linux-kernel - Re: [PATCH v2 bpf-next 5/5] bpf: trampoline: support FTRACE_OPS_FL_SHARE

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Message-ID: <20220706174049.6c60250f@gandalf.local.home>
Date:   Wed, 6 Jul 2022 17:40:49 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Song Liu <songliubraving@...com>
Cc:     Song Liu <song@...nel.org>, Networking <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, lkml <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Kernel Team <Kernel-team@...com>,
        "jolsa@...nel.org" <jolsa@...nel.org>,
        "mhiramat@...nel.org" <mhiramat@...nel.org>
Subject: Re: [PATCH v2 bpf-next 5/5] bpf: trampoline: support
 FTRACE_OPS_FL_SHARE_IPMODIFY

On Wed, 6 Jul 2022 21:37:52 +0000
Song Liu <songliubraving@...com> wrote:

> > Can you comment here that returning -EAGAIN will not cause this to repeat.
> > That it will change things where the next try will not return -EGAIN?  
> 
> Hmm.. this is not the guarantee here. This conflict is a real race condition 
> that an IPMODIFY function (i.e. livepatch) is being registered at the same time 
> when something else, for example bpftrace, is updating the BPF trampoline. 
> 
> This EAGAIN will propagate to the user of the IPMODIFY function (i.e. livepatch),
> and we need to retry there. In the case of livepatch, the retry is initiated 
> from user space. 

We need to be careful here then. If there's a userspace application that
runs at real-time and does a:

	do {
		errno = 0;
		regsiter_bpf();
	} while (errno != -EAGAIN);

it could in theory preempt the owner of the lock and never make any
progress.

-- Steve