Re: [PATCH] kasan: Always respect CONFIG_KASAN_STACK

[Nathan Chancellor <nathan@kernel.org>]

On Sun, Oct 03, 2021 at 08:04:53PM +0200, Andrey Konovalov wrote:
> On Wed, Sep 22, 2021 at 10:55 PM Nathan Chancellor <nathan@kernel.org> wrote:
> >
> > Currently, the asan-stack parameter is only passed along if
> > CFLAGS_KASAN_SHADOW is not empty, which requires KASAN_SHADOW_OFFSET to
> > be defined in Kconfig so that the value can be checked. In RISC-V's
> > case, KASAN_SHADOW_OFFSET is not defined in Kconfig, which means that
> > asan-stack does not get disabled with clang even when CONFIG_KASAN_STACK
> > is disabled, resulting in large stack warnings with allmodconfig:
> >
> > drivers/video/fbdev/omap2/omapfb/displays/panel-lgphilips-lb035q02.c:117:12:
> > error: stack frame size (14400) exceeds limit (2048) in function
> > 'lb035q02_connect' [-Werror,-Wframe-larger-than]
> > static int lb035q02_connect(struct omap_dss_device *dssdev)
> >            ^
> > 1 error generated.
> >
> > Ensure that the value of CONFIG_KASAN_STACK is always passed along to
> > the compiler so that these warnings do not happen when
> > CONFIG_KASAN_STACK is disabled.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1453
> > References: 6baec880d7a5 ("kasan: turn off asan-stack for clang-8 and earlier")
> > Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> > ---
> >  scripts/Makefile.kasan | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/scripts/Makefile.kasan b/scripts/Makefile.kasan
> > index 801c415bac59..b9e94c5e7097 100644
> > --- a/scripts/Makefile.kasan
> > +++ b/scripts/Makefile.kasan
> > @@ -33,10 +33,11 @@ else
> >         CFLAGS_KASAN := $(CFLAGS_KASAN_SHADOW) \
> >          $(call cc-param,asan-globals=1) \
> >          $(call cc-param,asan-instrumentation-with-call-threshold=$(call_threshold)) \
> > -        $(call cc-param,asan-stack=$(stack_enable)) \
> >          $(call cc-param,asan-instrument-allocas=1)
> >  endif
> 
> This part of code always looked weird to me.
> 
> Shouldn't we be able to pull all these options out of the else section?
> 
> Then, the code structure would make sense: first, try applying
> KASAN_SHADOW_OFFSET; if failed, use CFLAGS_KASAN_MINIMAL; and then try
> applying all these options one by one.

Prior to commit 1a69e7ce8391 ("kasan/Makefile: support LLVM style asan
parameters"), all the flags were run under one cc-option, meaning that
if $(KASAN_SHADOW_OFFSET) was not set, the whole call would fail.
However, after that commit, it is possible to do this but I was not sure
if that was intentional so I went for the minimal fix.

Cheers,
Nathan