On Wed, Nov 24, 2021 at 7:41 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Wed, Nov 24, 2021 at 8:56 PM Eric Dumazet <edumazet@google.com> wrote:
> >
> > On Wed, Nov 24, 2021 at 5:59 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> > >
> >
> > >
> > > Hi, I'm not sure if this is intentional or not, but I noticed that the output
> > > of 'csum_partial' is different after this patch. I figured that the checksum
> > > algorithm is fixed so just wanted mention it incase its a bug. If not sorry
> > > for the spam.
> > >
> > > Example on x86_64:
> > >
> > > Buff: [ 87, b3, 92, b7, 8b, 53, 96, db, cd, 0f, 7e, 7e ]
> > > len : 11
> > > sum : 0
> > >
> > > csum_partial new : 2480936615
> > > csum_partial HEAD: 2472089390
> >
> > No worries.
> >
> > skb->csum is 32bit, but really what matters is the 16bit folded value.
> >
> > So make sure to apply csum_fold() before comparing the results.
> >
> > A minimal C and generic version of csum_fold() would be something like
> >
> > static unsigned short csum_fold(u32 csum)
> > {
> >   u32 sum = csum;
> >   sum = (sum & 0xffff) + (sum >> 16);
> >   sum = (sum & 0xffff) + (sum >> 16);
> >   return ~sum;
> > }
> >
> > I bet that csum_fold(2480936615) == csum_fold(2472089390)
> >
>
> Correct :)
>
> The outputs seem to match if `buff` is aligned to 64-bit. Still see
> difference with `csum_fold(csum_partial())` if `buff` is not 64-bit aligned.
>
> The comment at the top says it's "best" to have `buff` 64-bit aligned but
> the code logic seems meant to support the misaligned case so not
> sure if it's an issue.
>

It is an issue in general, not in standard cases because network
headers are aligned.

I think it came when I folded csum_partial() and do_csum(), I forgot
to ror() the seed.

I suspect the following would help:

diff --git a/arch/x86/lib/csum-partial_64.c b/arch/x86/lib/csum-partial_64.c
index 1eb8f2d11f7c785be624eba315fe9ca7989fd56d..ee7b0e7a6055bcbef42d22f7e1d8f52ddbd6be6d
100644
--- a/arch/x86/lib/csum-partial_64.c
+++ b/arch/x86/lib/csum-partial_64.c
@@ -41,6 +41,7 @@ __wsum csum_partial(const void *buff, int len, __wsum sum)
        if (unlikely(odd)) {
                if (unlikely(len == 0))
                        return sum;
+               temp64 = ror32((__force u64)sum, 8);
                temp64 += (*(unsigned char *)buff << 8);
                len--;
                buff++;




> Example:
>
> csum_fold(csum_partial) new : 0x3764
> csum_fold(csum_partial) HEAD: 0x3a61
>
> buff        : [ 11, ea, 75, 76, e9, ab, 86, 48 ]
> buff addr   : ffff88eaf5fb0001
> len         : 8
> sum_in      : 25
>
> > It would be nice if we had a csum test suite, hint, hint ;)
>
> Where in the kernel would that belong?

This could be a module, like lib/test_csum.c