linux-kernel - Re: [PATCH v5 5/5] powerpc/inst: Optimise copy_inst_from_kernel

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Message-ID: <Ya7ntJ0ehZPy6HKA@archlinux-ax161>
Date:   Mon, 6 Dec 2021 21:48:52 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Bill Wendling <morbo@...gle.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        kernel test robot <lkp@...el.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        llvm@...ts.linux.dev, kbuild-all@...ts.01.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 5/5] powerpc/inst: Optimise
 copy_inst_from_kernel_nofault()

On Tue, Dec 07, 2021 at 02:37:26PM +1100, Michael Ellerman wrote:
> Bill Wendling <morbo@...gle.com> writes:
> > On Tue, Nov 30, 2021 at 10:38 AM Bill Wendling <morbo@...gle.com> wrote:
> >> On Tue, Nov 30, 2021 at 10:17 AM Nathan Chancellor <nathan@...nel.org> wrote:
> >> > On Tue, Nov 30, 2021 at 10:25:43PM +1100, Michael Ellerman wrote:
> >> > > Christophe Leroy <christophe.leroy@...roup.eu> writes:
> >> > > > Le 29/11/2021 à 23:55, kernel test robot a écrit :
> ...
> >> > > >> 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 = ppc_inst(val);
> >> > > >>                                      ^~~
> >> > > >>     arch/powerpc/include/asm/inst.h:53:22: note: expanded from macro '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;
> >> > > >>                             ^
> >> > > >>                              = 0
> >> > > >
> >> > > > I can't understand what's wrong here.
> ...
> >> > > >
> >> > > > I see no possibility, no alternative path where val wouldn't be set. 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 are
> >> > being migrated from Bugzilla to GitHub Issues right now so I cannot file
> >> > 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/inst.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_inst_t *inst, u32 *src)
>  	if (unlikely(!is_kernel_addr((unsigned long)src)))
>  		return -ERANGE;

Could we add a version check to this and a link to our bug tracker:

/* https://github.com/ClangBuiltLinux/linux/issues/1521 */
#if defined(CONFIG_CC_IS_CLANG) && CONFIG_CLANG_VERSION < 140000

> +#ifdef CONFIG_CC_IS_CLANG
> +	val = suffix = 0;
> +#endif
>  	__get_kernel_nofault(&val, src, u32, Efault);
>  	if (IS_ENABLED(CONFIG_PPC64) && get_op(val) == OP_PREFIX) {
>  		__get_kernel_nofault(&suffix, src + 1, u32, Efault);
> 
> 
> 
> cheers