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@xxxxxxxxxx>
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. Acked-by: Yonghong Song <yhs@xxxxxx>
