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

[Dave.Martin@arm.com (Dave Martin)]

On Tue, Mar 29, 2016 at 11:34:18AM +0100, Russell King - ARM Linux wrote:
> 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@rdamicro.com>
> > > > Signed-off-by: Chen Gang <chengang.beijing@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.

Hmm, you're right -- in which case the change looks reasonable.

I wonder whether the following would be cleaner than having these
aliased arguments:

	asm(	/* ... */
		"bl	__do_div64"
		: "+r" (__n), "=r" (__res)
		: "r" (__base)
		: "ip", "lr", "cc");
	*n = __res;
	return __n >> 32;

(providing that GCC doesn't make a mess of the "easy" shift).

Cheers
---Dave