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

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Date:   Wed, 2 Sep 2020 09:33:06 +0800
From:   "Leizhen (ThunderTown)" <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>
Subject: Re: [PATCH v2 1/1] samples/seccomp: eliminate two compile warnings in
 user-trap.c

Doesn't anyone care about this? Or is it that everyone hasn't encountered this problem?
Why do these two warnings occur every time I compiled?

On 2020/9/1 16:39, Zhen Lei wrote:
> 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)
>