[Devel] [PATCH VZ10] fs/fuse kio: replace zero-length array with flexible array syntax

[Konstantin Khorenko]

On 5/6/26 12:27, Liu Kui wrote:
> With FORTIFY_SOURCE, memcpy to a zero-length arrary field is flagged
> as field-spanning, causing kernel warning. Switch to the C99 flexible
> array [] syntax to fix this.
> 
> Changes:
>   - _data_buf[0] -> _data_buf[] in struct krpc_completion
>   - sync_resp[0] -> sync_resp[] in struct pcs_cs_iohdr
>   - nodes[0] -> nodes[] in struct pcs_cs_map_prop
>   - cs[0] -> cs[] in struct pcs_cs_list
>   - cs[0] -> cs[] in struct pcs_ioc_getmap
> 
> Link: https://virtuozzo.atlassian.net/browse/VSTOR-130548
> 
> Signed-off-by: Liu Kui <kui.liu at virtuozzo.com>
> ---
...

> diff --git a/fs/fuse/kio/pcs/pcs_krpc.h b/fs/fuse/kio/pcs/pcs_krpc.h
> index 6a090ef66185..d5a7a56d84c8 100644
> --- a/fs/fuse/kio/pcs/pcs_krpc.h
> +++ b/fs/fuse/kio/pcs/pcs_krpc.h
> @@ -99,7 +99,7 @@ struct krpc_completion {
>   
>   	void		*private;
>   	int			data_len;
> -	u8			_data_buf[0];
> +	u8			_data_buf[];
>   };

struct krpc_completion is embedded in struct krpc_req as a non-last member:

    fs/fuse/kio/pcs/pcs_krpc.h lines 134-137

       struct krpc_completion completion;
       u32             gen;
       struct llist_node       llist_link;

A struct with a C99 flexible array member embedded as a non-last member of another struct is formally 
undefined behavior. GCC's zero-length array extension [0] handled this correctly, but [] should 
trigger warnings.

May be the compiler is very smart and detects that _data_buf is not used when struct krpc_completion 
is embedded into struct krpc_req, but i think it's quite error prone.

May be better just to move struct krpc_completion to the end of struct krpc_req ?

Also, minor typo in the commit message: "arrary" should be "array".

>   
>   #define KRPC_MAX_DATA_PAGES			256
> diff --git a/fs/fuse/kio/pcs/pcs_map.h b/fs/fuse/kio/pcs/pcs_map.h
> index bf8a0e7177d0..788a8cb16d8e 100644
> --- a/fs/fuse/kio/pcs/pcs_map.h
> +++ b/fs/fuse/kio/pcs/pcs_map.h
> @@ -127,7 +127,7 @@ struct pcs_cs_list
>   	int			write_timeout;
>   	int			nsrv;
>   	PCS_MAP_VERSION_T	version;		/* version inherented from map */
> -	struct pcs_cs_record	cs[0];
> +	struct pcs_cs_record	cs[];
>   };
>   
>   /* TODO, LOCKING!!!!!