linux-kernel - Re: [RESEND,PATCH] ARM: fix __div64_32() error when compiling with clang

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Message-ID: <CAC5oF3W+RkcO-dSiKXGxVvhBGb0n7fQ-KvdjbPNJJVadv6qNBw@mail.gmail.com>
Date:   Tue, 24 Nov 2020 18:14:25 +0800
From:   Antony Yu <swpenim@...il.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Russell King <linux@...linux.org.uk>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com, Pen-Yung Yu <swpenim@...il.com>
Subject: Re: [RESEND,PATCH] ARM: fix __div64_32() error when compiling with clang

Antony Yu <swpenim@...il.com> 於 2020年11月24日 週二 下午3:43寫道：
>
> On Mon, Nov 23, 2020 at 11:16:02AM -0700, Nathan Chancellor wrote:
> > On Mon, Nov 23, 2020 at 03:36:32PM +0800, Antony Yu wrote:
> > > __do_div64 clobbers the input register r0 in little endian system.
> > > According to the inline assembly document, if an input operand is
> > > modified, it should be tied to a output operand. This patch can
> > > prevent compilers from reusing r0 register after asm statements.
> > >
> > > Signed-off-by: Antony Yu <swpenim@...il.com>
> > > ---
> > >  arch/arm/include/asm/div64.h | 5 +++--
> > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/arm/include/asm/div64.h b/arch/arm/include/asm/div64.h
> > > index 898e9c78a7e7..809efc51e90f 100644
> > > --- a/arch/arm/include/asm/div64.h
> > > +++ b/arch/arm/include/asm/div64.h
> > > @@ -39,9 +39,10 @@ static inline uint32_t __div64_32(uint64_t *n, uint32_t base)
> > >     asm(    __asmeq("%0", __xh)
> > >             __asmeq("%1", "r2")
> > >             __asmeq("%2", "r0")
> > > -           __asmeq("%3", "r4")
> > > +           __asmeq("%3", "r0")
> > > +           __asmeq("%4", "r4")
> > >             "bl     __do_div64"
> > > -           : "=r" (__rem), "=r" (__res)
> > > +           : "=r" (__rem), "=r" (__res), "=r" (__n)
> > >             : "r" (__n), "r" (__base)
> > >             : "ip", "lr", "cc");
> > >     *n = __res;
> > > --
> > > 2.23.0
> > >
> >
> > I am not sure that I am qualified to review this (my assembly knowledge
> > is not the best) but your commit title mentions an error when compiling
> > with clang. What is the exact error, what configuration generates it,
> > and what version of clang? We have done fairly decent testing for
> > 32-bit ARM, I would like to know what we are missing.
> >
> > Cheers,
> > Nathan
>
> We have run fail on android R vts vts_libsnapshot_test with kernel 4.14.
> This bug is triggered accidently by a workaround patch in our code base.
> It is fine on a pure clean 4.14 branch since __do_div64 may not be
> executed in skip_metadata.
>
> The attachment are .i and generated .s file. .s file can be reproduced
> with clang -target arm-linux-eabi -march=armv8.2-a -O2.
>
> In function skip_metadata, it loads some value to r0, calls __do_div64,
> adds 1 to r0 and stores it to [r5]. It gets wrong value since __do_div64
> clobbers r0 register.
>
> We have tried clang-10, clang-11 and android prebuilt clang-r383902b. All
> of them have the same problem.

Sorry for the large attachment.
I put .i and .s files on
https://gist.github.com/penyung/274b0c697062a1c776994bb40243cfff

Antony Yu