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Date: Wed, 31 Aug 2022 14:12:42 -0500
From: "Gustavo A. R. Silva" <gustavoars@...nel.org>
To: "David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
"Gustavo A. R. Silva" <gustavoars@...nel.org>,
linux-hardening@...r.kernel.org, Kees Cook <keescook@...omium.org>
Subject: [PATCH v2][next] net/ipv4: Use __DECLARE_FLEX_ARRAY() helper
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@...nel.org>
---
Changes in v2:
- Remove unnecessary internal anonymous struct. (Kees)
include/uapi/linux/in.h | 20 ++++++--------------
1 file changed, 6 insertions(+), 14 deletions(-)
diff --git a/include/uapi/linux/in.h b/include/uapi/linux/in.h
index 14168225cecd..578daa6f816b 100644
--- a/include/uapi/linux/in.h
+++ b/include/uapi/linux/in.h
@@ -188,21 +188,13 @@ struct ip_mreq_source {
};
struct ip_msfilter {
+ __be32 imsf_multiaddr;
+ __be32 imsf_interface;
+ __u32 imsf_fmode;
+ __u32 imsf_numsrc;
union {
- struct {
- __be32 imsf_multiaddr_aux;
- __be32 imsf_interface_aux;
- __u32 imsf_fmode_aux;
- __u32 imsf_numsrc_aux;
- __be32 imsf_slist[1];
- };
- struct {
- __be32 imsf_multiaddr;
- __be32 imsf_interface;
- __u32 imsf_fmode;
- __u32 imsf_numsrc;
- __be32 imsf_slist_flex[];
- };
+ __be32 imsf_slist[1];
+ __DECLARE_FLEX_ARRAY(__be32, imsf_slist_flex);
};
};
--
2.34.1
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