[PATCH][next] net/ipv4/ipv6: Replace one-element arraya with flexible-array members

From: Gustavo A. R. Silva
Date: Wed Aug 04 2021 - 16:43:07 EST

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There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Use an anonymous union with a couple of anonymous structs in order to
keep userspace unchanged and refactor the related code accordingly:

$ pahole -C group_filter net/ipv4/ip_sockglue.o
struct group_filter {
union {
struct {
__u32 gf_interface_aux; /* 0 4 */

/* XXX 4 bytes hole, try to pack */

struct __kernel_sockaddr_storage gf_group_aux; /* 8 128 */
/* --- cacheline 2 boundary (128 bytes) was 8 bytes ago --- */
__u32 gf_fmode_aux; /* 136 4 */
__u32 gf_numsrc_aux; /* 140 4 */
struct __kernel_sockaddr_storage gf_slist[1]; /* 144 128 */
}; /* 0 272 */
struct {
__u32 gf_interface; /* 0 4 */

/* XXX 4 bytes hole, try to pack */

struct __kernel_sockaddr_storage gf_group; /* 8 128 */
/* --- cacheline 2 boundary (128 bytes) was 8 bytes ago --- */
__u32 gf_fmode; /* 136 4 */
__u32 gf_numsrc; /* 140 4 */
struct __kernel_sockaddr_storage gf_slist_flex[0]; /* 144 0 */
}; /* 0 144 */
}; /* 0 272 */

/* size: 272, cachelines: 5, members: 1 */
/* last cacheline: 16 bytes */
};

$ pahole -C compat_group_filter net/ipv4/ip_sockglue.o
struct compat_group_filter {
union {
struct {
__u32 gf_interface_aux; /* 0 4 */
struct __kernel_sockaddr_storage gf_group_aux __attribute__((__aligned__(4))); /* 4 128 */
/* --- cacheline 2 boundary (128 bytes) was 4 bytes ago --- */
__u32 gf_fmode_aux; /* 132 4 */
__u32 gf_numsrc_aux; /* 136 4 */
struct __kernel_sockaddr_storage gf_slist[1] __attribute__((__aligned__(4))); /* 140 128 */
} __attribute__((__packed__)) __attribute__((__aligned__(4))); /* 0 268 */
struct {
__u32 gf_interface; /* 0 4 */
struct __kernel_sockaddr_storage gf_group __attribute__((__aligned__(4))); /* 4 128 */
/* --- cacheline 2 boundary (128 bytes) was 4 bytes ago --- */
__u32 gf_fmode; /* 132 4 */
__u32 gf_numsrc; /* 136 4 */
struct __kernel_sockaddr_storage gf_slist_flex[0] __attribute__((__aligned__(4))); /* 140 0 */
} __attribute__((__packed__)) __attribute__((__aligned__(4))); /* 0 140 */
} __attribute__((__aligned__(1))); /* 0 268 */

/* size: 268, cachelines: 5, members: 1 */
/* forced alignments: 1 */
/* last cacheline: 12 bytes */
} __attribute__((__packed__));

This helps with the ongoing efforts to globally enable -Warray-bounds
and get us closer to being able to tighten the FORTIFY_SOURCE routines
on memcpy().

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.10/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/109
Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
---
include/net/compat.h | 27 ++++++++++++++++++++-------
include/uapi/linux/in.h | 21 ++++++++++++++++-----
net/ipv4/ip_sockglue.c | 19 ++++++++++---------
net/ipv6/ipv6_sockglue.c | 18 +++++++++---------
4 files changed, 55 insertions(+), 30 deletions(-)

diff --git a/include/net/compat.h b/include/net/compat.h
index 84805bdc4435..595fee069b82 100644
--- a/include/net/compat.h
+++ b/include/net/compat.h
@@ -71,13 +71,26 @@ struct compat_group_source_req {
} __packed;

struct compat_group_filter {
- __u32 gf_interface;
- struct __kernel_sockaddr_storage gf_group
- __aligned(4);
- __u32 gf_fmode;
- __u32 gf_numsrc;
- struct __kernel_sockaddr_storage gf_slist[1]
- __aligned(4);
+ union {
+ struct {
+ __u32 gf_interface_aux;
+ struct __kernel_sockaddr_storage gf_group_aux
+ __aligned(4);
+ __u32 gf_fmode_aux;
+ __u32 gf_numsrc_aux;
+ struct __kernel_sockaddr_storage gf_slist[1]
+ __aligned(4);
+ } __packed;
+ struct {
+ __u32 gf_interface;
+ struct __kernel_sockaddr_storage gf_group
+ __aligned(4);
+ __u32 gf_fmode;
+ __u32 gf_numsrc;
+ struct __kernel_sockaddr_storage gf_slist_flex[]
+ __aligned(4);
+ } __packed;
+ };
} __packed;

