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

[Casey Schaufler]

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@xxxxxxxxxx> 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 
>