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

[Gustavo A. R. Silva]

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@xxxxxxxxxx>
> > >
> > > > ---
> > > >    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