On Fri, Feb 08, 2013 at 09:06:52AM -0800, H. Peter Anvin wrote:
> On 02/08/2013 07:28 AM, Russell King - ARM Linux wrote:
> >
> > Whether that's safe for x86 or not, I don't know, but my suspicions are
> > that it's unsafe on x86 as it's possible to refer to the various bytes/
> > half-words of eax separately.
> >
> > So, I came to the conclusion that if x86 remains a problem, there's
> > little point supporting it on ARM.
> >
>
> It is possible to access bytes separately, but gcc generally doesn't.
> However, whether or not that can be relied upon safely is a tricky question.
>
> It *is* also worth nothing that the x86 ABI does allow two words to be
> returned in registers from a normal C function, simply by returning a
> structure. That doesn't solve the problem at hand, though, which is how
> to make a type-neutral macro which can handle doublewords...
I just tried to compare a couple of options:
1)
unsigned long __val_gu;
unsigned long long __val_gu8;
2)
register typeof(x) __val_gu asm("eax");
I was still using a test app based on my orignal patch which
used eax and edx for the value.
3)
typeof(x) __val_gu;
While options 2 and 3 get rid of the warnings, they unfortunately
produce different results as well.
I've attached my test app in case you want to see it.
One other note is that if I include the memtest() call in the test,
option 1 and 2 produce the same results, but w/o the memtest() the
results differ. I didn't look into it any further.
--
Ville Syrjälä
Intel OTC
#ifdef AMD64
#define _ASM_AX rax
#define _ASM_CX rcx
#define _ASM_DX rdx
#else
#define _ASM_AX eax
#define _ASM_CX ecx
#define _ASM_DX edx
#endif
.text
.globl __get_user_1
.globl __get_user_2
.globl __get_user_4
#ifdef AMD64
.globl __get_user_8
#else
.globl __get_user_8_32
#endif
__get_user_1:
movzb (%_ASM_CX),%eax
xor %ecx,%ecx
ret
__get_user_2:
add $1,%_ASM_CX
movzwl -1(%_ASM_CX),%eax
xor %ecx,%ecx
ret
__get_user_4:
add $3,%_ASM_CX
mov -3(%_ASM_CX),%eax
xor %ecx,%ecx
ret
#ifdef AMD64
__get_user_8:
add $7,%_ASM_CX
movq -7(%_ASM_CX),%_ASM_AX
xor %ecx,%ecx
ret
#else
__get_user_8_32:
add $7,%_ASM_CX
movl -7(%_ASM_CX),%eax
movl -3(%_ASM_CX),%edx
xor %ecx,%ecx
ret
#endif
CFLAGS:=-std=gnu99 -O2
CPPFLAGS:=-DAMD64
#CFLAGS+=-m32
#ASFLAGS+=-m32
CPPFLAGS+=-Wint-to-pointer-cast
.PHONY: all clean
all: test
test: uaccess.o get_user.o
$(CC) $(CFLAGS) -o $@ $^
clean:
rm *.o
#include <stdio.h>
#include <stdint.h>
#include <inttypes.h>
#include <string.h>
#define OPT 1
#define __get_user_x(size, ret, x, ptr) \
asm volatile("call __get_user_" #size \
: "=c" (ret), "=a" (x) \
: "0" (ptr))
#ifdef AMD64
#define __get_user_8(__ret_gu, __val_gu, ptr) \
__get_user_x(8, __ret_gu, __val_gu, ptr)
#else
#define __get_user_8(ret, x, ptr) \
asm volatile("call __get_user_8_32" \
: "=c" (ret), "=A" (x) \
: "0" (ptr))
#endif
#if OPT == 1
#define get_user(x, ptr) \
({ \
int __ret_gu; \
unsigned long __val_gu; \
unsigned long long __val_gu8; \
switch (sizeof(*(ptr))) { \
case 1: \
__get_user_x(1, __ret_gu, __val_gu, ptr); \
break; \
case 2: \
__get_user_x(2, __ret_gu, __val_gu, ptr); \
break; \
case 4: \
__get_user_x(4, __ret_gu, __val_gu, ptr); \
break; \
case 8: \
__get_user_8(__ret_gu, __val_gu8, ptr); \
break; \
default: \
__get_user_x(X, __ret_gu, __val_gu, ptr); \
break; \
} \
if (sizeof(*(ptr)) == 8) \
(x) = (typeof(*(ptr)))__val_gu8; \
else \
(x) = (typeof(*(ptr)))__val_gu; \
__ret_gu; \
})
#endif
#if OPT == 2
#define get_user(x, ptr) \
({ \
int __ret_gu; \
register typeof(x) __val_gu asm("eax"); \
switch (sizeof(*(ptr))) { \
case 1: \
__get_user_x(1, __ret_gu, __val_gu, ptr); \
break; \
case 2: \
__get_user_x(2, __ret_gu, __val_gu, ptr); \
break; \
case 4: \
__get_user_x(4, __ret_gu, __val_gu, ptr); \
break; \
case 8: \
__get_user_8(__ret_gu, __val_gu, ptr); \
break; \
default: \
__get_user_x(X, __ret_gu, __val_gu, ptr); \
break; \
} \
(x) = (typeof(*(ptr)))__val_gu; \
__ret_gu; \
})
#endif
#if OPT == 3
#define get_user(x, ptr) \
({ \
int __ret_gu; \
typeof(x) __val_gu; \
switch (sizeof(*(ptr))) { \
case 1: \
__get_user_x(1, __ret_gu, __val_gu, ptr); \
break; \
case 2: \
__get_user_x(2, __ret_gu, __val_gu, ptr); \
break; \
case 4: \
__get_user_x(4, __ret_gu, __val_gu, ptr); \
break; \
case 8: \
__get_user_8(__ret_gu, __val_gu, ptr); \
break; \
default: \
__get_user_x(X, __ret_gu, __val_gu, ptr); \
break; \
} \
(x) = (typeof(*(ptr)))__val_gu; \
__ret_gu; \
})
#endif
int main(void)
{
uint64_t foo = 0x89abcdef76543210;
printf(" foo: %"PRIx64"\n", foo);
struct {
uint8_t sika1;
uint16_t sika2;
uint32_t sika4;
uint64_t sika8;
} __attribute__((__packed__)) x;
if (get_user(x.sika8, (const uint64_t*)&foo)) return 1;
if (get_user(x.sika4, (const uint64_t*)&foo)) return 1;
if (get_user(x.sika2, (const uint64_t*)&foo)) return 1;
if (get_user(x.sika1, (const uint64_t*)&foo)) return 1;
printf("sika8: %"PRIx64"\n", x.sika8);
printf("sika4: %x\n", x.sika4);
printf("sika2: %x\n", x.sika2);
printf("sika1: %x\n", x.sika1);
//memset(&x, 0, sizeof(x));
if (get_user(x.sika1, (const uint64_t*)&foo)) return 1;
if (get_user(x.sika2, (const uint64_t*)&foo)) return 1;
if (get_user(x.sika4, (const uint64_t*)&foo)) return 1;
if (get_user(x.sika8, (const uint64_t*)&foo)) return 1;
printf("sika8: %"PRIx64"\n", x.sika8);
printf("sika4: %x\n", x.sika4);
printf("sika2: %x\n", x.sika2);
printf("sika1: %x\n", x.sika1);
return 0;
}
