linux-kernel - Re: [PATCH] audit: check syscall bitmap on entry to avoid extra work

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Date:   Tue, 23 May 2023 14:50:27 -0700
From:   Ivan Babrou <ivan@...udflare.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     audit@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...udflare.com, Eric Paris <eparis@...hat.com>
Subject: Re: [PATCH] audit: check syscall bitmap on entry to avoid extra work

On Tue, May 23, 2023 at 12:59 PM Paul Moore <paul@...l-moore.com> wrote:
> Before seriously considering something like this, I would really like
> to see some time put into profiling the original overhead and some
> designs on how that could be improved.  Without that, patches like
> this look like drive-by band-aids which have already caused enough
> headaches for audit maintenance.

Hello Paul,

Could you elaborate on what exactly you would like to see added? It's
not clear to me what is missing.

There's a benchmark in the commit description with the numbers
attached, which quantifies and explains the existing overhead. In my
experience, people on Linux mailing lists frown upon external links to
images, but if it helps to visualize the effects, I have some
flamegraphs for the benchmark from the commit message:

* 10 rules, before:
https://r2.ivan.computer/audit-syscall-bitmap/flamegraph-before.svg?s=audit
* 10 rules, with patch applied:
https://r2.ivan.computer/audit-syscall-bitmap/flamegraph-after.svg?s=audit

Here's a couple extra:

* 0 rules, auditd running:
https://r2.ivan.computer/audit-syscall-bitmap/flamegraph-after.svg?s=audit
* 11 rules (extra rule matching the running syscalls):
https://r2.ivan.computer/audit-syscall-bitmap/flamegraph-after-match.svg?s=audit

The bitmap design mirrors what tracepoints implement for syscall entry/exit:

* https://elixir.bootlin.com/linux/v6.4-rc1/source/kernel/trace/trace_syscalls.c#L585

I am happy to consider a different design if you have one in mind.