netdev - Re: [PATCH v3 bpf-next 12/18] libbpf: support extern resolution for BTF-defined maps in .maps section

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Message-ID: <df3be1a8-a8b7-b79a-2264-b5ccb2d5a105@fb.com>
Date:   Fri, 23 Apr 2021 11:49:15 -0700
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andrii@...nel.org>, <bpf@...r.kernel.org>,
        <netdev@...r.kernel.org>, <ast@...com>, <daniel@...earbox.net>
CC:     <kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 12/18] libbpf: support extern resolution for
 BTF-defined maps in .maps section



On 4/23/21 11:13 AM, 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@...nel.org>

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