linux-kernel - Re: [PATCH bpf-next 2/4] bpf: Make bpf inode storage available to tracing program

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Message-ID: <1cd17944-8c1f-4b13-9ac5-912086fbead6@schaufler-ca.com>
Date: Wed, 13 Nov 2024 10:29:57 -0800
From: Casey Schaufler <casey@...aufler-ca.com>
To: Song Liu <songliubraving@...a.com>, Christian Brauner <brauner@...nel.org>
Cc: Song Liu <song@...nel.org>, "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
 "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-security-module@...r.kernel.org"
 <linux-security-module@...r.kernel.org>, Kernel Team <kernel-team@...a.com>,
 "andrii@...nel.org" <andrii@...nel.org>,
 "eddyz87@...il.com" <eddyz87@...il.com>, "ast@...nel.org" <ast@...nel.org>,
 "daniel@...earbox.net" <daniel@...earbox.net>,
 "martin.lau@...ux.dev" <martin.lau@...ux.dev>,
 "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
 "jack@...e.cz" <jack@...e.cz>, "kpsingh@...nel.org" <kpsingh@...nel.org>,
 "mattbobrowski@...gle.com" <mattbobrowski@...gle.com>,
 "amir73il@...il.com" <amir73il@...il.com>,
 "repnop@...gle.com" <repnop@...gle.com>,
 "jlayton@...nel.org" <jlayton@...nel.org>, Josef Bacik
 <josef@...icpanda.com>, "mic@...ikod.net" <mic@...ikod.net>,
 "gnoack@...gle.com" <gnoack@...gle.com>,
 Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH bpf-next 2/4] bpf: Make bpf inode storage available to
 tracing program

On 11/13/2024 6:15 AM, Song Liu wrote:
> Hi Christian, 
>
> Thanks for your review. 
>
>> On Nov 13, 2024, at 2:19 AM, Christian Brauner <brauner@...nel.org> wrote:
> [...]
>
>>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>>> index 3559446279c1..479097e4dd5b 100644
>>> --- a/include/linux/fs.h
>>> +++ b/include/linux/fs.h
>>> @@ -79,6 +79,7 @@ struct fs_context;
>>> struct fs_parameter_spec;
>>> struct fileattr;
>>> struct iomap_ops;
>>> +struct bpf_local_storage;
>>>
>>> extern void __init inode_init(void);
>>> extern void __init inode_init_early(void);
>>> @@ -648,6 +649,9 @@ struct inode {
>>> #ifdef CONFIG_SECURITY
>>> void *i_security;
>>> #endif
>>> +#ifdef CONFIG_BPF_SYSCALL
>>> + struct bpf_local_storage __rcu *i_bpf_storage;
>>> +#endif
>> Sorry, we're not growing struct inode for this. It just keeps getting
>> bigger. Last cycle we freed up 8 bytes to shrink it and we're not going
>> to waste them on special-purpose stuff. We already NAKed someone else's
>> pet field here.
> Would it be acceptable if we union i_bpf_storage with i_security?

No!

> IOW, if CONFIG_SECURITY is enabled, we will use existing logic. 
> If CONFIG_SECURITY is not enabled, we will use i_bpf_storage. 
> Given majority of default configs have CONFIG_SECURITY=y, this 
> will not grow inode for most users. OTOH, users with 
> CONFIG_SECURITY=n && CONFIG_BPF_SYSCALL=y combination can still 
> use inode local storage in the tracing BPF programs. 
>
> Does this make sense?

All it would take is one BPF programmer assuming that CONFIG_SECURITY=n
is the norm for this to blow up spectacularly.

>
> Thanks,
> Song 
>