linux-kernel - Re: [PATCH bpf-next] bpf: fix 'dubious one-bit signed bitfield' warnings

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Message-ID: <eafd8d7b-4aea-9ec0-6e3f-8e0afb290235@fb.com>
Date:   Sun, 10 Jul 2022 17:38:50 -0700
From:   Yonghong Song <yhs@...com>
To:     Matthieu Baerts <matthieu.baerts@...sares.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Eduard Zingerman <eddyz87@...il.com>
Cc:     mptcp@...ts.linux.dev, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next] bpf: fix 'dubious one-bit signed bitfield'
 warnings



On 7/10/22 1:19 PM, Matthieu Baerts wrote:
> Hi Yonghong,
> 
> Thank you for the review!
> 
> On 10/07/2022 18:59, Yonghong Song wrote:> On 7/10/22 1:35 AM, Matthieu
> Baerts wrote:
>>> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
>>> index 81b19669efba..2ac424641cc3 100644
>>> --- a/include/linux/bpf_verifier.h
>>> +++ b/include/linux/bpf_verifier.h
>>> @@ -345,10 +345,10 @@ struct bpf_verifier_state_list {
>>>    };
>>>      struct bpf_loop_inline_state {
>>> -    int initialized:1; /* set to true upon first entry */
>>> -    int fit_for_inline:1; /* true if callback function is the same
>>> -                   * at each call and flags are always zero
>>> -                   */
>>> +    bool initialized; /* set to true upon first entry */
>>> +    bool fit_for_inline; /* true if callback function is the same
>>> +                  * at each call and flags are always zero
>>> +                  */
>>
>> I think changing 'int' to 'unsigned' is a better alternative for
>> potentially adding more bitfields in the future. This is also a pattern
>> for many other kernel data structures.
> 
> There was room, I was not sure if it would be OK but I saw 'bool' were
> often used in structures from this bpf_verifier.h file.
> 
> I can of course switch to an unsigned one. I would have picked 'u8' when
> looking at the structures around but any preferences from you?
> 'unsigned', 'unsigned int', 'u8', 'u32'?

The original data structure is
   struct bpf_loop_inline_state {
         int initialized:1; /* set to true upon first entry */
         int fit_for_inline:1; /* true if callback function is the same
                                * at each call and flags are always zero
                                */
         u32 callback_subprogno; /* valid when fit_for_inline is true */
   };

So 'initialized' and 'fit_for_inline' and additional padding will take
4 bytes, so 'unsigned', 'unsigned int', 'u32' should be appropriate 
here. Later, if people want to add a u8 or u16 to utilize the padding,
the type of 'initialized' and 'fit_for_inlined' might be changed to
u8 or u16.

For which of 'unsigned', 'unsigned int', 'u32', checking with
   $ [~/work/bpf-next/include/linux] grep ":1" *.h
both 'unsigned' and 'unsigned int' are used in many places. I don't have
a preference. I saw one instance 'unsigned int' is used in this file,
so 'unsigned int' should be okay here.


> 
> Cheers,
> Matt