netdev - Re: [PATCH net-next 1/3] ipv4: Don't reset ->flowi4_scope in ip_rt_fix

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Date:   Fri, 22 Apr 2022 08:40:01 -0600
From:   David Ahern <dsahern@...nel.org>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        dccp@...r.kernel.org
Subject: Re: [PATCH net-next 1/3] ipv4: Don't reset ->flowi4_scope in
 ip_rt_fix_tos().

On 4/22/22 4:53 AM, Guillaume Nault wrote:
> On Thu, Apr 21, 2022 at 08:30:52PM -0600, David Ahern wrote:
>> On 4/20/22 5:21 PM, Guillaume Nault wrote:
>>> All callers already initialise ->flowi4_scope with RT_SCOPE_UNIVERSE,
>>> either by manual field assignment, memset(0) of the whole structure or
>>> implicit structure initialisation of on-stack variables
>>> (RT_SCOPE_UNIVERSE actually equals 0).
>>>
>>> Therefore, we don't need to always initialise ->flowi4_scope in
>>> ip_rt_fix_tos(). We only need to reduce the scope to RT_SCOPE_LINK when
>>> the special RTO_ONLINK flag is present in the tos.
>>>
>>> This will allow some code simplification, like removing
>>> ip_rt_fix_tos(). Also, the long term idea is to remove RTO_ONLINK
>>> entirely by properly initialising ->flowi4_scope, instead of
>>> overloading ->flowi4_tos with a special flag. Eventually, this will
>>> allow to convert ->flowi4_tos to dscp_t.
>>>
>>> Signed-off-by: Guillaume Nault <gnault@...hat.com>
>>> ---
>>> It's important for the correctness of this patch that all callers
>>> initialise ->flowi4_scope to 0 (in one way or another). Auditing all of
>>> them is long, although each case is pretty trivial.
>>>
>>> If it helps, I can send a patch series that converts implicit
>>> initialisation of ->flowi4_scope with an explicit assignment to
>>> RT_SCOPE_UNIVERSE. This would also have the advantage of making it
>>> clear to future readers that ->flowi4_scope _has_ to be initialised. I
>>> haven't sent such patch series to not overwhelm reviewers with trivial
>>> and not technically-required changes (there are 40+ places to modify,
>>> scattered over 30+ different files). But if anyone prefers explicit
>>> initialisation everywhere, then just let me know and I'll send such
>>> patches.
>>
>> There are a handful of places that open code the initialization of the
>> flow struct. I *think* I found all of them in 40867d74c374.
> 
> By open code, do you mean "doesn't use flowi4_init_output() nor
> ip_tunnel_init_flow()"? If so, I think there are many more.
> 

no, you made a comment about flow struct being initialized to 0 which
implicitly initializes scope. My comment is that there are only a few
places that do not use either `memset(flow, 0, sizeof())` or `struct
flowi4 fl4 = {}` to fully initialize the struct.