linux-kernel - Re: [PATCH 06/14] Fix CFLAGS for UBSAN

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Message-ID: <20200915102458.GA1650630@google.com>
Date:   Tue, 15 Sep 2020 10:24:58 +0000
From:   George Popescu <georgepope@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     maz@...nel.org, catalin.marinas@....com, will@...nel.org,
        masahiroy@...nel.org, michal.lkml@...kovi.net,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        clang-built-linux@...glegroups.com, james.morse@....com,
        julien.thierry.kdev@...il.com, suzuki.poulose@....com,
        natechancellor@...il.com, ndesaulniers@...gle.com,
        dbrazdil@...gle.com, broonie@...nel.org, maskray@...gle.com,
        ascull@...gle.com, akpm@...ux-foundation.org, dvyukov@...gle.com,
        elver@...gle.com, tglx@...utronix.de, arnd@...db.de
Subject: Re: [PATCH 06/14] Fix CFLAGS for UBSAN_BOUNDS on Clang

On Mon, Sep 14, 2020 at 03:13:14PM -0700, Kees Cook wrote:
> On Mon, Sep 14, 2020 at 05:27:42PM +0000, George-Aurelian Popescu wrote:
> > From: George Popescu <georgepope@...gle.com>
> > 
> > When the kernel is compiled with Clang, UBSAN_BOUNDS inserts a brk after
> > the handler call, preventing it from printing any information processed
> > inside the buffer.
> > For Clang -fsanitize=bounds expands to -fsanitize=array-bounds and
> > -fsanitize=local-bounds, and the latter adds a brk after the handler
> > call
> 
> That sounds like a compiler bug?
Actually in clang 12 documentation is written that -fsanitize=bounds
expands to that. GCC  doesn't have those two options, only the
-fsanitize=bounds which looks similar to -fsanitize=array-bounds from
clang. So I don't see it as a compiler bug, just a misuse of flags.

> > Signed-off-by: George Popescu <georgepope@...gle.com>
> > ---
> >  scripts/Makefile.ubsan | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/scripts/Makefile.ubsan b/scripts/Makefile.ubsan
> > index 27348029b2b8..3d15ac346c97 100644
> > --- a/scripts/Makefile.ubsan
> > +++ b/scripts/Makefile.ubsan
> > @@ -4,7 +4,14 @@ ifdef CONFIG_UBSAN_ALIGNMENT
> >  endif
> >  
> >  ifdef CONFIG_UBSAN_BOUNDS
> > -      CFLAGS_UBSAN += $(call cc-option, -fsanitize=bounds)
> > +      # For Clang -fsanitize=bounds translates to -fsanitize=array-bounds and
> > +      # -fsanitize=local-bounds; the latter adds a brk right after the
> > +      # handler is called.
> > +      ifdef CONFIG_CC_IS_CLANG
> > +            CFLAGS_UBSAN += $(call cc-option, -fsanitize=array-bounds)
> 
> This would mean losing the local-bounds coverage? Isn't that for locally
> defined arrays on the stack?
This would mean losing the local-bounds coverage. I tried to  test it without
local-bounds and with a locally defined array on the stack and it works fine
(the handler is called and the error reported). For me it feels like
--array-bounds and --local-bounds are triggered for the same type of
undefined_behaviours but they are handling them different.

> > +      else
> > +            CFLAGS_UBSAN += $(call cc-option, -fsanitize=bounds)
> > +      endif
> >  endif
> >  
> >  ifdef CONFIG_UBSAN_MISC
> > -- 
> > 2.28.0.618.gf4bc123cb7-goog
> > 
> 
> --
Thanks,
George