linux-kernel - Re: [PATCH 2/2] [v2] bpf: fix bpf_probe_read

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Message-Id: <a63a344c-e64e-4f2a-9082-f970ae9f963a@app.fastmail.com>
Date:   Wed, 24 May 2023 15:28:46 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Yonghong Song" <yhs@...a.com>, "Arnd Bergmann" <arnd@...nel.org>,
        "Alexei Starovoitov" <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        "Andrii Nakryiko" <andrii@...nel.org>,
        "Song Liu" <song@...nel.org>,
        "Steven Rostedt" <rostedt@...dmis.org>,
        "Masami Hiramatsu" <mhiramat@...nel.org>
Cc:     stable@...r.kernel.org, "Martin KaFai Lau" <martin.lau@...ux.dev>,
        "Yonghong Song" <yhs@...com>,
        "John Fastabend" <john.fastabend@...il.com>,
        "KP Singh" <kpsingh@...nel.org>,
        "Stanislav Fomichev" <sdf@...gle.com>,
        "Hao Luo" <haoluo@...gle.com>, "Jiri Olsa" <jolsa@...nel.org>,
        "Kumar Kartikeya Dwivedi" <memxor@...il.com>,
        "Dave Marchevsky" <davemarchevsky@...com>,
        "Joanne Koong" <joannelkoong@...il.com>,
        "Delyan Kratunov" <delyank@...com>,
        "Peter Zijlstra" <peterz@...radead.org>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] [v2] bpf: fix bpf_probe_read_kernel prototype mismatch

On Wed, May 24, 2023, at 05:12, Yonghong Song wrote:
> On 5/23/23 12:43 PM, Arnd Bergmann wrote:

>> Aside from the warning, this addresses a bug on 32-bit architectures
>> from incorrect argument passing with the mismatched prototype.
>
> Could you explain what is this '32-bit architectures ... incorrect 
> argument passing' thing?

I've expanded that paragraph now:

| Aside from the warning, this addresses a bug on 32-bit architectures
| from incorrect argument passing with the mismatched prototype:
| BPF_CALL_x() functions use 64-bit arguments that are passed in
| pairs of register or on the stack on 32-bit architectures, while the
| normal function uses one register per argument.

Let me know if you think I should put more details in there.

>> @@ -1635,11 +1636,13 @@ bool bpf_opcode_in_insntable(u8 code)
>>   }
>>   
>>   #ifndef CONFIG_BPF_JIT_ALWAYS_ON
>> -u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)
>> +#ifndef CONFIG_BPF_EVENTS
>> +int bpf_probe_read_kernel_common(void * dst, u32 size, const void *unsafe_ptr)
>
> void * dst => void *dst
>

Fixed now.

Thanks,

     Arnd