Re: [PATCH v4 15/17] module: Introduce hash-based integrity checking

["Thomas Weißschuh" <linux@weissschuh.net>]

On 2026-01-30 18:06:20+0100, Petr Pavlu wrote:
> On 1/13/26 1:28 PM, Thomas Weißschuh wrote:
> > Normally the .ko module files depend on a fully built vmlinux to be
> > available for modpost validation and BTF generation. With
> > CONFIG_MODULE_HASHES, vmlinux now depends on the modules
> > to build a merkle tree. This introduces a dependency cycle which is
> > impossible to satisfy. Work around this by building the modules during
> > link-vmlinux.sh, after vmlinux is complete enough for modpost and BTF
> > but before the final module hashes are
> 
> I wonder if this dependency cycle could be resolved by utilizing the
> split into vmlinux.unstripped and vmlinux that occurred last year.
> 
> The idea is to create the following ordering: vmlinux.unstripped ->
> modules -> vmlinux, and to patch in .module_hashes only when building
> the final vmlinux.
> 
> This would require the following:
> * Split scripts/Makefile.vmlinux into two Makefiles, one that builds the
>   current vmlinux.unstripped and the second one that builds the final
>   vmlinux from it.
> * Modify the top Makefile to recognize vmlinux.unstripped and update the
>   BTF generation rule 'modules: vmlinux' to
>   'modules: vmlinux.unstripped'.
> * Add the 'vmlinux: modules' ordering in the top Makefile for
>   CONFIG_MODULE_HASHES=y.
> * Remove the patching of vmlinux.unstripped in scripts/link-vmlinux.sh
>   and instead move it into scripts/Makefile.vmlinux when running objcopy
>   to produce the final vmlinux.
> 
> I think this approach has two main advantages:
> * CONFIG_MODULE_HASHES can be made orthogonal to
>   CONFIG_DEBUG_INFO_BTF_MODULES.
> * All dependencies are expressed at the Makefile level instead of having
>   scripts/link-vmlinux.sh invoke 'make -f Makefile modules'.
> 
> Below is a rough prototype that applies on top of this series. It is a
> bit verbose due to the splitting of part of scripts/Makefile.vmlinux
> into scripts/Makefile.vmlinux_unstripped.

That looks like a feasible alternative. Before adopting it, I'd like to
hear the preference of the kbuild folks.

> diff --git a/Makefile b/Makefile
> index 841772a5a260..19a3beb82fa7 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1259,7 +1259,7 @@ vmlinux_o: vmlinux.a $(KBUILD_VMLINUX_LIBS)
>  vmlinux.o modules.builtin.modinfo modules.builtin: vmlinux_o
>  	@:
>  
> -PHONY += vmlinux
> +PHONY += vmlinux.unstripped vmlinux
>  # LDFLAGS_vmlinux in the top Makefile defines linker flags for the top vmlinux,
>  # not for decompressors. LDFLAGS_vmlinux in arch/*/boot/compressed/Makefile is
>  # unrelated; the decompressors just happen to have the same base name,
> @@ -1270,9 +1270,11 @@ PHONY += vmlinux
>  #   https://savannah.gnu.org/bugs/?61463
>  # For Make > 4.4, the following simple code will work:
>  #  vmlinux: private export LDFLAGS_vmlinux := $(LDFLAGS_vmlinux)
> -vmlinux: private _LDFLAGS_vmlinux := $(LDFLAGS_vmlinux)
> -vmlinux: export LDFLAGS_vmlinux = $(_LDFLAGS_vmlinux)
> -vmlinux: vmlinux.o $(KBUILD_LDS) modpost
> +vmlinux.unstripped: private _LDFLAGS_vmlinux := $(LDFLAGS_vmlinux)
> +vmlinux.unstripped: export LDFLAGS_vmlinux = $(_LDFLAGS_vmlinux)
> +vmlinux.unstripped: vmlinux.o $(KBUILD_LDS) modpost
> +	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.vmlinux_unstripped
> +vmlinux: vmlinux.unstripped
>  	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.vmlinux

Maybe we could keep them together in a single Makefile,
and instead have different targets in it.

(...)

> @@ -98,70 +44,15 @@ remove-symbols := -w --strip-unneeded-symbol='__mod_device_table__*'
>  # To avoid warnings: "empty loadable segment detected at ..." from GNU objcopy,
>  # it is necessary to remove the PT_LOAD flag from the segment.
>  quiet_cmd_strip_relocs = OBJCOPY $@
> -      cmd_strip_relocs = $(OBJCOPY) $(patsubst %,--set-section-flags %=noload,$(remove-section-y)) $< $@; \
> -                         $(OBJCOPY) $(addprefix --remove-section=,$(remove-section-y)) $(remove-symbols) $@
> +      cmd_script_relocs = $(OBJCOPY) $(patsubst %,--set-section-flags %=noload,$(remove-section-y)) $< $@; \
> +                          $(OBJCOPY) $(addprefix --remove-section=,$(remove-section-y)) \
> +                                     $(remove-symbols) \
> +                                     $(patch-module-hashes) $@

cmd_script_relocs -> cmd_strip_relocs

(...)