Re: [PATCH bpf 2/2] bpf: selftests: Add non function pointer test to struct_ops

On Wed, Feb 10, 2021 at 06:07:04PM -0800, Andrii Nakryiko wrote:

On Wed, Feb 10, 2021 at 5:55 PM Martin KaFai Lau [off-list ref] wrote:

quoted

On Wed, Feb 10, 2021 at 02:54:40PM -0800, Andrii Nakryiko wrote:

quoted

On Wed, Feb 10, 2021 at 1:17 PM Martin KaFai Lau [off-list ref] wrote:

quoted

On Wed, Feb 10, 2021 at 12:27:38PM -0800, Andrii Nakryiko wrote:

quoted

On Tue, Feb 9, 2021 at 12:11 PM Martin KaFai Lau [off-list ref] wrote:

quoted

This patch adds a "void *owner" member.  The existing
bpf_tcp_ca test will ensure the bpf_cubic.o and bpf_dctcp.o
can be loaded.

Signed-off-by: Martin KaFai Lau <redacted>
---

Acked-by: Andrii Nakryiko <andrii@kernel.org>

What will happen if BPF code initializes such non-func ptr member?
Will libbpf complain or just ignore those values? Ignoring initialized
members isn't great.

The latter. libbpf will ignore non-func ptr member.  The non-func ptr
member stays zero when it is passed to the kernel.

libbpf can be changed to copy this non-func ptr value.
The kernel will decide what to do with it.  It will
then be consistent with int/array member like ".name"
and ".flags" where the kernel will verify the value.
I can spin v2 to do that.

I was thinking about erroring out on non-zero fields, but if you think
it's useful to pass through values, it could be done, but will require
more and careful code, probably. So, basically, don't feel obligated
to do this in this patch set.

You meant it needs different handling in copying ptr value
than copying int/char[]?

Hm.. If we are talking about copying pointer values, then I don't see
how you can provide a valid kernel pointer from the BPF program?...

I am thinking the kernel is already rejecting members that is supposed
to be zero (e.g. non func ptr here), so there is no need to add codes
to libbpf to do this again.

But if we are talking about copying field values in general, then
you'll need to handle enums, struct/union, etc, no? If int/char[] is
supported (I probably missed that it is), that might be the only
things you'd need to support. So for non function pointers, I'd just
enforce zeroes.

Sure, we can reject everything else for non zero in libbpf.
I think we can use a different patch set for that?