linux-kernel - Re: [PATCH bpf-next v1 0/3] add bpf_file_d

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Message-ID: <9642ad1d-e227-417e-a9ff-b69b2cb2d0d9@linux.dev>
Date: Thu, 18 Jul 2024 10:16:54 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Lin Yikai <yikai.lin@...o.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
 Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Matt Bobrowski <mattbobrowski@...gle.com>,
 Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
 linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
 opensource.kernel@...o.com
Subject: Re: [PATCH bpf-next v1 0/3] add bpf_file_d_path helper and selftests

On 7/18/24 4:51 AM, Lin Yikai wrote:
> v1:
>   - patch 2:
>     - [1/2] add bpf_file_d_path helper
>     - [2/2] add selftest to it
> 
> Hi, we are looking to add the "bpf_file_d_path" helper,
> used to retrieve the path from a struct file object.
> 	bpf_file_d_path(void *file, char *dst, u32 size);
> 	
> It's worth noting that the "file" parameter is defined as "void*" type.
> 
> * Our problems *
> Previously, we encountered issues
> on some user-space operating systems(OS):
> 
> 1.Difficulty using vmlinux.h
> (1) The OS lacks support for bpftool.
> We can not use:
> "bpftool btf dump file /sys/kernel/btf/vmlinux format c > vmlinux.h".
> Bpftool need a separate complex cross-compilation environment to build.
> 
> (2) Many duplicate definitions between OS and vmlinux.h.
> 
> (3) The vmlinux.h size is large (2.8MB on arm64/Android),
> causing increased ebpf prog size and user space consumption.

The compiled bpf prog size is increased by 2.8MB because it included vmlinux.h?

> 
> 2.The "struct file" has many internal variables and definitions,
> and maybe change along with Linux version iterations,
> making it hard to copy it to OS.

If vmlinux.h is not convenience in your use case, you can try to define "struct 
file" with __attribute__((preserve_access_index)) and the libbpf will adjust the 
bpf prog against the running kernel.

There was a discussion a year ago about bpf helpers freeze. No new helper can be 
added since then. The same goes for this one.

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