Messages in this thread | | | Date | Wed, 31 Aug 2022 12:26:28 -0700 | From | Kees Cook <> | Subject | Re: [PATCH v2][next] net/ipv4: Use __DECLARE_FLEX_ARRAY() helper |
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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
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