linux-kernel - Re: [PATCH] RISC-V/bpf: Enable bpf_probe

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Date:   Wed, 6 Jul 2022 00:01:51 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Yonghong Song <yhs@...com>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        Alan Maguire <alan.maguire@...cle.com>,
        Yixun Lan <dlan@...too.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        linux-riscv@...ts.infradead.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH] RISC-V/bpf: Enable bpf_probe_read{, str}()

On Tue, Jul 05, 2022 at 11:41:30PM -0700, Yonghong Song wrote:
> 
> 
> On 7/5/22 10:00 PM, Andrii Nakryiko wrote:
> > On Sun, Jul 3, 2022 at 10:53 PM Christoph Hellwig <hch@...radead.org> wrote:
> > > 
> > > On Sun, Jul 03, 2022 at 09:09:24PM +0800, Yixun Lan wrote:
> > > > Enable this option to fix a bcc error in RISC-V platform
> > > > 
> > > > And, the error shows as follows:
> > > 
> > > These should not be enabled on new platforms.  Use the proper helpers
> > > to probe kernel vs user pointers instead.
> > 
> > riscv existed as of [0], so I'd argue it is a proper bug fix, as
> > corresponding select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE should
> > have been added back then.
> > 
> > But I also agree that BCC tools should be updated to use proper
> > bpf_probe_read_{kernel,user}[_str()] helpers, please contribute such
> > fixes to BCC tools and BCC itself as well. Cc'ed Alan as his ksnoop in
> > libbpf-tools seems to be using bpf_probe_read() as well and needs to
> > be fixed.
> 
> Yixun, the bcc change looks like below:

No, this is broken.  bcc needs to stop using bpf_probe_read entirely
for user addresses and unconditionally use bpf_probe_read_user first
and only fall back to bpf_probe_read if not supported.