#endif /* NET_COMPAT_H */
diff --git a/include/uapi/linux/in.h b/include/uapi/linux/in.h
index d1b327036ae4..d8bd07215770 100644
--- a/include/uapi/linux/in.h
+++ b/include/uapi/linux/in.h
@@ -211,11 +211,22 @@ struct group_source_req {
};

struct group_filter {
- __u32 gf_interface; /* interface index */
- struct __kernel_sockaddr_storage gf_group; /* multicast address */
- __u32 gf_fmode; /* filter mode */
- __u32 gf_numsrc; /* number of sources */
- struct __kernel_sockaddr_storage gf_slist[1]; /* interface index */
+ union {
+ struct {
+ __u32 gf_interface_aux; /* interface index */
+ struct __kernel_sockaddr_storage gf_group_aux; /* multicast address */
+ __u32 gf_fmode_aux; /* filter mode */
+ __u32 gf_numsrc_aux; /* number of sources */
+ struct __kernel_sockaddr_storage gf_slist[1]; /* interface index */
+ };
+ struct {
+ __u32 gf_interface; /* interface index */
+ struct __kernel_sockaddr_storage gf_group; /* multicast address */
+ __u32 gf_fmode; /* filter mode */
+ __u32 gf_numsrc; /* number of sources */
+ struct __kernel_sockaddr_storage gf_slist_flex[]; /* interface index */
+ };
+ };
};

#define GROUP_FILTER_SIZE(numsrc) \
diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index ec6036713e2c..6ac6e4aec29f 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -791,7 +791,8 @@ static int ip_set_mcast_msfilter(struct sock *sk, sockptr_t optval, int optlen)
goto out_free_gsf;

err = set_mcast_msfilter(sk, gsf->gf_interface, gsf->gf_numsrc,
- gsf->gf_fmode, &gsf->gf_group, gsf->gf_slist);
+ gsf->gf_fmode, &gsf->gf_group,
+ gsf->gf_slist_flex);
out_free_gsf:
kfree(gsf);
return err;
@@ -800,7 +801,7 @@ static int ip_set_mcast_msfilter(struct sock *sk, sockptr_t optval, int optlen)
static int compat_ip_set_mcast_msfilter(struct sock *sk, sockptr_t optval,
int optlen)
{
- const int size0 = offsetof(struct compat_group_filter, gf_slist);
+ const int size0 = offsetof(struct compat_group_filter, gf_slist_flex);
struct compat_group_filter *gf32;
unsigned int n;
void *p;
@@ -814,7 +815,7 @@ static int compat_ip_set_mcast_msfilter(struct sock *sk, sockptr_t optval,
p = kmalloc(optlen + 4, GFP_KERNEL);
if (!p)
return -ENOMEM;
- gf32 = p + 4; /* we want ->gf_group and ->gf_slist aligned */
+ gf32 = p + 4; /* we want ->gf_group and ->gf_slist_flex aligned */

err = -EFAULT;
if (copy_from_sockptr(gf32, optval, optlen))
@@ -827,7 +828,7 @@ static int compat_ip_set_mcast_msfilter(struct sock *sk, sockptr_t optval,
goto out_free_gsf;

err = -EINVAL;
- if (offsetof(struct compat_group_filter, gf_slist[n]) > optlen)
+ if (offsetof(struct compat_group_filter, gf_slist_flex[n]) > optlen)
goto out_free_gsf;

/* numsrc >= (4G-140)/128 overflow in 32 bits */
@@ -835,7 +836,7 @@ static int compat_ip_set_mcast_msfilter(struct sock *sk, sockptr_t optval,
if (n > sock_net(sk)->ipv4.sysctl_igmp_max_msf)
goto out_free_gsf;
err = set_mcast_msfilter(sk, gf32->gf_interface, n, gf32->gf_fmode,
- &gf32->gf_group, gf32->gf_slist);
+ &gf32->gf_group, gf32->gf_slist_flex);
out_free_gsf:
kfree(p);
return err;
@@ -1456,7 +1457,7 @@ static bool getsockopt_needs_rtnl(int optname)
static int ip_get_mcast_msfilter(struct sock *sk, void __user *optval,
int __user *optlen, int len)
{
- const int size0 = offsetof(struct group_filter, gf_slist);
+ const int size0 = offsetof(struct group_filter, gf_slist_flex);
struct group_filter __user *p = optval;
struct group_filter gsf;
int num;
@@ -1468,7 +1469,7 @@ static int ip_get_mcast_msfilter(struct sock *sk, void __user *optval,
return -EFAULT;

num = gsf.gf_numsrc;
- err = ip_mc_gsfget(sk, &gsf, p->gf_slist);
+ err = ip_mc_gsfget(sk, &gsf, p->gf_slist_flex);
if (err)
return err;
if (gsf.gf_numsrc < num)
@@ -1482,7 +1483,7 @@ static int ip_get_mcast_msfilter(struct sock *sk, void __user *optval,
static int compat_ip_get_mcast_msfilter(struct sock *sk, void __user *optval,
int __user *optlen, int len)
{
- const int size0 = offsetof(struct compat_group_filter, gf_slist);
+ const int size0 = offsetof(struct compat_group_filter, gf_slist_flex);
struct compat_group_filter __user *p = optval;
struct compat_group_filter gf32;
struct group_filter gf;
@@ -1499,7 +1500,7 @@ static int compat_ip_get_mcast_msfilter(struct sock *sk, void __user *optval,
num = gf.gf_numsrc = gf32.gf_numsrc;
gf.gf_group = gf32.gf_group;

