Re: [PATCH bpf-next v4 06/12] bpf: defer vmlinux kfunc and struct_ops registrations

With CONFIG_DEBUG_INFO_BTF=m the vmlinux BTF is loaded on first use.  For
that to save anything, nothing may pull it in at boot.  The verifier no
longer does since the previous patches, but register_btf_kfunc_id_set(),
register_btf_id_dtor_kfuncs() and register_bpf_struct_ops() for vmlinux
run from initcalls and need the parsed BTF.

Queue them instead (btf_defer_reg()) and apply them in
btf_parse_vmlinux(), before the BTF is published, so that no program can
see a vmlinux BTF without its kfuncs and struct_ops, not even by id: the
id is reserved before the queue is applied, so that nothing can fail once
it is, and installed afterwards (btf_reserve_id(), btf_install_id()).
Applying a struct_ops runs its ->init(), which registers the kfunc sets
of its hook; those land back on the queue, so it is drained in a loop
until a pass adds nothing, and only then are new registrations applied
directly.  The dtor arrays are copied: every caller in the tree builds
[ ... ]
together with deferring the module BTF itself.  With =y the BTF is
present from boot and nothing is queued.  Nothing here is reachable
until the Kconfig symbol becomes a tristate.

Signed-off-by: Jay Wang <redacted>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20261001225214.12351-1-wanjay@amazon.com?part=6