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Date:   Fri, 18 Nov 2022 16:26:34 +0000
From:   Mark Rutland <mark.rutland@arm.com>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Masami Hiramatsu <mhiramat@kernel.org>,
        Florent Revest <revest@chromium.org>,
        bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        KP Singh <kpsingh@kernel.org>,
        Brendan Jackman <jackmanb@google.com>, markowsky@google.com,
        Steven Rostedt <rostedt@goodmis.org>,
        Xu Kuohai <xukuohai@huawei.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFC 0/1] BPF tracing for arm64 using fprobe
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Hi Alexei,

On Thu, Nov 17, 2022 at 08:50:12AM -0800, Alexei Starovoitov wrote:
> There might not be a task available where bpf trampoline is running.

I'm not sure what you mean by "there might not be a task available"; do you
mean that there might not be space in the per-task shadow stack, or that the
BPF program can be invoked inan IRQ context?

> rcu protection might not be there either.

We've spent a lot of time reworking entry/idle sequences with noinstr, so any
time BPF can be invoked, we should have a regular kernel environment available,
with RCU watching (but not necessarily in an RCU read-side critical section).
If BPF is being invoked without RCU watching, that is a bug that needs to be
fixed.

Do you have a specific example in mind?

Thanks,
Mark.