linux-kernel - Re: [PATCH][next] nfsd: Replace one-element array with flexible-array member

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Message-ID: <136d72e5-287c-ebaf-c1e1-92b10e68ba8d@embeddedor.com>
Date:   Tue, 23 May 2023 19:42:20 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Chuck Lever <cel@...nel.org>
Cc:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Jeff Layton <jlayton@...nel.org>, linux-nfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
        dai.ngo@...cle.com
Subject: Re: [PATCH][next] nfsd: Replace one-element array with flexible-array
 member



On 5/23/23 19:31, Chuck Lever wrote:
> On Tue, May 23, 2023 at 07:11:37PM -0600, Gustavo A. R. Silva wrote:
>>
>> On 5/23/23 19:01, Chuck Lever wrote:
>>> On Tue, May 23, 2023 at 06:44:23PM -0600, Gustavo A. R. Silva wrote:
>>>> One-element arrays are deprecated, and we are replacing them with
>>>> flexible array members instead. So, replace a one-element array
>>>> with a flexible-arrayº member in struct vbi_anc_data and refactor
>>>
>>> I don't know what "struct vbi_anc_data" is. Is the patch description
>>> correct?
>>
>> Oops, copy/paste error. I'll fix it up. :)
>>
>>>
>>>
>>>> the rest of the code, accordingly.
>>>>
>>>> This results in no differences in binary output.
>>>>
>>>> Link: https://github.com/KSPP/linux/issues/79
>>>> Link: https://github.com/KSPP/linux/issues/298
>>>> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
>>>
>>>> ---
>>>>    fs/nfsd/nfs4callback.c | 2 +-
>>>>    fs/nfsd/xdr4.h         | 2 +-
>>>>    2 files changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
>>>> index 4039ffcf90ba..2c688d51135d 100644
>>>> --- a/fs/nfsd/nfs4callback.c
>>>> +++ b/fs/nfsd/nfs4callback.c
>>>> @@ -353,7 +353,7 @@ encode_cb_recallany4args(struct xdr_stream *xdr,
>>>>    {
>>>>    	encode_nfs_cb_opnum4(xdr, OP_CB_RECALL_ANY);
>>>>    	encode_uint32(xdr, ra->ra_keep);
>>>> -	encode_bitmap4(xdr, ra->ra_bmval, ARRAY_SIZE(ra->ra_bmval));
>>>> +	encode_bitmap4(xdr, ra->ra_bmval, 1);
>>>
>>> I find the new code less self-documenting.
>>>
>>>
>>>>    	hdr->nops++;
>>>>    }
>>>> diff --git a/fs/nfsd/xdr4.h b/fs/nfsd/xdr4.h
>>>> index 510978e602da..68072170eac8 100644
>>>> --- a/fs/nfsd/xdr4.h
>>>> +++ b/fs/nfsd/xdr4.h
>>>> @@ -899,7 +899,7 @@ struct nfsd4_operation {
>>>>    struct nfsd4_cb_recall_any {
>>>>    	struct nfsd4_callback	ra_cb;
>>>>    	u32			ra_keep;
>>>> -	u32			ra_bmval[1];
>>>> +	u32			ra_bmval[];
>>>
>>> This is not a placeholder for "1 or more elements". We actually want
>>> just a single u32 element in this array. Doesn't this change the
>>> sizeof(struct nfsd4_cb_recall_any) ?
>>
>> I see. Yes, it does change the size. Can we replace it with a simple
>> object of type u32? or do you actually need this to stay an array?
> 
> It's not impossible to make it a scalar u32, however...
> 
> In this area of code, *_bmval is always a bitmap -- an array of u32s.
> Helpers like encode_bitmap4() assume an array. I think it would be
> less confusing overall to human readers if it remained an array.
> 
> In this case, it is a single element array because CB_RECALL_ANY
> doesn't happen to use bits above the first 32-bit word of the
> bitmap.

I see. If this is never going to be treated as a flexible array, then
it can stay as is.

-fstrict-flex-arrays=3 should not warn about this because the array
will never ever be tried to be accessed beyond element 1. :)

Thanks for the feedback!
--
Gustavo

> 
> We could make it a 2-element array, I think, without harm. Send a
> patch for that, and Dai can test it to make sure there are no
> unexpected interoperability consequences.
> 
> I hope that would avoid suspicious-looking array definitions.
> 
> 
>>>>    };
>>>>    #endif
>>>> -- 
>>>> 2.34.1
>>>>