linux-kernel - Re: [PATCH bpf-next v5 4/5] bpf: Make fs kfuncs available for SYSCALL and TRACING program types

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Message-ID:
 <AM6PR03MB508072B5D29C8BD433AD186E993E2@AM6PR03MB5080.eurprd03.prod.outlook.com>
Date: Wed, 11 Dec 2024 21:29:12 +0000
From: Juntong Deng <juntong.deng@...look.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>,
 Christian Brauner <brauner@...nel.org>
Cc: Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 John Fastabend <john.fastabend@...il.com>,
 Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, Eddy Z <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>,
 KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
 Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
 Kumar Kartikeya Dwivedi <memxor@...il.com>, snorcht@...il.com,
 bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
 Linux-Fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v5 4/5] bpf: Make fs kfuncs available for SYSCALL
 and TRACING program types

On 2024/12/10 18:58, Alexei Starovoitov wrote:
> On Tue, Dec 10, 2024 at 6:43 AM Christian Brauner <brauner@...nel.org> wrote:
>>
>> On Tue, Dec 10, 2024 at 02:03:53PM +0000, Juntong Deng wrote:
>>> Currently fs kfuncs are only available for LSM program type, but fs
>>> kfuncs are generic and useful for scenarios other than LSM.
>>>
>>> This patch makes fs kfuncs available for SYSCALL and TRACING
>>> program types.
>>
>> I would like a detailed explanation from the maintainers what it means
>> to make this available to SYSCALL program types, please.
> 
> Sigh.
> This is obviously not safe from tracing progs.
> 
>  From BPF_PROG_TYPE_SYSCALL these kfuncs should be safe to use,
> since those progs are not attached to anything.
> Such progs can only be executed via sys_bpf syscall prog_run command.
> They're sleepable, preemptable, faultable, in task ctx.
> 
> But I'm not sure what's the value of enabling these kfuncs for
> BPF_PROG_TYPE_SYSCALL.

Thanks for your reply.

Song said here that we need some of these kfuncs to be available for
tracing functions [0].

If Song saw this email, could you please join the discussion?

[0]: 
https://lore.kernel.org/bpf/CAPhsuW6ud21v2xz8iSXf=CiDL+R_zpQ+p8isSTMTw=EiJQtRSw@mail.gmail.com/

For BPF_PROG_TYPE_SYSCALL, I think BPF_PROG_TYPE_SYSCALL has now
exceeded its original designed purpose and has become a more general
program type.

Currently BPF_PROG_TYPE_SYSCALL is widely used in HID drivers, and there
are some use cases in sched-ext (CRIB is also a use case, although still
in infancy).

As BPF_PROG_TYPE_SYSCALL becomes more general, it would be valuable to
make more kfuncs available for BPF_PROG_TYPE_SYSCALL.