linux-kernel - Re: [PATCH v2 bpf-next 2/4] bpf: Make bpf inode storage available to tracing program

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Message-ID: <CAPhsuW4xEOhpU87kGvO45TBW0aLwCKvFi9hRfJ90FgGUq8OAcA@mail.gmail.com>
Date: Tue, 12 Nov 2024 15:10:04 -0800
From: Song Liu <song@...nel.org>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: bpf@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org, 
	kernel-team@...a.com, andrii@...nel.org, eddyz87@...il.com, ast@...nel.org, 
	daniel@...earbox.net, viro@...iv.linux.org.uk, brauner@...nel.org, 
	jack@...e.cz, kpsingh@...nel.org, mattbobrowski@...gle.com, 
	amir73il@...il.com, repnop@...gle.com, jlayton@...nel.org, 
	josef@...icpanda.com, mic@...ikod.net, gnoack@...gle.com
Subject: Re: [PATCH v2 bpf-next 2/4] bpf: Make bpf inode storage available to
 tracing program

On Tue, Nov 12, 2024 at 3:00 PM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
>
> On 11/12/24 12:36 AM, Song Liu wrote:
> >   void __destroy_inode(struct inode *inode)
> >   {
> >       BUG_ON(inode_has_buffers(inode));
> > +     bpf_inode_storage_free(inode);
>
> Not sure if this is done in the rcu callback (i.e. after the rcu gp). Please check.
>
> >       inode_detach_wb(inode);
> >       security_inode_free(inode);
> >       fsnotify_inode_delete(inode);
>
> [ ... ]
>
> > @@ -136,12 +119,7 @@ BPF_CALL_5(bpf_inode_storage_get, struct bpf_map *, map, struct inode *, inode,
> >       if (flags & ~(BPF_LOCAL_STORAGE_GET_F_CREATE))
> >               return (unsigned long)NULL;
> >
> > -     /* explicitly check that the inode_storage_ptr is not
> > -      * NULL as inode_storage_lookup returns NULL in this case and
> > -      * bpf_local_storage_update expects the owner to have a
> > -      * valid storage pointer.
> > -      */
> > -     if (!inode || !inode_storage_ptr(inode))
> > +     if (!inode)
> >               return (unsigned long)NULL;
>
> There is an atomic_read in this function:
>
>         /* only allocate new storage, when the inode is refcounted */
>         if (atomic_read(&inode->i_count) &&
>             flags & BPF_LOCAL_STORAGE_GET_F_CREATE) {
>
> If the bpf_inode_storage_free is not done after rcu gp, this will need a
> inc_not_zero like how the sk storage does. I think moving the storage_free to
> the inode rcu call back may be easier if it is not the case now.

This is a great catch!

I will move bpf_inode_storage_free to i_callback().

Thanks,
Song