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References: <20201230154749.746641-1-arnd@kernel.org> <202101061350.913E1FDF6@keescook>
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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Wed, 6 Jan 2021 23:12:18 +0100
Message-ID: <CAK8P3a1tSaUE2uzb2JbQ1f7LWmkiHQtSxzJHmfa=fqT3fNXOPA@mail.gmail.com>
Subject: Re: [PATCH] ubsan: disable unsigned-integer-overflow sanitizer with clang
To:     Kees Cook <keescook@chromium.org>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Marco Elver <elver@google.com>,
        George Popescu <georgepope@android.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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On Wed, Jan 6, 2021 at 10:57 PM Kees Cook <keescook@chromium.org> wrote:
> On Wed, Dec 30, 2020 at 04:47:35PM +0100, Arnd Bergmann wrote:
> > diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
> > index 8b635fd75fe4..e23873282ba7 100644
> > --- a/lib/Kconfig.ubsan
> > +++ b/lib/Kconfig.ubsan
> > @@ -122,6 +122,8 @@ config UBSAN_SIGNED_OVERFLOW
> >
> >  config UBSAN_UNSIGNED_OVERFLOW
> >       bool "Perform checking for unsigned arithmetic overflow"
> > +     # clang hugely expands stack usage with -fsanitize=object-size
> > +     depends on !CC_IS_CLANG
> >       depends on $(cc-option,-fsanitize=unsigned-integer-overflow)
>
> Because of Clang implementation issues (see commit c637693b20da), this is
> already "default n" (and not supported under GCC at all). IIUC, setting
> this to "depends on !COMPILE_TEST" won't work for randconfigs, yes?

Ah, I had not realized this is clang specific. Adding the !COMPILE_TEST
dependency would hide it for me, which may be good enough for me.

> Is there some better way to mark this as "known to have issues, please
> don't include in randconfig?"
>
> I'd like to keep it around so people can continue to work out the
> problems with it, but not have unexpecting folks trip over it. ;)

I've reverted that patch locally now and default-enabled for randconfigs.
Now that I have an otherwise clean build, this should provide me
with a full set of files that produce the warning. If the number is
small enough, I could try opening individual github issues.

      Arnd