[PATCH v4] kbuild: rpm-pkg: Preserve BTF sections in kernel modules during debuginfo stripping

[Yafang Shao <laoar.shao@gmail.com>]

After switching to the kernel's default package scripts for our local
kernel RPM builds, we noticed that module BTF entries were missing:

  $ ls /sys/kernel/btf/
  vmlinux    <<<< only vmlinux, no module BTF

Root cause: find-debuginfo.sh (from the debugedit package) prefers
eu-strip over strip when elfutils is installed, which is the common
case on RHEL 9. eu-strip removes non-allocated ELF sections, including
the .BTF section that contains BPF Type Format information for kernel
modules. Without .BTF, BPF tools (bpftool, bcc, bpftrace) cannot resolve
kernel types at runtime, and /sys/kernel/btf/<module> entries are not
created when modules are loaded.

Additionally, since commit 8646db238997 ("libbpf,bpf: Share BTF
relocate-related code with kernel"), modules contain a .BTF.base section
that maps distilled type IDs to vmlinux types. If .BTF.base is stripped,
btf_parse_module() falls back to vmlinux BTF directly, causing type ID
mismatches and rejecting the module's BTF entirely.

Fix by passing --keep-section .BTF and --keep-section .BTF.base via
_find_debuginfo_opts, which adds -K .BTF and -K .BTF.base to the
eu-strip/strip command, preserving both sections while allowing normal
debuginfo extraction to proceed.

After this change, all module BTF files are properly generated:

  $ ls /sys/kernel/btf/
  aesni_intel         drm                 i2c_i801            mfd_core
  ahci                drm_client_lib      i2c_mux             net_failover
  backlight           drm_kms_helper      i2c_smbus           pcspkr
  ccp                 drm_shmem_helper    input_leds          qemu_fw_cfg
  dm_log              failover            intel_rapl_common   sch_fq_codel
  dm_mirror           fat                 intel_rapl_msr      serio_raw
  dm_mod              fuse                irqbypass           sunrpc
  dm_region_hash      gf128mul            iTCO_wdt            vfat
  virtio_balloon      virtio_console      virtio_dma_buf      virtio_gpu
  virtio_net          virtio_rng          virtio_blk          vmlinux
  xfs

Suggested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Alan Maguire <alan.maguire@oracle.com>
---
 scripts/package/kernel.spec | 11 +++++++++++
 1 file changed, 11 insertions(+)

v3->v4: also preserve .BTF.base (sashiko-bot)
v2->v3: use %{__find_debuginfo} (Nathan)
v1->v2: check if find-debuginfo.sh supports --keep-section (sashiko-bot)

diff --git a/scripts/package/kernel.spec b/scripts/package/kernel.spec
index c732415662ef..46e80970f723 100644
--- a/scripts/package/kernel.spec
+++ b/scripts/package/kernel.spec
@@ -67,7 +67,18 @@ This package provides debug information for the kernel image and modules from th
 %undefine _unique_debug_srcs
 %undefine _debugsource_packages
 %undefine _debuginfo_subpackages
+
+# Preserve .BTF and .BTF.base sections in kernel modules during debuginfo
+# stripping. find-debuginfo.sh uses eu-strip which removes non-allocated ELF
+# sections like .BTF by default. .BTF.base is required for BTF distillation
+# support; without it, module BTF validation fails.
+%global with_keep_section %(%{__find_debuginfo} --help 2>&1 | grep -c keep-section)
+%if %{with_keep_section}
+%global _find_debuginfo_opts -r --keep-section .BTF --keep-section .BTF.base
+%else
 %global _find_debuginfo_opts -r
+%endif
+
 %global _missing_build_ids_terminate_build 1
 %global _no_recompute_build_ids 1
 %{debug_package}
-- 
2.52.0