Re: [PATCH v3 12/28] kbuild: Defer running objtool to link time for all CFG features

[Nicolas Schier]

On Mon, Sep 21, 2026 at 03:31:13PM -0700, Josh Poimboeuf wrote:
> noreturns.h file is hand-maintained file which hard codes all the known
> __noreturn functions in the kernel.  It's fragile, often missing
> entries, and keeping it up to date is a maintenance burden.
> 
> For LTO, IBT, and klp-build configs, the majority of those entries
> aren't needed, because objtool runs on vmlinux.o, so it already has
> visibility into whether a given function returns (with its dead end
> detection).
> 
> Remove the need for many of the noreturns.h entries by just always
> deferring objtool for all features which rely on having the noreturn
> information: namely the ones which rely on the reverse-engineered
> control flow graph (CFG).
> 
> Note that NOINSTR_VALIDATION is a special case, where objtool runs on
> individual TUs (with all the objtool-args-y), and then again on
> vmlinux.o (without objtool-args-y except for --werror).
> 
> This change makes NOINSTR_VALIDATION just another OBJTOOL_DEFERRED
> feature which runs deferred, and *not* on TUs, so now it gets all the
> args like the others, and the special case for adding --werror on
> !OBJTOOL_DEFERRED is no longer needed.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> ---
>  lib/Kconfig.debug          |  3 ++-
>  scripts/Makefile.vmlinux_o | 14 +++-----------
>  tools/objtool/noreturns.h  | 24 ------------------------
>  3 files changed, 5 insertions(+), 36 deletions(-)
> 
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 798a10ded19fd..e5aea1c0e54a2 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -575,7 +575,8 @@ config OBJTOOL_WERROR
>  config OBJTOOL_DEFERRED
>  	def_bool y
>  	depends on OBJTOOL
> -	depends on LTO_CLANG || X86_KERNEL_IBT || KLP_BUILD
> +	depends on OBJTOOL_CONTROL_FLOW || NOINSTR_VALIDATION || LTO_CLANG || \
> +		   X86_KERNEL_IBT || KLP_BUILD
>  
>  # Objtool reverse-engineers the control flow graph
>  config OBJTOOL_CONTROL_FLOW
> diff --git a/scripts/Makefile.vmlinux_o b/scripts/Makefile.vmlinux_o
> index 9bac917e8b819..df1e3584883e6 100644
> --- a/scripts/Makefile.vmlinux_o
> +++ b/scripts/Makefile.vmlinux_o
> @@ -30,20 +30,12 @@ endif
>  # objtool for vmlinux.o
>  # ---------------------------------------------------------------------------
>  #
> -# For CONFIG_OBJTOOL_DEFERRED (IBT or LTO), objtool doesn't run on individual
> -# translation units.  Instead it runs on vmlinux.o.
> -#
> -# For !CONFIG_OBJTOOL_DEFERRED + CONFIG_NOINSTR_VALIDATION, it runs on both
> -# translation units and vmlinux.o, with the latter only used for noinstr/unret
> -# validation.
> +# For CONFIG_OBJTOOL_DEFERRED, objtool doesn't run on individual translation
> +# units.  Instead it runs on vmlinux.o.

According to the proposed commit message that's not true if 
CONFIG_NOINSTR_VALIDATION=y.  Is that (still) correct?  Might it make 
sense to mention the exception here, too?

for kbuild:

Acked-by: Nicolas Schier <n.schier@xxxxxxxxx>

-- 
Nicolas