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@xxxxxxxxxxxxxx/ > > 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.
