Re: [PATCH] NFS/flexfiles: Replace one-element array with flexible-array member

[Trond Myklebust <trondmy@hammerspace.com>]

On Sat, 2024-08-17 at 16:20 +0200, Thorsten Blum wrote:
> Replace the deprecated one-element array with a modern flexible-array
> member in the struct nfs4_flexfile_layoutreturn_args.
> 
> Adjust the struct size accordingly.
> 
> There are no binary differences after this conversion.
> 
> Link: https://github.com/KSPP/linux/issues/79
> Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
> ---
>  fs/nfs/flexfilelayout/flexfilelayout.c | 2 +-
>  fs/nfs/flexfilelayout/flexfilelayout.h | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/nfs/flexfilelayout/flexfilelayout.c
> b/fs/nfs/flexfilelayout/flexfilelayout.c
> index 39ba9f4208aa..fc698fa9aaea 100644
> --- a/fs/nfs/flexfilelayout/flexfilelayout.c
> +++ b/fs/nfs/flexfilelayout/flexfilelayout.c
> @@ -2224,7 +2224,7 @@ ff_layout_prepare_layoutreturn(struct
> nfs4_layoutreturn_args *args)
>  	struct nfs4_flexfile_layoutreturn_args *ff_args;
>  	struct nfs4_flexfile_layout *ff_layout =
> FF_LAYOUT_FROM_HDR(args->layout);
>  
> -	ff_args = kmalloc(sizeof(*ff_args), nfs_io_gfp_mask());
> +	ff_args = kmalloc(struct_size(ff_args, pages, 1),
> nfs_io_gfp_mask());
>  	if (!ff_args)
>  		goto out_nomem;
>  	ff_args->pages[0] = alloc_page(nfs_io_gfp_mask());
> diff --git a/fs/nfs/flexfilelayout/flexfilelayout.h
> b/fs/nfs/flexfilelayout/flexfilelayout.h
> index f84b3fb0dddd..a269753f9a46 100644
> --- a/fs/nfs/flexfilelayout/flexfilelayout.h
> +++ b/fs/nfs/flexfilelayout/flexfilelayout.h
> @@ -115,7 +115,7 @@ struct nfs4_flexfile_layoutreturn_args {
>  	struct nfs42_layoutstat_devinfo
> devinfo[FF_LAYOUTSTATS_MAXDEV];
>  	unsigned int num_errors;
>  	unsigned int num_dev;
> -	struct page *pages[1];
> +	struct page *pages[];
>  };
>  
>  static inline struct nfs4_flexfile_layout *

NACK. There is no advantage to using a flexible array here. Indeed,
you're replacing something that is correctly dimensioned (we only ever
use 1 page), and that can be easily bounds checked by the compiler with
something that has neither property.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com