Date: Mon, 28 Jul 2003 03:59:00 +0200 From: Thomas Moestl <t.moestl@tu-bs.de> To: Jun Kuriyama <kuriyama@imgsrc.co.jp> Cc: Current <freebsd-current@freebsd.org> Subject: Re: dereferencing type-punned pointer will break strict-aliasing rules Message-ID: <20030728015900.GB5628@crow.dom2ip.de> In-Reply-To: <7mwue3v6gf.wl@black.imgsrc.co.jp> References: <7mwue3v6gf.wl@black.imgsrc.co.jp>
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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@tu-bs.de> http://www.tu-bs.de/~y0015675/
<tmm@FreeBSD.org> http://people.FreeBSD.org/~tmm/
PGP fingerprint: 1C97 A604 2BD0 E492 51D0 9C0F 1FE6 4F1D 419C 776C
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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; \
} \
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