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Date:   Wed, 22 Jul 2020 17:40:10 +0200
From:   Peter Zijlstra <>
To:     Song Liu <>
Cc:     Alexei Starovoitov <>,
        open list <>,
        bpf <>, Networking <>,
        Alexei Starovoitov <>,
        Daniel Borkmann <>,
        Kernel Team <>,
        "" <>,
        "" <>,
        "" <>
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

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