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: <CAPhsuW7zZuHf6dgDpYnibONoKt0p=zb0wCta1R1MtLv=Q=4FfA@mail.gmail.com>
Date: Wed, 11 Dec 2024 14:06:13 -0800
From: Song Liu <song@...nel.org>
To: Juntong Deng <juntong.deng@...look.com>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, Christian Brauner <brauner@...nel.org>, 
	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>, 
	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 Wed, Dec 11, 2024 at 1:29 PM Juntong Deng <juntong.deng@...look.com> wrote:
>
> 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].

I meant we can put the new kfuncs, such as bpf_get_file_ops_type, in
bpf_fs_kfuncs.c, and make it available to tracing programs. But we
cannot blindly make all of these kfuncs available to tracing programs.
Instead, we need to review each kfunc and check whether it is safe
for tracing programs.

Thanks,
Song

> 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/