From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Bill Wendling <morbo@google.com>,
        Nathan Chancellor <nathan@kernel.org>
Cc:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        kernel test robot <lkp@intel.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        llvm@lists.linux.dev, kbuild-all@lists.01.org,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 5/5] powerpc/inst: Optimise
 copy_inst_from_kernel_nofault()
In-Reply-To: <CAGG=3QVQ9bwWWyKDN3_C2B0v7H6iZ4ZpNybXGCqbzwWrPjuPrg@mail.gmail.com>
References: <0d5b12183d5176dd702d29ad94c39c384e51c78f.1638208156.git.christophe.leroy@csgroup.eu>
 <202111300652.0yDBNvyJ-lkp@intel.com>
 <e7b67ca6-8cd1-da3c-c0f3-e05f7e592828@csgroup.eu>
 <87a6hlq408.fsf@mpe.ellerman.id.au> <YaZqs2tPxMzhgkAW@archlinux-ax161>
 <CAGG=3QX4k6MZ1qkT+sVAroJeBpbZBnOJauM_uJsu2uV1vnVObQ@mail.gmail.com>
 <CAGG=3QVQ9bwWWyKDN3_C2B0v7H6iZ4ZpNybXGCqbzwWrPjuPrg@mail.gmail.com>
Date:   Tue, 07 Dec 2021 14:37:26 +1100
Message-ID: <87o85tnkzt.fsf@mpe.ellerman.id.au>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

Bill Wendling <morbo@google.com> writes:
> On Tue, Nov 30, 2021 at 10:38 AM Bill Wendling <morbo@google.com> wrote:
>> On Tue, Nov 30, 2021 at 10:17 AM Nathan Chancellor <nathan@kernel.org> w=
rote:
>> > On Tue, Nov 30, 2021 at 10:25:43PM +1100, Michael Ellerman wrote:
>> > > Christophe Leroy <christophe.leroy@csgroup.eu> writes:
>> > > > Le 29/11/2021 =C3=A0 23:55, kernel test robot a =C3=A9crit :
...
>> > > >> All warnings (new ones prefixed by >>):
>> > > >>
>> > > >>     In file included from arch/powerpc/kernel/asm-offsets.c:71:
>> > > >>     In file included from arch/powerpc/kernel/../xmon/xmon_bpts.h=
:7:
>> > > >>>> arch/powerpc/include/asm/inst.h:165:20: warning: variable 'val'=
 is uninitialized when used here [-Wuninitialized]
>> > > >>                     *inst =3D ppc_inst(val);
>> > > >>                                      ^~~
>> > > >>     arch/powerpc/include/asm/inst.h:53:22: note: expanded from ma=
cro 'ppc_inst'
>> > > >>     #define ppc_inst(x) (x)
>> > > >>                          ^
>> > > >>     arch/powerpc/include/asm/inst.h:155:18: note: initialize the =
variable 'val' to silence this warning
>> > > >>             unsigned int val, suffix;
>> > > >>                             ^
>> > > >>                              =3D 0
>> > > >
>> > > > I can't understand what's wrong here.
...
>> > > >
>> > > > I see no possibility, no alternative path where val wouldn't be se=
t. The
>> > > > asm clearly has *addr as an output param so it is always set.
>> > >
>> > > I guess clang can't convince itself of that?
...
>> >
>> > It certainly looks like there is something wrong with how clang is
>> > tracking the initialization of the variable because it looks to me like
>> > val is only used in the fallthrough path, which happens after it is
>> > initialized via lwz.  Perhaps something is wrong with the logic of
>> > https://reviews.llvm.org/D71314?  I've added Bill to CC (LLVM issues a=
re
>> > being migrated from Bugzilla to GitHub Issues right now so I cannot fi=
le
>> > this upstream at the moment).
>> >
>> If I remove the casts of "val" the warning doesn't appear. I suspect
>> that when I wrote that patch I forgot to remove those when checking.
>> #include "Captain_Picard_facepalm.h"
>>
>> I'll look into it.
>>
> Small retraction. It's the "*(<cast>)&val" that's the issue. (I.e. the "*=
&")

I guess for now I'll just squash this in as a workaround?


diff --git a/arch/powerpc/include/asm/inst.h b/arch/powerpc/include/asm/ins=
t.h
index 631436f3f5c3..5b591c51fec9 100644
--- a/arch/powerpc/include/asm/inst.h
+++ b/arch/powerpc/include/asm/inst.h
@@ -157,6 +157,9 @@ static inline int copy_inst_from_kernel_nofault(ppc_ins=
t_t *inst, u32 *src)
 	if (unlikely(!is_kernel_addr((unsigned long)src)))
 		return -ERANGE;
=20
+#ifdef CONFIG_CC_IS_CLANG
+	val =3D suffix =3D 0;
+#endif
 	__get_kernel_nofault(&val, src, u32, Efault);
 	if (IS_ENABLED(CONFIG_PPC64) && get_op(val) =3D=3D OP_PREFIX) {
 		__get_kernel_nofault(&suffix, src + 1, u32, Efault);


cheers