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Date:   Tue, 5 Jul 2022 23:41:30 -0700
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Subject: Re: [PATCH] RISC-V/bpf: Enable bpf_probe_read{, str}()
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To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        Christoph Hellwig <hch@infradead.org>,
        Alan Maguire <alan.maguire@oracle.com>
Cc:     Yixun Lan <dlan@gentoo.org>, Palmer Dabbelt <palmer@dabbelt.com>,
        linux-riscv@lists.infradead.org,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>
References: <20220703130924.57240-1-dlan@gentoo.org>
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From:   Yonghong Song <yhs@fb.com>
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On 7/5/22 10:00 PM, Andrii Nakryiko wrote:
> On Sun, Jul 3, 2022 at 10:53 PM Christoph Hellwig <hch@infradead.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:

--- a/src/cc/frontends/clang/b_frontend_action.cc
+++ b/src/cc/frontends/clang/b_frontend_action.cc
@@ -132,7 +132,7 @@ static std::string 
check_bpf_probe_read_user(llvm::StringRef probe,

      /* For arch with overlapping address space, dont use 
bpf_probe_read for
       * user read. Just error out */
-#if defined(__s390x__)
+#if defined(__s390x__) || defined(__riscv_)
      overlap_addr = true;
      return "";
  #endif


Basically, prevent using bpf_probe_read() helper, so it will force user
to use bpf_probe_read_user() or bpf_probe_read_kernel(). and this should
make it work for old kernels.

> 
>    [0] 0ebeea8ca8a4 ("bpf: Restrict bpf_probe_read{, str}() only to
> archs where they work")