dereferencing type-punned pointer will break strict-aliasing rules

[Jun Kuriyama]

Hmm, it seems this macro is John's baby.  John?

At Mon, 28 Jul 2003 02:00:50 +0000 (UTC),
Thomas Moestl wrote:
> 
> [1  <text/plain; us-ascii (7bit)>]
> On Mon, 2003/07/28 at 09:30:08 +0900, Jun Kuriyama wrote:
> > 
> > Is this caused by -oS option?
> > 
> > ----- in making BOOTMFS in make release
> > cc -c -Os -pipe -Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes  -Wmissing-prototypes -Wpointer-arith -Winline -Wcast-qual  -fformat-extensions -std=c99  -nostdinc -I-  -I. -I/usr/src/sys -I/usr/src/sys/dev -I/usr/src/sys/contrib/dev/acpica -I/usr/src/sys/contrib/ipfilter -I/usr/src/sys/contrib/dev/ath -I/usr/src/sys/contrib/dev/ath/freebsd -D_KERNEL -include opt_global.h -fno-common -finline-limit=15000  -mno-align-long-strings -mpreferred-stack-boundary=2 -ffreestanding -Werror  /usr/src/sys/geom/geom_dev.c
> > /usr/src/sys/geom/geom_dev.c: In function `g_dev_open':
> > /usr/src/sys/geom/geom_dev.c:198: warning: dereferencing type-punned pointer will break strict-aliasing rules
> > [...]
> 
> Yes, by implying -fstrict-aliasing, so using -fno-strict-aliasing is a
> workaround. The problem is caused by the i386 PCPU_GET/PCPU_SET
> implementation:
> 
> 	#define	__PCPU_GET(name) ({						\
> 		__pcpu_type(name) __result;					\
> 										\
> 	[...]
> 		} else if (sizeof(__result) == 4) {				\
> 			u_int __i;						\
> 			__asm __volatile("movl %%fs:%1,%0"			\
> 			    : "=r" (__i)					\
> 			    : "m" (*(u_int *)(__pcpu_offset(name))));		\
> 			__result = *(__pcpu_type(name) *)&__i;			\
> 	[...]
> 
> In this case, the PCPU_GET is used to retrieve curthread, causing
> sizeof(__result) to be 4, so the cast at the end of the code snippet
> is from a u_int * to struct thread *, and __i is accessed through the
> casted pointer, which violates the C99 aliasing rules.
> An alternative is to type-pun via a union, which is also a bit ugly,
> but explicitly allowed by C99. Patch attached (but only superficially
> tested).
> 
> 	- Thomas
> 
> -- 
> Thomas Moestl <t.moestl at tu-bs.de>	http://www.tu-bs.de/~y0015675/
>               <tmm at FreeBSD.org>		http://people.FreeBSD.org/~tmm/
> PGP fingerprint: 1C97 A604 2BD0 E492 51D0  9C0F 1FE6 4F1D 419C 776C
> [2 pcpu.diff <text/plain; us-ascii (7bit)>]
> Index: pcpu.h
> ===================================================================
> RCS file: /vol/ncvs/src/sys/i386/include/pcpu.h,v
> retrieving revision 1.36
> diff -u -r1.36 pcpu.h
> --- pcpu.h	27 Jun 2003 21:50:52 -0000	1.36
> +++ pcpu.h	28 Jul 2003 01:37:57 -0000
> @@ -96,23 +96,32 @@
>  	__pcpu_type(name) __result;					\
>  									\
>  	if (sizeof(__result) == 1) {					\
> -		u_char __b;						\
> +		union {							\
> +			u_char __b;					\
> +			__pcpu_type(name) __r;				\
> +		} __u;							\
>  		__asm __volatile("movb %%fs:%1,%0"			\
> -		    : "=r" (__b)					\
> +		    : "=r" (__u.__b)					\
>  		    : "m" (*(u_char *)(__pcpu_offset(name))));		\
> -		__result = *(__pcpu_type(name) *)&__b;			\
> +		__result = __u.__r;					\
>  	} else if (sizeof(__result) == 2) {				\
> -		u_short __w;						\
> +		union {							\
> +			u_short __w;					\
> +			__pcpu_type(name) __r;				\
> +		} __u;							\
>  		__asm __volatile("movw %%fs:%1,%0"			\
> -		    : "=r" (__w)					\
> +		    : "=r" (__u.__w)					\
>  		    : "m" (*(u_short *)(__pcpu_offset(name))));		\
> -		__result = *(__pcpu_type(name) *)&__w;			\
> +		__result = __u.__r;					\
>  	} else if (sizeof(__result) == 4) {				\
> -		u_int __i;						\
> +		union {							\
> +			u_int __i;					\
> +			__pcpu_type(name) __r;				\
> +		} __u;							\
>  		__asm __volatile("movl %%fs:%1,%0"			\
> -		    : "=r" (__i)					\
> +		    : "=r" (__u.__i)					\
>  		    : "m" (*(u_int *)(__pcpu_offset(name))));		\
> -		__result = *(__pcpu_type(name) *)&__i;			\
> +		__result = __u.__r;					\
>  	} else {							\
>  		__result = *__PCPU_PTR(name);				\
>  	}								\
> @@ -127,23 +136,32 @@
>  	__pcpu_type(name) __val = (val);				\
>  									\
>  	if (sizeof(__val) == 1) {					\
> -		u_char __b;						\
> -		__b = *(u_char *)&__val;				\
> +		union {							\
> +			u_char __b;					\
> +			__pcpu_type(name) __v;				\
> +		} __u;							\
> +		__u.__v = __val;					\
>  		__asm __volatile("movb %1,%%fs:%0"			\
>  		    : "=m" (*(u_char *)(__pcpu_offset(name)))		\
> -		    : "r" (__b));					\
> +		    : "r" (__u.__b));					\
>  	} else if (sizeof(__val) == 2) {				\
> -		u_short __w;						\
> -		__w = *(u_short *)&__val;				\
> +		union {							\
> +			u_short __w;					\
> +			__pcpu_type(name) __v;				\
> +		} __u;							\
> +		__u.__v = __val;					\
>  		__asm __volatile("movw %1,%%fs:%0"			\
>  		    : "=m" (*(u_short *)(__pcpu_offset(name)))		\
> -		    : "r" (__w));					\
> +		    : "r" (__u.__w));					\
>  	} else if (sizeof(__val) == 4) {				\
> -		u_int __i;						\
> -		__i = *(u_int *)&__val;					\
> +		union {							\
> +			u_int __i;					\
> +			__pcpu_type(name) __v;				\
> +		} __u;							\
> +		__u.__v = __val;					\
>  		__asm __volatile("movl %1,%%fs:%0"			\
>  		    : "=m" (*(u_int *)(__pcpu_offset(name)))		\
> -		    : "r" (__i));					\
> +		    : "r" (__u.__i));					\
>  	} else {							\
>  		*__PCPU_PTR(name) = __val;				\
>  	}								\