[PATCH 1/2] kcsan: xtensa: Add atomic builtin stubs for 32-bit systems

[Marco Elver]

On Thu, Feb 16, 2023 at 07:12AM +0000, Christophe Leroy wrote:
> 
> 
> Le 16/02/2023 à 06:09, Rohan McLure a écrit :
> > KCSAN instruments calls to atomic builtins, and will in turn call these
> > builtins itself. As such, architectures supporting KCSAN must have
> > compiler support for these atomic primitives.
> > 
> > Since 32-bit systems are unlikely to have 64-bit compiler builtins,
> > provide a stub for each missing builtin, and use BUG() to assert
> > unreachability.
> > 
> > In commit 725aea873261 ("xtensa: enable KCSAN"), xtensa implements these
> > locally. Move these definitions to be accessible to all 32-bit
> > architectures that do not provide the necessary builtins, with opt in
> > for PowerPC and xtensa.
> > 
> > Signed-off-by: Rohan McLure <rmclure at linux.ibm.com>
> > Reviewed-by: Max Filippov <jcmvbkbc at gmail.com>
> 
> This series should also be addressed to KCSAN Maintainers, shouldn't it ?
> 
> KCSAN
> M:	Marco Elver <elver at google.com>
> R:	Dmitry Vyukov <dvyukov at google.com>
> L:	kasan-dev at googlegroups.com
> S:	Maintained
> F:	Documentation/dev-tools/kcsan.rst
> F:	include/linux/kcsan*.h
> F:	kernel/kcsan/
> F:	lib/Kconfig.kcsan
> F:	scripts/Makefile.kcsan
> 
> 
> > ---
> > Previously issued as a part of a patch series adding KCSAN support to
> > 64-bit.
> > Link: https://lore.kernel.org/linuxppc-dev/167646486000.1421441.10070059569986228558.b4-ty@ellerman.id.au/T/#t
> > v1: Remove __has_builtin check, as gcc is not obligated to inline
> > builtins detected using this check, but instead is permitted to supply
> > them in libatomic:
> > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108734
> > Instead, opt-in PPC32 and xtensa.
> > ---
> >   arch/xtensa/lib/Makefile                              | 1 -
> >   kernel/kcsan/Makefile                                 | 2 ++
> >   arch/xtensa/lib/kcsan-stubs.c => kernel/kcsan/stubs.c | 0
> >   3 files changed, 2 insertions(+), 1 deletion(-)
> >   rename arch/xtensa/lib/kcsan-stubs.c => kernel/kcsan/stubs.c (100%)
> > 
> > diff --git a/arch/xtensa/lib/Makefile b/arch/xtensa/lib/Makefile
> > index 7ecef0519a27..d69356dc97df 100644
> > --- a/arch/xtensa/lib/Makefile
> > +++ b/arch/xtensa/lib/Makefile
> > @@ -8,5 +8,4 @@ lib-y	+= memcopy.o memset.o checksum.o \
> >   	   divsi3.o udivsi3.o modsi3.o umodsi3.o mulsi3.o umulsidi3.o \
> >   	   usercopy.o strncpy_user.o strnlen_user.o
> >   lib-$(CONFIG_PCI) += pci-auto.o
> > -lib-$(CONFIG_KCSAN) += kcsan-stubs.o
> >   KCSAN_SANITIZE_kcsan-stubs.o := n
> > diff --git a/kernel/kcsan/Makefile b/kernel/kcsan/Makefile
> > index 8cf70f068d92..86dd713d8855 100644
> > --- a/kernel/kcsan/Makefile
> > +++ b/kernel/kcsan/Makefile
> > @@ -12,6 +12,8 @@ CFLAGS_core.o := $(call cc-option,-fno-conserve-stack) \
> >   	-fno-stack-protector -DDISABLE_BRANCH_PROFILING
> >   
> >   obj-y := core.o debugfs.o report.o
> > +obj-$(CONFIG_PPC32) += stubs.o
> > +obj-$(CONFIG_XTENSA) += stubs.o
> 
> Not sure it is acceptable to do it that way.
> 
> There should likely be something like a CONFIG_ARCH_WANTS_KCSAN_STUBS in 
> KCSAN's Kconfig then PPC32 and XTENSA should select it.

The longer I think about it, since these stubs all BUG() anyway, perhaps
we ought to just avoid them altogether. If you delete all the stubs from
ppc and xtensa, but do this:

 | diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
 | index 54d077e1a2dc..8169d6dadd0e 100644
 | --- a/kernel/kcsan/core.c
 | +++ b/kernel/kcsan/core.c
 | @@ -1261,7 +1261,9 @@ static __always_inline void kcsan_atomic_builtin_memorder(int memorder)
 |  DEFINE_TSAN_ATOMIC_OPS(8);
 |  DEFINE_TSAN_ATOMIC_OPS(16);
 |  DEFINE_TSAN_ATOMIC_OPS(32);
 | +#ifdef CONFIG_64BIT
 |  DEFINE_TSAN_ATOMIC_OPS(64);
 | +#endif
 |  
 |  void __tsan_atomic_thread_fence(int memorder);
 |  void __tsan_atomic_thread_fence(int memorder)

Does that work?