math-emu: correct test for downshifting fraction in _FP_FROM_INT()

The kernel's math-emu code contains a macro _FP_FROM_INT() which is
used to convert an integer to a raw normalized floating-point value.
It does this basically in three steps:

1. Compute the exponent from the number of leading zero bits.
2. Downshift large fractions to put the MSB in the right position
   for normalized fractions.
3. Upshift small fractions to put the MSB in the right position.

There is an boundary error in step 2, causing a fraction with its
MSB exactly one bit above the normalized MSB position to not be
downshifted.  This results in a non-normalized raw float, which when
packed becomes a massively inaccurate representation for that input.

The impact of this depends on a number of arch-specific factors,
but it is known to have broken emulation of FXTOD instructions
on UltraSPARC III, which was originally reported as GCC bug 44631
<http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44631>.

Any arch which uses math-emu to emulate conversions from integers to
same-size floats may be affected.

The fix is simple: the exponent comparison used to determine if the
fraction should be downshifted must be "<=" not "<".

I'm sending a kernel module to test this as a reply to this message.
There are also SPARC user-space test cases in the GCC bug entry.

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
---
 include/math-emu/op-common.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

On Tue, 20 Jul 2010 00:12:02 +0200
Mikael Pettersson <mikpe@it.uu.se> wrote:

> Unfortunately it seems difficult to write a generic module
> which uses math-emu:
> - <math-emu/soft-fp.h> includes <asm/sfp-machine.h>,
>   but only a handful of archs have it
> - <asm/sfp-machine.h> isn't always self-contained and may depend
>   on various $arch-specific declarations being present
> 
> The given test module works on sparc64 and ppc64, where it uses
> the kernel's sfp-machine.h, and on x86 where it uses a stub
> sfp-machine.h supplied by itself.  I tried to cross-compile it
> for alpha, but that failed due to its sfp-machine.h not being
> self-contained.  I didn't try sh or s390.

It would be challange to try this on s390. The math emulation code is
only used for really old 31 bit machines. Starting with the G5 the fpu
can do IEEE754, I would say the math emulation code is irrelevant for
s390 by now.

Mikael Pettersson writes:
 > The kernel's math-emu code contains a macro _FP_FROM_INT() which is
 > used to convert an integer to a raw normalized floating-point value.
 > It does this basically in three steps:
 > 
 > 1. Compute the exponent from the number of leading zero bits.
 > 2. Downshift large fractions to put the MSB in the right position
 >    for normalized fractions.
 > 3. Upshift small fractions to put the MSB in the right position.
 > 
 > There is an boundary error in step 2, causing a fraction with its
 > MSB exactly one bit above the normalized MSB position to not be
 > downshifted.  This results in a non-normalized raw float, which when
 > packed becomes a massively inaccurate representation for that input.
 > 
 > The impact of this depends on a number of arch-specific factors,
 > but it is known to have broken emulation of FXTOD instructions
 > on UltraSPARC III, which was originally reported as GCC bug 44631
 > <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44631>.
 > 
 > Any arch which uses math-emu to emulate conversions from integers to
 > same-size floats may be affected.
 > 
 > The fix is simple: the exponent comparison used to determine if the
 > fraction should be downshifted must be "<=" not "<".
 > 
 > I'm sending a kernel module to test this as a reply to this message.
 > There are also SPARC user-space test cases in the GCC bug entry.
 > 
 > Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>

I forgot to mention that this needs to be backported to older kernels,
so can the maintainer who picks this up please add

	Cc: stable@kernel.org

Thanks,

/Mikael

 > ---
 >  include/math-emu/op-common.h |    2 +-
 >  1 file changed, 1 insertion(+), 1 deletion(-)
 > 
 > diff -rupN linux-2.6.35-rc5/include/math-emu/op-common.h linux-2.6.35-rc5.mathemu-FP_FROM_INT-fraction-downshift-condition/include/math-emu/op-common.h
 > --- linux-2.6.35-rc5/include/math-emu/op-common.h	2010-05-17 19:51:32.000000000 +0200
 > +++ linux-2.6.35-rc5.mathemu-FP_FROM_INT-fraction-downshift-condition/include/math-emu/op-common.h	2010-07-18 22:33:46.000000000 +0200
 > @@ -799,7 +799,7 @@ do {									\
 >  		X##_e -= (_FP_W_TYPE_SIZE - rsize);			\
 >  	X##_e = rsize - X##_e - 1;					\
 >  									\
 > -	if (_FP_FRACBITS_##fs < rsize && _FP_WFRACBITS_##fs < X##_e)	\
 > +	if (_FP_FRACBITS_##fs < rsize && _FP_WFRACBITS_##fs <= X##_e)	\
 >  	  __FP_FRAC_SRS_1(ur_, (X##_e - _FP_WFRACBITS_##fs + 1), rsize);\
 >  	_FP_FRAC_DISASSEMBLE_##wc(X, ur_, rsize);			\
 >  	if ((_FP_WFRACBITS_##fs - X##_e - 1) > 0)			\
 > --
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 >

From: Mikael Pettersson <mikpe@it.uu.se>
Date: Mon, 19 Jul 2010 23:58:42 +0200

> The kernel's math-emu code contains a macro _FP_FROM_INT() which is
> used to convert an integer to a raw normalized floating-point value.
> It does this basically in three steps:
 ...
> The fix is simple: the exponent comparison used to determine if the
> fraction should be downshifted must be "<=" not "<".
> 
> I'm sending a kernel module to test this as a reply to this message.
> There are also SPARC user-space test cases in the GCC bug entry.
> 
> Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>

Applied and I'll make sure this gets into -stable too.

Thanks!