On Tue, Jan 27, 2015 at 04:25:16PM +0100, Karl Beldan wrote: > The carry from the 64->32bits folding was dropped, e.g with: > saddr=0xFFFFFFFF daddr=0xFF0000FF len=0xFFFF proto=0 sum=1 > > Signed-off-by: Karl Beldan <karl.beldan@xxxxxxxxxxxxxxxx> > Cc: Mike Frysinger <vapier@xxxxxxxxxx> > Cc: Arnd Bergmann <arnd@xxxxxxxx> > Cc: linux-kernel@xxxxxxxxxxxxxxx > Cc: Stable <stable@xxxxxxxxxxxxxxx> > --- > lib/checksum.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/lib/checksum.c b/lib/checksum.c > index 129775e..4b5adf2 100644 > --- a/lib/checksum.c > +++ b/lib/checksum.c > @@ -195,8 +195,8 @@ __wsum csum_tcpudp_nofold(__be32 saddr, __be32 daddr, > #else > s += (proto + len) << 8; > #endif > - s += (s >> 32); > - return (__force __wsum)s; > + s += (s << 32) + (s >> 32); > + return (__force __wsum)(s >> 32); Umm... I _think_ it's correct, but it needs a better commit message. AFAICS, what we have is that s is guaranteed to be (a << 32) + b, with a being small. What we want is something congruent to a + b modulo 0xffff. And yes, in case when a + b >= 2^32, the original variant fails - it yields a + b - 2^32, which is one less than what's needed. New one results first in (a + b)(2^32+1)mod 2^64, then that divided by 2^32. If a + b <= 2^32 - 1, the first product is less than 2^64 and dividing it by 2^32 yields a + b. If a + b = 2^32 + c, c is guaranteed to be small and we first get 2^32 * c + 2^32 + 1, then c + 1, i.e. a + b - 0xffffffff, i.e. a + b - 0x10001 * 0xffff, so the congruence holds in all cases. IOW, I think the fix is correct, but it really needs analysis in the commit message. -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html