Paul Mackerras <pau...@ozlabs.org> a écrit :
On Tue, Mar 27, 2018 at 05:22:32PM +0200, LEROY Christophe wrote:
Shile Zhang <shile.zh...@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.zh...@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.
It is the sum of the two arguments of csum_add() which can't exceed
0x1fffffffe
So the sum of the two halves will be 0xffffffff which is the expected result.
The issue would start with res = 0x1ffffffff, in that case the sum of
the two halves would be 0x100000000 which would have been truncated to
0 whereas we expect the result to be 1 in that case.
But in order to obtain res = 0x1ffffffff or higher, you have to sum at
least three 32 bits numbers. You can't obtain such a value with a sum
of two 32 bits numbers.
Christophe
