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This means that > some sections may start out as ephemeral (if only externs are present and thus > there is not corresponding ELF section), but will be "upgraded" to actual ELF > section as symbols are resolved or new non-extern definitions are appended. > > Additional care is taken to not duplicate extern entries in sections like > .kconfig and .ksyms. > > Given libbpf requires BTF type to always be present for .kconfig/.ksym > externs, linker extends this requirement to all the externs, even those that > are supposed to be resolved during static linking and which won't be visible > to libbpf. With BTF information always present, static linker will check not > just ELF symbol matches, but entire BTF type signature match as well. That > logic is stricter that BPF CO-RE checks. It probably should be re-used by > .ksym resolution logic in libbpf as well, but that's left for follow up > patches. > > To make it unnecessary to rewrite ELF symbols and minimize BTF type > rewriting/removal, ELF symbols that correspond to externs initially will be > updated in place once they are resolved. Similarly for BTF type info, VAR/FUNC > and var_secinfo's (sec_vars in struct bpf_linker) are staying stable, but > types they point to might get replaced when extern is resolved. This might > leave some left-over types (even though we try to minimize this for common > cases of having extern funcs with not argument names vs concrete function with > names properly specified). That can be addresses later with a generic BTF > garbage collection. That's left for a follow up as well. > > Given BTF type appending phase is separate from ELF symbol > appending/resolution, special struct glob_sym->underlying_btf_id variable is > used to communicate resolution and rewrite decisions. 0 means > underlying_btf_id needs to be appended (it's not yet in final linker->btf), <0 > values are used for temporary storage of source BTF type ID (not yet > rewritten), so -glob_sym->underlying_btf_id is BTF type id in obj-btf. But by > the end of linker_append_btf() phase, that underlying_btf_id will be remapped > and will always be > 0. This is the uglies part of the whole process, but > keeps the other parts much simpler due to stability of sec_var and VAR/FUNC > types, as well as ELF symbol, so please keep that in mind while reviewing. > > BTF-defined maps require some extra custom logic and is addressed separate in > the next patch, so that to keep this one smaller and easier to review. > > Signed-off-by: Andrii Nakryiko Acked-by: Yonghong Song