[PATCH 1/1] ARM : missing corrupted reg in __do_div_asm

[Russell King - ARM Linux]

On Tue, Mar 29, 2016 at 11:26:05AM +0100, Dave Martin wrote:
> On Tue, Mar 29, 2016 at 12:19:49PM +0200, Arnd Bergmann wrote:
> > On Monday 28 March 2016 12:19:03 Chen Gang wrote:
> > > __xl(R0 in little endian system, or R1 in big endian system) is corrupted
> > > after calling __do_div64 and compiler is not informed about this in
> > > macro __do_div_asm. If n is used again afterwards, __xl won't be
> > > reloaded and n will contain incorrect value.
> > > 
> > > Signed-off-by: Chen Gang <gangchen at rdamicro.com>
> > > Signed-off-by: Chen Gang <chengang.beijing at gmail.com>
> > > ---
> > 
> > How did you find this? Did you run into this problem on a live system
> > or see it through inspection?
> > 
> > >  arch/arm/include/asm/div64.h | 6 ++++--
> > >  1 file changed, 4 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/arch/arm/include/asm/div64.h b/arch/arm/include/asm/div64.h
> > > index e1f0776..1a6e91a 100644
> > > --- a/arch/arm/include/asm/div64.h
> > > +++ b/arch/arm/include/asm/div64.h
> > > @@ -35,12 +35,14 @@ static inline uint32_t __div64_32(uint64_t *n, uint32_t base)
> > >         register unsigned long long __n   asm("r0") = *n;
> > >         register unsigned long long __res asm("r2");
> > >         register unsigned int __rem       asm(__xh);
> > > +       register unsigned int __clobber   asm(__xl);
> > >         asm(    __asmeq("%0", __xh)
> > >                 __asmeq("%1", "r2")
> > > +               __asmeq("%3", "r0")
> > > +               __asmeq("%4", "r4")
> > >                 __asmeq("%2", "r0")
> > > -               __asmeq("%3", "r4")
> > >                 "bl     __do_div64"
> > > -               : "=r" (__rem), "=r" (__res)
> > > +               : "=r" (__rem), "=r" (__res), "=r" (__clobber)
> > >                 : "r" (__n), "r" (__base)
> > >                 : "ip", "lr", "cc");
> > >         *n = __res;
> > 
> > Doesn't the clobber normally go in the third line along with
> > "ip" and "lr"?
> 
> Since __xl is not used for any real argument to the asm, I think
> we can just add __xl to the clobber list directly, without needing
> to introduce an extra register variable ... no?

No, you can't.  The clobber list is not allowed to specify registers
that may be used for input or output operands, and since __xl may be
r0, and __n _is_ r0, you can't specify r0 in the clobber list.

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