Re: [PATCH] scm: remove use CMSG{_COMPAT}_ALIGN(sizeof(struct {compat_}cmsghdr))

[David Miller <davem@davemloft.net>]

From: yuan linyu <cugyly@163.com>
Date: Tue,  3 Jan 2017 20:42:17 +0800

> From: yuan linyu <Linyu.Yuan@alcatel-sbell.com.cn>
> 
> sizeof(struct cmsghdr) and sizeof(struct compat_cmsghdr) already aligned.
> remove use CMSG_ALIGN(sizeof(struct cmsghdr)) and
> CMSG_COMPAT_ALIGN(sizeof(struct compat_cmsghdr)) keep code consistent.
> 
> Signed-off-by: yuan linyu <Linyu.Yuan@alcatel-sbell.com.cn>

Applied, and I added the following commit which will make sure our
analysis is accurate.

====================
[PATCH] net: Assert at build time the assumptions we make about the CMSG header.

It must always be the case that CMSG_ALIGN(sizeof(hdr)) == sizeof(hdr).

Otherwise there are missing adjustments in the various calculations
that parse and build these things.

Signed-off-by: David S. Miller <davem@davemloft.net>
---
 net/compat.c | 3 +++
 net/socket.c | 2 ++
 2 files changed, 5 insertions(+)

diff --git a/net/compat.c b/net/compat.c
index 4e27dd1..ba3ac72 100644
--- a/net/compat.c
+++ b/net/compat.c
@@ -130,6 +130,9 @@ int cmsghdr_from_user_compat_to_kern(struct msghdr *kmsg, struct sock *sk,
 	__kernel_size_t kcmlen, tmp;
 	int err = -EFAULT;
 
+	BUILD_BUG_ON(sizeof(struct compat_cmsghdr) !=
+		     CMSG_COMPAT_ALIGN(sizeof(struct compat_cmsghdr)));
+
 	kcmlen = 0;
 	kcmsg_base = kcmsg = (struct cmsghdr *)stackbuf;
 	ucmsg = CMSG_COMPAT_FIRSTHDR(kmsg);
diff --git a/net/socket.c b/net/socket.c
index 8487bf1..5f3b5a2 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -1948,6 +1948,8 @@ static int ___sys_sendmsg(struct socket *sock, struct user_msghdr __user *msg,
 		ctl_buf = msg_sys->msg_control;
 		ctl_len = msg_sys->msg_controllen;
 	} else if (ctl_len) {
+		BUILD_BUG_ON(sizeof(struct cmsghdr) !=
+			     CMSG_ALIGN(sizeof(struct cmsghdr)));
 		if (ctl_len > sizeof(ctl)) {
 			ctl_buf = sock_kmalloc(sock->sk, ctl_len, GFP_KERNEL);
 			if (ctl_buf == NULL)
-- 
2.4.11