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: <CAADnVQKdBrX6pSJrgBY0SvFZQLpu+CMSshwD=21NdFaoAwW_eg@mail.gmail.com>
Date: Tue, 10 Dec 2024 10:58:52 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Christian Brauner <brauner@...nel.org>
Cc: Juntong Deng <juntong.deng@...look.com>, 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 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.