linux-kernel - Re: [PATCH] kernel.h: Disable -Wreturn-stack-address for _THIS_IP

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Date:   Mon, 30 Jul 2018 12:48:06 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     natechancellor@...il.com
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthias Kaehlcke <mka@...omium.org>, ghackmann@...roid.com,
        Greg Hackmann <ghackmann@...gle.com>, stable@...r.kernel.org,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Wei Wang <wvw@...gle.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        rdunlap@...radead.org, neilb@...e.com,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernel.h: Disable -Wreturn-stack-address for _THIS_IP_

On Mon, Jul 30, 2018 at 11:39 AM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> On Mon, Jul 30, 2018 at 10:06:20AM -0700, Nick Desaulniers wrote:
> > Starting with Clang-7.0, _THIS_IP_ generates -Wreturn-stack-address
> > warnings for almost every translation unit. In general, I'd prefer to
> > leave this on (returning the address of a stack allocated variable is in
> > general a bad idea) and disable it only at whitelisted call sites.
> >
> > We can't do something like:
> >  #pragma clang diagnostic push
> >  #pragma clang diagnostic ignored "-Wreturn-stack-address"
> >  <code>
> >  #pragma clang diagnostic pop
> >
> > in a GNU Statement Expression or macro, hence we use _Pragma, which is
> > its raison d'être: https://gcc.gnu.org/onlinedocs/cpp/Pragmas.html
> >
> > Cc: stable@...r.kernel.org # 4.17, 4.14, 4.9, 4.4
> > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > ---
> >  include/linux/kernel.h | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > index 941dc0a5a877..5906f5727f90 100644
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -168,7 +168,15 @@
> >
> >
> >  #define _RET_IP_             (unsigned long)__builtin_return_address(0)
> > -#define _THIS_IP_  ({ __label__ __here; __here: (unsigned long)&&__here; })
> > +#define _THIS_IP_  (                                                 \
> > +{                                                                    \
> > +     _Pragma("clang diagnostic push")                                \
> > +     _Pragma("clang diagnostic ignored \"-Wreturn-stack-address\"")  \
> > +     __label__ __here;                                               \
> > +__here: (unsigned long)&&__here;                                     \
> > +     _Pragma("clang diagnostic pop")                                 \
> > +}                                                                    \
> > +)
> >
> >  #ifdef CONFIG_LBDAF
> >  # include <asm/div64.h>
> > --
> > 2.18.0.233.g985f88cf7e-goog
> >
>
> This generates a ton of warnings with GCC:
>
> In file included from ./include/linux/spinlock.h:58,
>                  from ./include/linux/mmzone.h:8,
>                  from ./include/linux/gfp.h:6,
>                  from ./include/linux/slab.h:15,
>                  from ./include/linux/crypto.h:24,
>                  from arch/x86/kernel/asm-offsets.c:9:
> ./include/linux/bottom_half.h: In function ‘local_bh_disable’:
> ./include/linux/bottom_half.h:19: error: ignoring #pragma clang diagnostic [-Werror=unknown-pragmas]
>   __local_bh_disable_ip(_THIS_IP_, SOFTIRQ_DISABLE_OFFSET);
>
> ./include/linux/bottom_half.h:19: error: ignoring #pragma clang diagnostic [-Werror=unknown-pragmas]
> ./include/linux/bottom_half.h:19: error: ignoring #pragma clang diagnostic [-Werror=unknown-pragmas]
> ./include/linux/bottom_half.h: In function ‘local_bh_enable’:
> ./include/linux/bottom_half.h:32: error: ignoring #pragma clang diagnostic [-Werror=unknown-pragmas]
>   __local_bh_enable_ip(_THIS_IP_, SOFTIRQ_DISABLE_OFFSET);
>
> ./include/linux/bottom_half.h:32: error: ignoring #pragma clang diagnostic [-Werror=unknown-pragmas]
> ./include/linux/bottom_half.h:32: error: ignoring #pragma clang diagnostic [-Werror=unknown-pragmas]
> cc1: all warnings being treated as errors
> make[1]: *** [Kbuild:56: arch/x86/kernel/asm-offsets.s] Error 1
> make: *** [Makefile:1081: prepare0] Error 2

Ah, good catch.  I thought I had tested locally with gcc-8, but it
seems that it was likely only godbolt.  I do see the errors locally
when building with gcc-8.

>
> A proper solution is probably going to involve what was done for the
> -Wattribute-alias warnings from GCC 8 in commits 8793bb7f4a9d ("kbuild:
> add macro for controlling warnings to linux/compiler.h") and
> bee20031772a ("disable -Wattribute-alias warning for SYSCALL_DEFINEx()")
>
> I'll take a look at it in a bit unless someone beats me to it.

I'll fix this up, triple check with gcc-8, and attribute you in the
Suggested-by tag.  Thank you for verifying.
-- 
Thanks,
~Nick Desaulniers