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@xxxxxxxx> 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@xxxxxxxxxx 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) \ > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > -- To unsubscribe from this list: send the line "unsubscribe sparclinux" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
