Re: [PATCH v2][next] net/ipv4: Use __DECLARE_FLEX_ARRAY() helper

[Kees Cook <keescook@chromium.org>]

On Wed, Aug 31, 2022 at 02:12:42PM -0500, Gustavo A. R. Silva wrote:
> We now have a cleaner way to keep compatibility with user-space
> (a.k.a. not breaking it) when we need to keep in place a one-element
> array (for its use in user-space) together with a flexible-array
> member (for its use in kernel-space) without making it hard to read
> at the source level. This is through the use of the new
> __DECLARE_FLEX_ARRAY() helper macro.
> 
> The size and memory layout of the structure is preserved after the
> changes. See below.
> 
> Before changes:
> 
> $ pahole -C ip_msfilter net/ipv4/igmp.o
> struct ip_msfilter {
> 	union {
> 		struct {
> 			__be32     imsf_multiaddr_aux;   /*     0     4 */
> 			__be32     imsf_interface_aux;   /*     4     4 */
> 			__u32      imsf_fmode_aux;       /*     8     4 */
> 			__u32      imsf_numsrc_aux;      /*    12     4 */
> 			__be32     imsf_slist[1];        /*    16     4 */
> 		};                                       /*     0    20 */
> 		struct {
> 			__be32     imsf_multiaddr;       /*     0     4 */
> 			__be32     imsf_interface;       /*     4     4 */
> 			__u32      imsf_fmode;           /*     8     4 */
> 			__u32      imsf_numsrc;          /*    12     4 */
> 			__be32     imsf_slist_flex[0];   /*    16     0 */
> 		};                                       /*     0    16 */
> 	};                                               /*     0    20 */
> 
> 	/* size: 20, cachelines: 1, members: 1 */
> 	/* last cacheline: 20 bytes */
> };
> 
> After changes:
> 
> $ pahole -C ip_msfilter net/ipv4/igmp.o
> struct ip_msfilter {
> 	__be32                     imsf_multiaddr;       /*     0     4 */
> 	__be32                     imsf_interface;       /*     4     4 */
> 	__u32                      imsf_fmode;           /*     8     4 */
> 	__u32                      imsf_numsrc;          /*    12     4 */
> 	union {
> 		__be32             imsf_slist[1];        /*    16     4 */
> 		struct {
> 			struct {
> 			} __empty_imsf_slist_flex;       /*    16     0 */
> 			__be32     imsf_slist_flex[0];   /*    16     0 */
> 		};                                       /*    16     0 */
> 	};                                               /*    16     4 */
> 
> 	/* size: 20, cachelines: 1, members: 5 */
> 	/* last cacheline: 20 bytes */
> };
> 
> In the past, we had to duplicate the whole original structure within
> a union, and update the names of all the members. Now, we just need to
> declare the flexible-array member to be used in kernel-space through
> the __DECLARE_FLEX_ARRAY() helper together with the one-element array,
> within a union. This makes the source code more clean and easier to read.
> 
> Link: https://github.com/KSPP/linux/issues/193
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook