Re: [PATCH v2 bpf-next 12/17] libbpf: support extern resolution for BTF-defined maps in .maps section

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Apr 22, 2021 at 3:56 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 4/16/21 1:23 PM, Andrii Nakryiko wrote:
> > Add extra logic to handle map externs (only BTF-defined maps are supported for
> > linking). Re-use the map parsing logic used during bpf_object__open(). Map
> > externs are currently restricted to always match complete map definition. So
> > all the specified attributes will be compared (down to pining, map_flags,
> > numa_node, etc). In the future this restriction might be relaxed with no
> > backwards compatibility issues. If any attribute is mismatched between extern
> > and actual map definition, linker will report an error, pointing out which one
> > mismatches.
> >
> > The original intent was to allow for extern to specify attributes that matters
> > (to user) to enforce. E.g., if you specify just key information and omit
> > value, then any value fits. Similarly, it should have been possible to enforce
> > map_flags, pinning, and any other possible map attribute. Unfortunately, that
> > means that multiple externs can be only partially overlapping with each other,
> > which means linker would need to combine their type definitions to end up with
> > the most restrictive and fullest map definition. This requires an extra amount
> > of BTF manipulation which at this time was deemed unnecessary and would
> > require further extending generic BTF writer APIs. So that is left for future
> > follow ups, if there will be demand for that. But the idea seems intresting
> > and useful, so I want to document it here.
> >
> > Weak definitions are also supported, but are pretty strict as well, just
> > like externs: all weak map definitions have to match exactly. In the follow up
> > patches this most probably will be relaxed, with __weak map definitions being
> > able to differ between each other (with non-weak definition always winning, of
> > course).
> >
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
>
> I think strict enforcement of extern/global map definitions is good.
> If library want people will use its maps, it may put the map definition
> into one of its headers and application can include and have
> exact the same definition.

In a lot of cases yes. But imagine I, as BPF library creator, started
out with just a typical hashmap definition, and then decided to add
pinning and maybe map_flags BPF_F_NO_PREALLOC. Why would that change
necessitate extern definition? But as you said, library provider can
(and should) provide extern definition that will be kept 100% in sync,
so this is not something that I urgently want to change.

>
> Acked-by: Yonghong Song <yhs@fb.com>