Re: [PATCH] kasan: Always respect CONFIG_KASAN_STACK

[Nathan Chancellor <nathan@kernel.org>]

On Fri, Oct 08, 2021 at 11:46:55AM -0700, Palmer Dabbelt wrote:
> On Thu, 23 Sep 2021 07:59:46 PDT (-0700), nathan@kernel.org wrote:
> > On Thu, Sep 23, 2021 at 12:07:17PM +0200, Marco Elver wrote:
> > > On Wed, 22 Sept 2021 at 22:55, 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>
> > > 
> > > Reviewed-by: Marco Elver <elver@google.com>
> > 
> > Thanks!
> > 
> > > [ Which tree are you planning to take it through? ]
> > 
> > Gah, I was intending for it to go through -mm, then I cc'd neither
> > Andrew nor linux-mm... :/ Andrew, do you want me to resend or can you
> > grab it from LKML?
> 
> Acked-by: Palmer Dabbelt <palmerdabbelt@google.com>
> 
> (assuming you still want it through somewhere else)

Thanks, it is now in mainline as commit 19532869feb9 ("kasan: always
respect CONFIG_KASAN_STACK").

> > > Note, arch/riscv/include/asm/kasan.h mentions KASAN_SHADOW_OFFSET in
> > > comment (copied from arm64). Did RISC-V just forget to copy over the
> > > Kconfig option?
> > 
> > I do see it defined in that file as well but you are right that they did
> > not copy the Kconfig logic, even though it was present in the tree when
> > RISC-V KASAN was implemented. Perhaps they should so that they get
> > access to the other flags in the "else" branch?
> 
> Ya, looks like we just screwed this up.  I'm seeing some warnings like
> 
>    cc1: warning: ‘-fsanitize=kernel-address’ with stack protection is not supported without ‘-fasan-shadow-offset=’ for this target

Hmmm, I thought I did a GCC build with this change but I must not have
:/ 

> which is how I ended up here, I'm assuming that's what you're talking about
> here?  LMK if you were planning on sending along a fix or if you want me to
> go figure it out.

I took a look at moving the logic into Kconfig like arm64 before sending
this change and I did not really understand it well enough to do so. I
think it would be best if you were able to do that so that nothing gets
messed up.

Cheers,
Nathan