netdev - Re: [PATCH v3 bpf-next 1/2] bpf: separate bpf_get

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Date:   Wed, 22 Jul 2020 17:40:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        open list <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "kpsingh@...omium.org" <kpsingh@...omium.org>,
        "brouer@...hat.com" <brouer@...hat.com>
Subject: Re: [PATCH v3 bpf-next 1/2] bpf: separate bpf_get_[stack|stackid]
 for perf events BPF

On Tue, Jul 21, 2020 at 10:40:19PM +0000, Song Liu wrote:

> We only need to block precise_ip >= 2. precise_ip == 1 is OK. 

Uuuh, how? Anything PEBS would have the same problem. Sure, precise_ip
== 1 will not correct the IP, but the stack will not match regardless.

You need IP,SP(,BP) to be a consistent set _AND_ have it match the
current stack, PEBS simply cannot do that, because the regs get recorded
(much) earlier than the PMI and the stack can have changed in the
meantime.