linux-kernel - Re: [PATCH v4] tracepoint: Do not fail unregistering a probe due to memory failure

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20210127141617.047d5b78@gandalf.local.home>
Date:   Wed, 27 Jan 2021 14:16:17 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     paulmck <paulmck@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Matt Mullins <mmullins@...x.us>,
        Ingo Molnar <mingo@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Alexey Kardashevskiy <aik@...abs.ru>
Subject: Re: [PATCH v4] tracepoint: Do not fail unregistering a probe due to
 memory failure

On Wed, 27 Jan 2021 13:13:22 -0500 (EST)
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> > Thanks for bringing that up.  
> 
> Requiring an RCU synchronize on element removal is quite intrusive, and can
> be problematic if tracepoint removal is called from e.g. preempt-off context.

But how often do you remove more than one callback from the same
tracepoint? Or should I say, from a lot of tracepoints?

This will only synchronize for the following case:

 Add three callbacks to a single tracepoint.
 Remove the first one.
    <rcu callback to update the counters>
 Remove the second one
   <triggers a synchronization if the counters have not been finished
    updating>
 Remove the third one.
   <no synchronization needed, because it's being freed>

And we may be able to make this work in batch too.

More to come, but I really like this approach over the others because it
does not increase the size of the kernel for a failure that should never
happen in practice.

-- Steve