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

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Date:   Wed, 6 Jul 2022 21:50:34 +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);
> 
> it could in theory preempt the owner of the lock and never make any
> progress.

We can probably workaround this with some trick on tr->indirect_call. However, 
I don't think this is a real concern from livepatch side. We have seen many 
other issues that cause live patch to fail and requires retry. This race 
condition in theory shouldn't cause real world issues. 

Thanks,
Song