Re: [PATCH bpf-next 2/5] bpf, fsverity: Add kfunc bpf_get_fsverity_digest

[Eric Biggers <ebiggers@kernel.org>]

On Mon, Oct 16, 2023 at 01:10:40PM -0700, Song Liu wrote:
> On Sun, Oct 15, 2023 at 12:07 AM Eric Biggers <ebiggers@kernel.org> wrote:
> >
> [...]
> > > + */
> > > +__bpf_kfunc int bpf_get_fsverity_digest(struct file *file, struct bpf_dynptr_kern *digest_ptr)
> > > +{
> > > +     const struct inode *inode = file_inode(file);
> > > +     struct fsverity_digest *arg = digest_ptr->data;
> >
> > What alignment is guaranteed here?
> 
> drnptr doesn't not provide alignment guarantee for digest_ptr->data.
> If we need alignment guarantee, we need to add it here.

So technically it's wrong to cast it to struct fsverity_digest, then.

> >
> > Also, it looks like I'm being signed up to maintain this.  This isn't a stable
> > UAPI, right?  No need to document this in Documentation/?
> 
> BPF kfuncs are not UAPI. They are as stable as exported symbols.
> We do have some documents for BPF kfuncs, for example in
> Documentation/bpf/cpumasks.rst.
> 
> Do you have a recommendation or preference on where we should
> document this? AFAICT, we can either add it to fsverity.rst or somewhere
> in Documentation/bpf/.

The BPF documentation seems like the right place.

- Eric