Re: [RFC PATCH bpf-next 2/3] bpf: Implement bpf_get_kern_btf_id() kfunc

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Wed, Nov 16, 2022 at 01:13:08AM IST, Alexei Starovoitov wrote:
> On Mon, Nov 14, 2022 at 08:23:39AM -0800, Yonghong Song wrote:
> > The signature of bpf_get_kern_btf_id() function looks like
> >   void *bpf_get_kern_btf_id(obj, expected_btf_id)
> > The obj has a pointer type. The expected_btf_id is 0 or
> > a btf id to be returned by the kfunc. The function
> > currently supports two kinds of obj:
> >   - obj: ptr_to_ctx, expected_btf_id: 0
> >     return the expected kernel ctx btf id
> >   - obj: ptr to char/unsigned char, expected_btf_id: a struct btf id
> >     return expected_btf_id
> > The second case looks like a type casting, e.g., in kernel we have
> >   #define skb_shinfo(SKB) ((struct skb_shared_info *)(skb_end_pointer(SKB)))
> > bpf program can get a skb_shared_info btf id ptr with bpf_get_kern_btf_id()
> > kfunc.
>
> Kumar has proposed
> bpf_rdonly_cast(any_64bit_value, btf_id) -> PTR_TO_BTF_ID | PTR_UNTRUSTED.
> The idea of bpf_get_kern_btf_id(ctx) looks complementary.
> The bpf_get_kern_btf_id name is too specific imo.
> How about two kfuncs:
>
> bpf_cast_to_kern_ctx(ctx) -> ptr_to_btf_id | ptr_trusted
> bpf_rdonly_cast(any_scalar, btf_id) -> ptr_to_btf_id | ptr_untrusted
>
> ptr_trusted flag will have semantics as discsused with David and Kumar in:
> https://lore.kernel.org/bpf/CAADnVQ+KZcFZdC=W_qZ3kam9yAjORtpN-9+Ptg_Whj-gRxCZNQ@mail.gmail.com/
>
> The verifier knows how to cast safe pointer 'ctx' to kernel 'mirror' structure.
> No need for additional btf_id argument.
> We can express it as ptr_to_btf_id | ptr_trusted and safely pass to kfuncs.
> bpf_rdonly_cast() can accept any 64-bit value.
> There is no need to limit it to 'char *' arg. Since it's ptr_to_btf_id | ptr_untrusted
> it cannot be passed to kfuncs and only rdonly acccess is allowed.
> Both kfuncs need to be cap_perfmon gated, of course.
> Thoughts?

Here is the PoC I wrote when we discussed this:
It still uses bpf_unsafe_cast naming, but that was before Alexei suggested the
bpf_rdonly_cast name.
https://github.com/kkdwivedi/linux/commits/unsafe-cast (see the 2 latest commits)
The selftest showcases how it will be useful.