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: <20241217-bespucken-beimischen-339f3cc03dc2@brauner>
Date: Tue, 17 Dec 2024 13:30:08 +0100
From: Christian Brauner <brauner@...nel.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
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 10:58:52AM -0800, 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.

Hm? Was that directed at my question? I don't have the background to
judge this and this whole api looks like a giant footgun so far for
questionable purposes.

I have a hard time seeing parts of CRIU moved into bpf especially
because all of the userspace stuff exists.

> 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.