Re: [PATCH net-next v2 1/7] net: ip: make fib_validate_source() return drop reason

[Paolo Abeni]

On 10/11/24 08:42, Menglong Dong wrote:
> On Thu, Oct 10, 2024 at 5:18 PM Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:
> > On Thu, Oct 10, 2024 at 4:25 PM Paolo Abeni <pabeni@xxxxxxxxxx> wrote:
> > > On 10/7/24 09:46, Menglong Dong wrote:
> > > > In this commit, we make fib_validate_source/__fib_validate_source return
> > > > -reason instead of errno on error. As the return value of them can be
> > > > -errno, 0, and 1, we can't make it return enum skb_drop_reason directly.
> > > >
> > > > In the origin logic, if __fib_validate_source() return -EXDEV,
> > > > LINUX_MIB_IPRPFILTER will be counted. And now, we need to adjust it by
> > > > checking "reason == SKB_DROP_REASON_IP_RPFILTER". However, this will take
> > > > effect only after the patch "net: ip: make ip_route_input_noref() return
> > > > drop reasons", as we can't pass the drop reasons from
> > > > fib_validate_source() to ip_rcv_finish_core() in this patch.
> > > >
> > > > We set the errno to -EINVAL when fib_validate_source() is called and the
> > > > validation fails, as the errno can be checked in the caller and now its
> > > > value is -reason, which can lead misunderstand.
> > > >
> > > > Following new drop reasons are added in this patch:
> > > >
> > > >     SKB_DROP_REASON_IP_LOCAL_SOURCE
> > > >     SKB_DROP_REASON_IP_INVALID_SOURCE
> > > >
> > > > Signed-off-by: Menglong Dong <dongml2@xxxxxxxxxxxxxxx>
> > >
> > > Looking at the next patches, I'm under the impression that the overall
> > > code will be simpler if you let __fib_validate_source() return directly
> > > a drop reason, and fib_validate_source(), too. Hard to be sure without
> > > actually do the attempt... did you try such patch by any chance?
> >
> > I analysed the usages of fib_validate_source() before. The
> > return value of fib_validate_source() can be -errno, "0", and "1".
> > And the value "1" can be used by the caller, such as
> > __mkroute_input(). Making it return drop reasons can't cover this
> > case.
> >
> > It seems that __mkroute_input() is the only case that uses the
> > positive returning value of fib_validate_source(). Let me think
> > about it more in this case.
>
> Hello,
>
> After digging into the code of __fib_validate_source() and __mkroute_input(),
> I think it's hard to make __fib_validate_source() return drop reasons
> directly.
>
> The __fib_validate_source() will return 1 if the scope of the
> source(revert) route is HOST. And the __mkroute_input()
> will mark the skb with IPSKB_DOREDIRECT in this
> case (combine with some other conditions). And then, a REDIRECT
> ICMP will be sent in ip_forward() if this flag exists.
>
> I don't find a way to pass this information to __mkroute_input
> if we make __fib_validate_source() return drop reasons. Can we?
>
> An option is to add a wrapper for fib_validate_source(), such as
> fib_validate_source_reason(), which returns drop reasons. And in
> __mkroute_input(), we still call fib_validate_source().
>
> What do you think?

Thanks for the investigation. I see that let __fib_validate_source() 
returning drop reasons does not look like a good design.

I think the additional helper will not help much, so I guess you can 
retain the current implementation here, but please expand the commit 
message with the above information.

Thanks!

Paolo