linux-kernel - [PATCH v2 1/1] samples/seccomp: eliminate two compile warnings in user-trap.c

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Date:   Tue, 1 Sep 2020 16:39:03 +0800
From:   Zhen Lei <thunder.leizhen@...wei.com>
To:     Kees Cook <keescook@...omium.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
CC:     Zhen Lei <thunder.leizhen@...wei.com>
Subject: [PATCH v2 1/1] samples/seccomp: eliminate two compile warnings in user-trap.c

samples/seccomp/user-trap.c is compiled with $(userccflags), and the
latter does not contain -fno-strict-aliasing, so the warnings reported as
below. Due to add "userccflags += -fno-strict-aliasing" will impact other
files, so use __attribute__((__may_alias__)) to suppress it exactly.

My gcc version is 5.5.0 20171010.

----------
samples/seccomp/user-trap.c: In function ‘send_fd’:
samples/seccomp/user-trap.c:50:2: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  *((int *)CMSG_DATA(cmsg)) = fd;
  ^
samples/seccomp/user-trap.c: In function ‘recv_fd’:
samples/seccomp/user-trap.c:83:2: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  return *((int *)CMSG_DATA(cmsg));
  ^

Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
---
 samples/seccomp/user-trap.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/samples/seccomp/user-trap.c b/samples/seccomp/user-trap.c
index 20291ec6489f31e..e36696b7f41517f 100644
--- a/samples/seccomp/user-trap.c
+++ b/samples/seccomp/user-trap.c
@@ -23,6 +23,8 @@
 
 #define ARRAY_SIZE(x) (sizeof(x) / sizeof(*(x)))
 
+typedef int __attribute__((__may_alias__)) __int_alias_t;
+
 static int seccomp(unsigned int op, unsigned int flags, void *args)
 {
 	errno = 0;
@@ -47,7 +49,7 @@ static int send_fd(int sock, int fd)
 	cmsg->cmsg_level = SOL_SOCKET;
 	cmsg->cmsg_type = SCM_RIGHTS;
 	cmsg->cmsg_len = CMSG_LEN(sizeof(int));
-	*((int *)CMSG_DATA(cmsg)) = fd;
+	*(__int_alias_t *)CMSG_DATA(cmsg) = fd;
 	msg.msg_controllen = cmsg->cmsg_len;
 
 	if (sendmsg(sock, &msg, 0) < 0) {
@@ -80,7 +82,7 @@ static int recv_fd(int sock)
 
 	cmsg = CMSG_FIRSTHDR(&msg);
 
-	return *((int *)CMSG_DATA(cmsg));
+	return *(__int_alias_t *)CMSG_DATA(cmsg);
 }
 
 static int user_trap_syscall(int nr, unsigned int flags)
-- 
1.8.3