linux-kernel - Re: [PATCH v2 bpf-next 1/3] bpf: Add bpf_perf_prog_read

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Message-ID: <2b11467f-9d93-8109-4561-d25ac605ef10@fb.com>
Date:   Thu, 23 Jan 2020 22:41:15 +0000
From:   Andrii Nakryiko <andriin@...com>
To:     Daniel Xu <dxu@...uu.xyz>, Daniel Borkmann <daniel@...earbox.net>,
        "John Fastabend" <john.fastabend@...il.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>
Subject: Re: [PATCH v2 bpf-next 1/3] bpf: Add bpf_perf_prog_read_branches()
 helper

On 1/23/20 2:30 PM, Daniel Xu wrote:
> On Thu Jan 23, 2020 at 11:23 PM, Daniel Borkmann wrote:
> [...]
>>
>> Yes, so we've been following this practice for all the BPF helpers no
>> matter
>> which program type. Though for tracing it may be up to debate whether it
>> makes
>> still sense given there's nothing to be leaked here since you can read
>> this data
>> anyway via probe read if you'd wanted to. So we might as well get rid of
>> the
>> clearing for all tracing helpers.
> 
> Right, that makes sense. Do you want me to leave it in for this patchset
> and then remove all of them in a followup patchset?
> 

I don't think we can remove that for existing tracing helpers (e.g., 
bpf_probe_read). There are applications that explicitly expect 
destination memory to be zeroed out on failure. It's a BPF world's 
memset(0).

I also wonder if BPF verifier has any extra assumptions for 
ARG_PTR_TO_UNINIT_MEM w.r.t. it being initialized after helper call 
(e.g., for liveness tracking).

>>
>> Different question related to your set. It looks like br_stack is only
>> available
>> on x86, is that correct? For other archs this will always bail out on
>> !br_stack
>> test. Perhaps we should document this fact so users are not surprised
>> why their
>> prog using this helper is not working on !x86. Wdyt?
> 
> I think perf_event_open() should fail on !x86 if a user tries to configure
> it with branch stack collection. So there would not be the opportunity for
> the bpf prog to be attached and run. I haven't tested this, though. I'll
> look through the code / install a VM and test it.
> 
> [...]
> 
> Thanks,
> Daniel
>