linux-kernel - Re: [PATCH] [v5] bpf: fix bpf_probe_read

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Message-ID: <c643db9c-c9fb-9e8d-f35c-e5c9316f657d@linux.dev>
Date:   Tue, 1 Aug 2023 07:41:26 -0700
From:   Yonghong Song <yonghong.song@...ux.dev>
To:     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:     Arnd Bergmann <arnd@...db.de>, stable@...r.kernel.org,
        John Fastabend <john.fastabend@...il.com>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        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>,
        David Vernet <void@...ifault.com>,
        Peter Zijlstra <peterz@...radead.org>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] [v5] bpf: fix bpf_probe_read_kernel prototype mismatch



On 8/1/23 4:13 AM, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> bpf_probe_read_kernel() has a __weak definition in core.c and another
> definition with an incompatible prototype in kernel/trace/bpf_trace.c,
> when CONFIG_BPF_EVENTS is enabled.
> 
> Since the two are incompatible, there cannot be a shared declaration in
> a header file, but the lack of a prototype causes a W=1 warning:
> 
> kernel/bpf/core.c:1638:12: error: no previous prototype for 'bpf_probe_read_kernel' [-Werror=missing-prototypes]
> 
> On 32-bit architectures, the local prototype
> 
> u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)
> 
> passes arguments in other registers as the one in bpf_trace.c
> 
> BPF_CALL_3(bpf_probe_read_kernel, void *, dst, u32, size,
>              const void *, unsafe_ptr)
> 
> which uses 64-bit arguments in pairs of registers.
> 
> As both versions of the function are fairly simple and only really
> differ in one line, just move them into a header file as an inline
> function that does not add any overhead for the bpf_trace.c callers
> and actually avoids a function call for the other one.
> 
> Cc: stable@...r.kernel.org
> Link: https://lore.kernel.org/all/ac25cb0f-b804-1649-3afb-1dc6138c2716@iogearbox.net/
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Acked-by: Yonghong Song <yonghong.song@...ux.dev>