From: Mikael Pettersson <mikpe@xxxxxxxx> 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: > > 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> Thanks for fixing this Mikael: Acked-by: David S. Miller <davem@xxxxxxxxxxxxx> Has anyone done an audit to compare the copy of math-emu in glibc, gcc, and the linux kernel so that we don't have bugs living in some places but not others? These sources really need to be consolidated somehow. -- 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
