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

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Date:   Wed, 6 Jul 2022 22:15:47 +0000
From:   Song Liu <songliubraving@...com>
To:     Steven Rostedt <rostedt@...dmis.org>
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 Jul 6, 2022, at 2:40 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> 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);

Actually, do you mean:

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

(== -EAGAIN) here?

In this specific race condition, register_bpf() will succeed, as it already
got tr->mutex. But the IPMODIFY (livepatch) side will fail and retry. 

Since both livepatch and bpf trampoline changes are rare operations, I think 
the chance of the race condition is low enough. 

Does this make sense?

Thanks,
Song