linux-kernel - Re: [PATCH] skbuff: suppress clang object-size-mismatch error

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Date:   Thu, 11 Nov 2021 08:01:26 -0800
From:   Tadeusz Struk <tadeusz.struk@...aro.org>
To:     Marco Elver <elver@...gle.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Alexander Lobakin <alobakin@...me>,
        Willem de Bruijn <willemb@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Cong Wang <cong.wang@...edance.com>,
        Kevin Hao <haokexin@...il.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, Kees Cook <keescook@...omium.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH] skbuff: suppress clang object-size-mismatch error

On 11/11/21 07:52, Marco Elver wrote:
>> The other way to fix it would be to make the struct sk_buff_head
>> equal in size with struct sk_buff:
>>
>>    struct sk_buff_head {
>> -       /* These two members must be first. */
>> -       struct sk_buff  *next;
>> -       struct sk_buff  *prev;
>> +       union {
>> +               struct {
>> +                       /* These two members must be first. */
>> +                       struct sk_buff  *next;
>> +                       struct sk_buff  *prev;
>>
>> -       __u32           qlen;
>> -       spinlock_t      lock;
>> +                       __u32           qlen;
>> +                       spinlock_t      lock;
>> +               };
>> +               struct sk_buff  __prv;
>> +       };
>>    };
>>
>> but that's much more invasive, and I don't even have means to
>> quantify this in terms of final binary size and performance
>> impact. I think that would be a flat out no go.
>>
>>   From the other hand if you look at the __skb_queue functions
>> they don't do much and at all so there is no much room for
>> other issues really. I followed the suggestion in [1]:
>>
>> "if your function deliberately contains possible ..., you can
>>    use __attribute__((no_sanitize... "
> That general advice might not be compatible with what the kernel
> wants, especially since UBSAN_OBJECT_SIZE is normally disabled and I
> think known to cause these issues in the kernel.
> 
> I'll defer to maintainers to decide what would be the preferred way of
> handling this.

Sure, I would also like to know if there is a better way of fixing this.
Thanks for your feedback.

-- 
Thanks,
Tadeusz