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SubjectRe: [PATCH][next] uapi: net/ipv4: Use __DECLARE_FLEX_ARRAY() helper
On Tue, Aug 30, 2022 at 12:24:44PM -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 {
> struct {
> __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 */
> }; /* 0 20 */
>
> /* size: 20, cachelines: 1, members: 1 */
> /* 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>
> ---
> include/uapi/linux/in.h | 20 +++++++-------------
> 1 file changed, 7 insertions(+), 13 deletions(-)
>
> diff --git a/include/uapi/linux/in.h b/include/uapi/linux/in.h
> index 14168225cecd..fa4dc8f8f081 100644
> --- a/include/uapi/linux/in.h
> +++ b/include/uapi/linux/in.h
> @@ -188,20 +188,14 @@ struct ip_mreq_source {
> };
>
> struct ip_msfilter {
> - union {
> - struct {
> - __be32 imsf_multiaddr_aux;
> - __be32 imsf_interface_aux;
> - __u32 imsf_fmode_aux;
> - __u32 imsf_numsrc_aux;
> + struct {

I don't think this internal anonymous struct is needed any more?

> + __be32 imsf_multiaddr;
> + __be32 imsf_interface;
> + __u32 imsf_fmode;
> + __u32 imsf_numsrc;
> + union {
> __be32 imsf_slist[1];
> - };
> - struct {
> - __be32 imsf_multiaddr;
> - __be32 imsf_interface;
> - __u32 imsf_fmode;
> - __u32 imsf_numsrc;
> - __be32 imsf_slist_flex[];
> + __DECLARE_FLEX_ARRAY(__be32, imsf_slist_flex);
> };
> };
> };

I.e. can't this now just be:

struct ip_msfilter {
__be32 imsf_multiaddr;
__be32 imsf_interface;
__u32 imsf_fmode;
__u32 imsf_numsrc;
union {
__be32 imsf_slist[1];
__DECLARE_FLEX_ARRAY(__be32, imsf_slist_flex);
};
};


> --
> 2.34.1
>

--
Kees Cook

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