Re: [PATCH] kasan: turn off asan-stack for clang-8 and earlier

[Nick Desaulniers]

+ Evgenii, Kostya for KASAN

On Tue, Feb 19, 2019 at 2:17 PM Qian Cai <cai@xxxxxx> wrote:
>
> On Tue, 2019-02-19 at 22:49 +0100, Arnd Bergmann wrote:
> > Building an arm64 allmodconfig kernel with clang results in over 140 warnings
> > about overly large stack frames, the worst ones being:
> >
> > drivers/gpu/drm/panel/panel-sitronix-st7789v.c:196:12: error: stack frame size
> > of 20224 bytes in function 'st7789v_prepare'
> > drivers/video/fbdev/omap2/omapfb/displays/panel-tpo-td028ttec1.c:196:12:
> > error: stack frame size of 13120 bytes in function 'td028ttec1_panel_enable'
> > drivers/usb/host/max3421-hcd.c:1395:1: error: stack frame size of 10048 bytes
> > in function 'max3421_spi_thread'
> > drivers/net/wan/slic_ds26522.c:209:12: error: stack frame size of 9664 bytes
> > in function 'slic_ds26522_probe'
> > drivers/crypto/ccp/ccp-ops.c:2434:5: error: stack frame size of 8832 bytes in
> > function 'ccp_run_cmd'
> > drivers/media/dvb-frontends/stv0367.c:1005:12: error: stack frame size of 7840
> > bytes in function 'stv0367ter_algo'
> >
> > None of these happen with gcc today, and almost all of these are the result
> > of a single known bug in llvm.  Hopefully it will eventually get fixed with
> > the
> > clang-9 release.

Would it be better to disable outright if CC_IS_CLANG, then go back
and adjust it once we know explicitly which version it lands in?
(Maybe I'm being too pessimistic about when it may be fixed).

> >
> > In the meantime, the best idea I have is to turn off asan-stack for clang-8
> > and earlier, so we can produce a kernel that is safe to run.
> >
> > I have posted three patches that address the frame overflow warnings that are
> > not addressed by turning off asan-stack, so in combination with this change,
> > we get much closer to a clean allmodconfig build, which in turn is necessary
> > to do meaningful build regression testing.
>
> Well, I am using clang 8.0 on arm64 and running the kernel just fine for a few
> weeks now and never trigger a single stack overflow (THREAD_SHIFT = 15) because
> I never use any of those drivers you mentioned above. I don't think it is a good
> idea to blankly remove the testing coverage here and affect people don't use all
> those offensive functions at all.

Thanks for the patch, Arnd!  Hopefully we can fix that up in Clang
soon.  Qian, I guess the alternative would be to add `-mllvm
-asan-stack=0` on potentially up to 140 Makefiles?

-- 
Thanks,
~Nick Desaulniers