Re: [PATCH] kbuild/btf: Avoid relinking modules when only vmlinux changes

[Petr Pavlu]

On 4/2/26 4:17 PM, Petr Pavlu wrote:
> Commit 5f9ae91f7c0d ("kbuild: Build kernel module BTFs if BTF is enabled
> and pahole supports it") in 2020 introduced CONFIG_DEBUG_INFO_BTF_MODULES
> to enable generation of split BTF for kernel modules. This change required
> the %.ko Makefile rule to additionally depend on vmlinux, which is used as
> a base for deduplication. The regular ld_ko_o command executed by the rule
> was then modified to be skipped if only vmlinux changes. This was done by
> introducing a new if_changed_except command and updating the original call
> to '+$(call if_changed_except,ld_ko_o,vmlinux)'.
> 
> Later, commit 214c0eea43b2 ("kbuild: add $(objtree)/ prefix to some
> in-kernel build artifacts") in 2024 updated the rule's reference to vmlinux
> from 'vmlinux' to '$(objtree)/vmlinux'. This accidentally broke the
> previous logic to skip relinking modules if only vmlinux changes. The issue
> is that '$(objtree)' is typically '.' and GNU Make normalizes the resulting
> prerequisite './vmlinux' to just 'vmlinux', while the exclusion logic
> retains the raw './vmlinux'. As a result, if_changed_except doesn't
> correctly filter out vmlinux. Consequently, with
> CONFIG_DEBUG_INFO_BTF_MODULES=y, modules are relinked even if only vmlinux
> changes.
> 
> Additionally, commit 522397d05e7d ("resolve_btfids: Change in-place update
> with raw binary output") in 2025 reworked the method for patching BTF data
> into the resulting modules by using 'objcopy --add-section'. This command
> fails if a section already exists.
> 
> Fix the unnecessary relinking issue by also excluding the normalized form
> 'vmlinux' when invoking ld_ko_o. Adjust embed_btf_data() to first use the
> --remove-section option to remove the patched BTF section if it is already
> present.

I noticed that sorting id+flags in BTF_SET8 by resolve_btfids doesn't
seem to be idempotent, so this requires additional work.

-- Petr