Re: [PATCH bpf-next v4 03/12] bpf: fetch the vmlinux BTF where kernel types enter a program

bpf_check() fetches the vmlinux BTF up front for every program, whether
the program uses kernel types or not.  With the upcoming
CONFIG_DEBUG_INFO_BTF=m the BTF is a module that is loaded on demand,
and since systemd loads socket filters at boot, a program that needs the
BTF only because bpf_check() asked for it would pull it in on every
system, whether anything uses BTF or not.

Stop fetching up front and fetch at the points where kernel types enter
the verifier state instead:

 - bpf_add_kfunc_call(), for the first kfunc call of a program;
 - check_pseudo_btf_id(), for ldimm64 of a kernel variable;
 - check_ptr_to_map_access(), for accessing a map pointer's fields;
 - check_helper_call(), when the helper's prototype takes or returns a
   PTR_TO_BTF_ID, or is bpf_snprintf_btf()/bpf_seq_printf_btf(), which
[ ... ]
With CONFIG_DEBUG_INFO_BTF=y the vmlinux BTF is parsed at boot by the
first kfunc registration, and without BTF the new helper check is
skipped, so nothing changes for either.

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=3