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

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Date:   Tue, 23 May 2023 21:01:56 -0400
From:   Chuck Lever <cel@...nel.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     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
Subject: Re: [PATCH][next] nfsd: Replace one-element array with
 flexible-array member

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?


> 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) ?


>  };
>  
>  #endif
> -- 
> 2.34.1
>