing to try is probably to
just do "hash_long(x, N)" to get N bits instead.

Assuming the input is somewhat ok in one word, it does a fairly good
job of mixing the bits with a simple multiply-and-shift.

Yes, yes, jhash is a fine hash, but it does a quite *lot* of (simple)
ALU ops. While "hash_long()" is often small enough to be inlined.

I also note that filenametr_hash() does the old "one byte at a time"
hash and partial_name_hash(). Is there any reason that code doesn't
use the "full_name_hash()" which does things a word at a time?

Probably doesn't matter, but since I looked at this to see what the
new hashing was, I noticed...

            Linus