linux-kernel - Re: [PATCH early RFC 2/4] kbuild: kasan: refactor open coded cflags for kasan test

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Message-ID: <CAG48ez2_HrKjRuH+5KSB+vK_9dGeNnh2O6qAN0ePr4BRnt3xzw@mail.gmail.com>
Date: Tue, 5 Aug 2025 15:49:45 +0200
From: Jann Horn <jannh@...gle.com>
To: Marco Elver <elver@...gle.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Nicolas Schier <nicolas.schier@...ux.dev>, Andrey Ryabinin <ryabinin.a.a@...il.com>, 
	Alexander Potapenko <glider@...gle.com>, Andrey Konovalov <andreyknvl@...il.com>, 
	Dmitry Vyukov <dvyukov@...gle.com>, Vincenzo Frascino <vincenzo.frascino@....com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Christoph Lameter <cl@...two.org>, 
	David Rientjes <rientjes@...gle.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Harry Yoo <harry.yoo@...cle.com>, 
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, 
	kasan-dev@...glegroups.com, linux-mm@...ck.org
Subject: Re: [PATCH early RFC 2/4] kbuild: kasan: refactor open coded cflags
 for kasan test

On Tue, Aug 5, 2025 at 2:31 PM Marco Elver <elver@...gle.com> wrote:
> On Mon, 4 Aug 2025 at 21:18, Jann Horn <jannh@...gle.com> wrote:
> > In the Makefile for mm/kasan/, KASAN is broadly disabled to prevent the
> > KASAN runtime from recursing into itself; but the KASAN tests must be
> > exempt from that.
> >
> > This is currently implemented by duplicating the same logic that is also
> > in scripts/Makefile.lib. In preparation for changing that logic,
> > refactor away the duplicate logic - we already have infrastructure for
> > opting in specific files inside directories that are opted out.
> >
> > Signed-off-by: Jann Horn <jannh@...gle.com>
> > ---
> >  mm/kasan/Makefile | 12 ++----------
> >  1 file changed, 2 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/kasan/Makefile b/mm/kasan/Makefile
> > index dd93ae8a6beb..922b2e6f6d14 100644
> > --- a/mm/kasan/Makefile
> > +++ b/mm/kasan/Makefile
> > @@ -35,18 +35,10 @@ CFLAGS_shadow.o := $(CC_FLAGS_KASAN_RUNTIME)
> >  CFLAGS_hw_tags.o := $(CC_FLAGS_KASAN_RUNTIME)
> >  CFLAGS_sw_tags.o := $(CC_FLAGS_KASAN_RUNTIME)
> >
> > -CFLAGS_KASAN_TEST := $(CFLAGS_KASAN)
> > -ifndef CONFIG_CC_HAS_KASAN_MEMINTRINSIC_PREFIX
> > -# If compiler instruments memintrinsics by prefixing them with __asan/__hwasan,
> > -# we need to treat them normally (as builtins), otherwise the compiler won't
> > -# recognize them as instrumentable. If it doesn't instrument them, we need to
> > -# pass -fno-builtin, so the compiler doesn't inline them.
> > -CFLAGS_KASAN_TEST += -fno-builtin
>
> Has the -fno-builtin passed to test if
> !CONFIG_CC_HAS_KASAN_MEMINTRINSIC_PREFIX become redundant?

... oh, bleh, good catch. Somehow I had convinced myself that
scripts/Makefile.kasan did this, but no, that only sets -fno-builtin
for uninstrumented code... I misunderstood what was going on here.