Re: [PATCH] kbuild: add -fdiagnostics-show-inlining-chain for FORTIFY_SOURCE

[Nathan Chancellor <nathan@kernel.org>]

On Fri, Mar 27, 2026 at 02:59:20PM -0700, Justin Stitt wrote:
> Clang recently added -fdiagnostics-show-inlining-chain [1] to improve
> the visibility of inlining chains in diagnostics. This is particularly
> useful for CONFIG_FORTIFY_SOURCE where detections can happen deep in
> inlined functions.
> 
> Add this flag to KBUILD_CFLAGS when CONFIG_FORTIFY_SOURCE is enabled
> and the compiler supports it.
> 
> Link: https://github.com/llvm/llvm-project/pull/174892 [1]
> Link: https://github.com/ClangBuiltLinux/linux/issues/1571
> Signed-off-by: Justin Stitt <justinstitt@google.com>
> ---
>  Makefile | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/Makefile b/Makefile
> index e1279c4d5b24..978726aeb1ef 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -973,6 +973,12 @@ KBUILD_CFLAGS	+= $(call cc-option, -fno-stack-clash-protection)
>  # Get details on warnings generated due to GCC value tracking.
>  KBUILD_CFLAGS	+= $(call cc-option, -fdiagnostics-show-context=2)
>  
> +# Show inlining chain notes for FORTIFY_SOURCE-related diagnostics.
> +# GCC does this by default while Clang 23+ supports a flag.
> +ifdef CONFIG_FORTIFY_SOURCE

While this is indeed particularly useful for CONFIG_FORTIFY_SOURCE, this
impacts the warning and error attributes, which can be used anywhere
(see __bad_copy_from() for example). Is this being wrapped due to the
potential compile time impact? Can we use something like hyperfine to
quantify it and see if the impact is worth the trade off of always
having it enabled for friendlier diagnostics? If not, maybe worth
adding a Kconfig option that is force selected by FORTIFY_SOURCE with
clang or can be optionally enabled by a user?

> +KBUILD_CFLAGS	+= $(call cc-option, -fdiagnostics-show-inlining-chain)
> +endif
> +
>  # Clear used registers at func exit (to reduce data lifetime and ROP gadgets).
>  ifdef CONFIG_ZERO_CALL_USED_REGS
>  KBUILD_CFLAGS	+= -fzero-call-used-regs=used-gpr
> 
> ---
> base-commit: 7df48e36313029e4c0907b2023905dd7213fd678
> change-id: 20260327-kbuild-show-inlining-557d31d2293a
> 
> Best regards,
> -- 
> Justin Stitt <justinstitt@google.com>
>