On Thu, Apr 22, 2021 at 3:56 PM Yonghong Song <yhs@xxxxxx> 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@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. 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@xxxxxx>
