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

[Kees Cook <kees@kernel.org>]

On Fri, Mar 27, 2026 at 03:29:18PM -0700, Justin Stitt wrote:
> On Fri, Mar 27, 2026 at 3:18 PM Nathan Chancellor <nathan@kernel.org> wrote:
> >
> > 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?
> 
> The compile time impact is not measurable (within expected noise). The
> peak memory usage may increase by somewhere in the 0.5% to 1.5% range
> depending on build configuration.
> 
> I bundled this under fortify to limit initial impact as its had
> virtually no real-world testing and may produce unhelpful diagnostic
> notes under its heuristic mode. I don't expect folks to use `-g1` in
> the kernel (which would enable full-proof diagnostic notes).
> 
> > 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?
> 
> I'll defer to you on this one. We could add
> CONFIG_SHOW_INLINING_CHAIN_NOTES or something similar?

I would prefer to just unconditionally enable this when it is supported.

-- 
Kees Cook