Re: [PATCH bpf-next v4 04/12] bpf: take the vmlinux BTF from the btf_vmlinux module

Add the runtime side of delivering the vmlinux BTF as a module: with
CONFIG_DEBUG_INFO_BTF=m the BTF is carried by a module named btf_vmlinux
and installed by the BTF module notifier when it loads.  Nothing in this
patch is reachable yet: CONFIG_DEBUG_INFO_BTF is still a bool and every
new path is under IS_MODULE(CONFIG_DEBUG_INFO_BTF); the kbuild side and
the Kconfig change follow.

With CONFIG_DEBUG_INFO_BTF=y the vmlinux BTF, 5.4 MiB on x86-64 with a
distribution config, is part of the kernel image and resident from boot
whether anything uses it or not.  Most systems never do.  Carrying it in
a module that is loaded on first use makes the memory a cost of using
BTF rather than of having a kernel that supports it.

btf_vmlinux_data() hands out the raw vmlinux BTF: from __start_BTF with
=y, or with =m from a vmalloc_user() copy that the notifier makes when
[ ... ]
maps the probed file.  Before the BTF is loaded mmap() fails, and libbpf,
which tries mmap() first, falls back to read(); afterwards it maps the
vmalloc_user() copy with remap_vmalloc_range().

Signed-off-by: Jay Wang <redacted>

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

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261001225214.12351-1-wanjay@amazon.com?part=4