[PATCH 2/3] sys_personality: make sure (int)personality >= 0

[Oleg Nesterov <oleg@redhat.com>]

Not sure about this patch. The kernel/libc part is correct, but
since user-space declares "int personality(unsigned long persona)"
the current behaviour can confuse the (poor written) applications
even on 64-bit machines.

Consider:

	personality(0xffffffff - 1);	// == (int)-2

	...

	int ret = personality(0);	// returns the old personality
	if (ret < 0)
		oops_we_cant_set_PER_LINUX(errno);

And, since libc correctly detects the successful return from syscall,
errno is random.

Change sys_personality() to ensure personality can not look like a
negative int. This disallows the MSB, it is not used for PER_ flags.

Suggested-by: Wenming Zhang <wezhang@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---

 kernel/exec_domain.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- 34-rc1/kernel/exec_domain.c~2_MAKE_IT_POSITIVE	2010-05-27 15:15:12.000000000 +0200
+++ 34-rc1/kernel/exec_domain.c	2010-05-27 15:54:33.000000000 +0200
@@ -193,7 +193,8 @@ SYSCALL_DEFINE1(personality, u_long, per
 	u_long old = current->personality;
 
 	if (personality != 0xffffffff) {
-		if ((unsigned int)personality != personality)
+		/* ensure it never looks like a negative int to user-space */
+		if (personality > 0x7fffffff)
 			return -EINVAL;
 		set_personality(personality);
 	}