Re: [PATCH] powerpc/64: Fix checksum folding in csum_add

[Paul Mackerras <paulus@ozlabs.org>]

On Tue, Mar 27, 2018 at 05:22:32PM +0200, LEROY Christophe wrote:
> Shile Zhang <shile.zhang@nokia.com> a écrit :
> 
> >fix the missed point in Paul's patch:
> >"powerpc/64: Fix checksum folding in csum_tcpudp_nofold and
> >ip_fast_csum_nofold"
> >
> >Signed-off-by: Shile Zhang <shile.zhang@nokia.com>
> >---
> > arch/powerpc/include/asm/checksum.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >diff --git a/arch/powerpc/include/asm/checksum.h
> >b/arch/powerpc/include/asm/checksum.h
> >index 5b1a6e3..430d038 100644
> >--- a/arch/powerpc/include/asm/checksum.h
> >+++ b/arch/powerpc/include/asm/checksum.h
> >@@ -108,7 +108,7 @@ static inline __wsum csum_add(__wsum csum, __wsum addend)
> >
> > #ifdef __powerpc64__
> > 	res += (__force u64)addend;
> >-	return (__force __wsum)((u32)res + (res >> 32));
> >+	return (__force __wsum) from64to32(res);
> 
> Did you encounter a bug due to that ?
> As far as i understand, csum and addend are 32 bits so can't exceed 0xffffffff
> Then their sum won't exceed 0x1fffffffe. So the sum of upper and lower part
> won't carry

If the sum of the two halves was 0x1fffffffe, then that previously got
truncated to 32 bits and returned as 0xfffffffe, which is wrong - the
result should be 0xffffffff.

Paul.