Compiling older Linux Kernels can fail with load BTF from vmlinux: invalid argument

This article describes how to solve the error load BTF from vmlinux: invalid argument when compiling older kernels.

Problem

As a kernel developer it is not uncommon to bisect in which kernel release and with which git commit a problem first occured. However when compiling 5.x kernels with the current version of the build environment, the following error can happen:

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  KSYMS   .tmp_vmlinux.kallsyms1.S
  AS      .tmp_vmlinux.kallsyms1.S
  LD      .tmp_vmlinux.kallsyms2
  KSYMS   .tmp_vmlinux.kallsyms2.S
  AS      .tmp_vmlinux.kallsyms2.S
  LD      vmlinux
  BTFIDS  vmlinux
FAILED: load BTF from vmlinux: Invalid argument
make: *** [Makefile:1187: vmlinux] Error 255
make: *** Deleting file 'vmlinux'

It turns out the problem is caused CONFIG_DEBUG_INFO_BTF configuration option and when the pahole utility has been updated to 1.24. If the version of the pahole utility is less than 1.24, the build is successful. The new pahole (version 1.24) generates by default the new BTF_KIND_ENUM64 BTF tag, which is not supported by stable kernel.

Solution

There are three ways to address the problem.

Disable the configuration setting

By disabling the CONFIG_DEBUG_INFO_BTF configuration option, the kernel will build successfully. However this has a downside, as BPF programs cannot be run.

Downgrade the pahole package

The pahole program is part of the pahole package. If your Linux distribution still provides an earlier version, it is the easiest to downgrade to the previous version.

Patch pahole-flags.sh

The new pahole provides the --skip_encoding_btf_enum64 option to skip BTF_KIND_ENUM64 generation. By specifying this option, a stable kernel will be created.

Applying the following patch to scripts/pahole-flags.sh will achieve enabling the new flag.

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--- a/scripts/pahole-flags.sh
+++ b/scripts/pahole-flags.sh
@@ -14,4 +14,8 @@ if [ "${pahole_ver}" -ge "118" ] && [ "$
 	extra_paholeopt="${extra_paholeopt} --skip_encoding_btf_vars"
 fi
 
+if [ "${pahole_ver}" -ge "124" ]; then
+	extra_paholeopt="${extra_paholeopt} --skip_encoding_btf_enum64"
+fi
+
 echo ${extra_paholeopt}