The PR_GET_ENDIAN and PR_SET_ENDIAN prctl() calls have been implemented to

allow to switch processes at runtime from big-endian to little-endian mode
(and vice versa) on PowerPC processors.  Since the other architectures
don't support this feature, they currently will just fail and return
-EINVAL.

This patch adds just minimal overhead and implements the PR_GET_ENDIAN
call for all other architectures by returning the native endianess of the
architecture.  Furthermore, calling prctl(PR_SET_ENDIAN) with the native
endianess of the architecture will succeed, while trying to set another
(not-supported) endianess, will fail.

The patch can be tested with the following program:

#include <stdio.h>
#include <linux/prctl.h>

int main(int argc, char **argv)
{
	int endian, ret;

	ret = prctl(PR_GET_ENDIAN, &endian);
	if (ret)
		perror("prctl(PR_GET_ENDIAN) not implemented");
	printf("current process/machine is running in %s endian mode (%d)\n",
		endian == PR_ENDIAN_LITTLE ? "little":"big", endian);

	/* setting native endianess should succeed */
	ret = prctl(PR_SET_ENDIAN, endian);
	printf("prctl(PR_SET_ENDIAN,%d) should succeed: %s\n",
		endian, ret == 0 ? "OK":"FAIL");

	/* setting foreign endianess should fail */
	endian = (endian == PR_ENDIAN_LITTLE) ?
		PR_ENDIAN_BIG : PR_ENDIAN_LITTLE;
	ret = prctl(PR_SET_ENDIAN, endian);
	printf("prctl(PR_SET_ENDIAN,%d) should fail: %s\n",
		endian, ret == 0 ? "OK":"FAIL");
}
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: Anton Blanchard <anton@samba.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

kernel/sys.c

@@ -46,6 +46,15 @@
 #include <asm/io.h>
 #include <asm/unistd.h>
 
+#if defined(__BIG_ENDIAN)
+# define PRCTL_ENDIAN_DEFAULT	(PR_ENDIAN_BIG)
+#elif defined(__LITTLE_ENDIAN)
+# define PRCTL_ENDIAN_DEFAULT	(PR_ENDIAN_LITTLE)
+#else
+# error "No endianess?"
+#endif
+
+
 #ifndef SET_UNALIGN_CTL
 # define SET_UNALIGN_CTL(a,b)	(-EINVAL)
 #endif
@@ -65,10 +74,12 @@
 # define GET_FPEXC_CTL(a,b)	(-EINVAL)
 #endif
 #ifndef GET_ENDIAN
-# define GET_ENDIAN(a,b)	(-EINVAL)
+# define GET_ENDIAN(task,addr)	\
+	put_user(PRCTL_ENDIAN_DEFAULT, (int __user *) (addr))
 #endif
 #ifndef SET_ENDIAN
-# define SET_ENDIAN(a,b)	(-EINVAL)
+# define SET_ENDIAN(task,value)	\
+	( (value) == PRCTL_ENDIAN_DEFAULT ? 0 : -EINVAL )
 #endif
 #ifndef GET_TSC_CTL
 # define GET_TSC_CTL(a)		(-EINVAL)