- err = ip_mc_gsfget(sk, &gf, p->gf_slist);
+ err = ip_mc_gsfget(sk, &gf, p->gf_slist_flex);
if (err)
return err;
if (gf.gf_numsrc < num)
diff --git a/net/ipv6/ipv6_sockglue.c b/net/ipv6/ipv6_sockglue.c
index a6804a7e34c1..e4bdb09c5586 100644
--- a/net/ipv6/ipv6_sockglue.c
+++ b/net/ipv6/ipv6_sockglue.c
@@ -225,7 +225,7 @@ static int ipv6_set_mcast_msfilter(struct sock *sk, sockptr_t optval,
if (GROUP_FILTER_SIZE(gsf->gf_numsrc) > optlen)
goto out_free_gsf;

- ret = ip6_mc_msfilter(sk, gsf, gsf->gf_slist);
+ ret = ip6_mc_msfilter(sk, gsf, gsf->gf_slist_flex);
out_free_gsf:
kfree(gsf);
return ret;
@@ -234,7 +234,7 @@ static int ipv6_set_mcast_msfilter(struct sock *sk, sockptr_t optval,
static int compat_ipv6_set_mcast_msfilter(struct sock *sk, sockptr_t optval,
int optlen)
{
- const int size0 = offsetof(struct compat_group_filter, gf_slist);
+ const int size0 = offsetof(struct compat_group_filter, gf_slist_flex);
struct compat_group_filter *gf32;
void *p;
int ret;
@@ -249,7 +249,7 @@ static int compat_ipv6_set_mcast_msfilter(struct sock *sk, sockptr_t optval,
if (!p)
return -ENOMEM;

- gf32 = p + 4; /* we want ->gf_group and ->gf_slist aligned */
+ gf32 = p + 4; /* we want ->gf_group and ->gf_slist_flex aligned */
ret = -EFAULT;
if (copy_from_sockptr(gf32, optval, optlen))
goto out_free_p;
@@ -261,14 +261,14 @@ static int compat_ipv6_set_mcast_msfilter(struct sock *sk, sockptr_t optval,
goto out_free_p;

ret = -EINVAL;
- if (offsetof(struct compat_group_filter, gf_slist[n]) > optlen)
+ if (offsetof(struct compat_group_filter, gf_slist_flex[n]) > optlen)
goto out_free_p;

ret = ip6_mc_msfilter(sk, &(struct group_filter){
.gf_interface = gf32->gf_interface,
.gf_group = gf32->gf_group,
.gf_fmode = gf32->gf_fmode,
- .gf_numsrc = gf32->gf_numsrc}, gf32->gf_slist);
+ .gf_numsrc = gf32->gf_numsrc}, gf32->gf_slist_flex);

out_free_p:
kfree(p);
@@ -1048,7 +1048,7 @@ static int ipv6_getsockopt_sticky(struct sock *sk, struct ipv6_txoptions *opt,
static int ipv6_get_msfilter(struct sock *sk, void __user *optval,
int __user *optlen, int len)
{
- const int size0 = offsetof(struct group_filter, gf_slist);
+ const int size0 = offsetof(struct group_filter, gf_slist_flex);
struct group_filter __user *p = optval;
struct group_filter gsf;
int num;
@@ -1062,7 +1062,7 @@ static int ipv6_get_msfilter(struct sock *sk, void __user *optval,
return -EADDRNOTAVAIL;
num = gsf.gf_numsrc;
lock_sock(sk);
- err = ip6_mc_msfget(sk, &gsf, p->gf_slist);
+ err = ip6_mc_msfget(sk, &gsf, p->gf_slist_flex);
if (!err) {
if (num > gsf.gf_numsrc)
num = gsf.gf_numsrc;
@@ -1077,7 +1077,7 @@ static int ipv6_get_msfilter(struct sock *sk, void __user *optval,
static int compat_ipv6_get_msfilter(struct sock *sk, void __user *optval,
int __user *optlen)
{
- const int size0 = offsetof(struct compat_group_filter, gf_slist);
+ const int size0 = offsetof(struct compat_group_filter, gf_slist_flex);
struct compat_group_filter __user *p = optval;
struct compat_group_filter gf32;
struct group_filter gf;
@@ -1100,7 +1100,7 @@ static int compat_ipv6_get_msfilter(struct sock *sk, void __user *optval,
return -EADDRNOTAVAIL;

lock_sock(sk);
- err = ip6_mc_msfget(sk, &gf, p->gf_slist);
+ err = ip6_mc_msfget(sk, &gf, p->gf_slist_flex);
release_sock(sk);
if (err)
return err;
--
2.27.0

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