x86: Fix rflags in FAKE_STACK_FRAME

The x86_64 kernel pushes the fake kernel stack in
arch/x86/kernel/entry_64.S:FAKE_STACK_FRAME, and
rflags register in it does not conform to the specification.

Although Intel's manual[1] says bit 1 of it shall be set to 1,
this bit is cleared to 0 on pushing the fake stack.

[1] Intel(R) 64 and IA-32 Architectures Software Developer's Manual
    Vol.1 3-21 Figure 3-8. EFLAGS Register

If it is not on purpose, it is better to be fixed, because
it can lead some tools misunderstanding the stack frame. For example,
"crash" utility[2] actually detects it and warns you like
below:

       RIP: ffffffff8005dfa2  RSP: ffff8104ce0c7f58  RFLAGS: 00000200
       [...]

       bt: WARNING: possibly bogus exception frame
Signed-off-by: Seiichi Ikarashi <s.ikarashi@jp.fujitsu.com>
Tested-by: Masayoshi MIZUMA <m.mizuma@jp.fujitsu.com>
Cc: Jan Beulich <JBeulich@suse.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/include/asm/processor-flags.h

@@ -6,6 +6,7 @@
  * EFLAGS bits
  */
 #define X86_EFLAGS_CF	0x00000001 /* Carry Flag */
+#define X86_EFLAGS_BIT1	0x00000002 /* Bit 1 - always on */
 #define X86_EFLAGS_PF	0x00000004 /* Parity Flag */
 #define X86_EFLAGS_AF	0x00000010 /* Auxiliary carry Flag */
 #define X86_EFLAGS_ZF	0x00000040 /* Zero Flag */

arch/x86/kernel/entry_64.S

@@ -221,7 +221,7 @@ ENDPROC(native_usergs_sysret64)
 	/*CFI_REL_OFFSET	ss,0*/
 	pushq_cfi %rax /* rsp */
 	CFI_REL_OFFSET	rsp,0
-	pushq_cfi $X86_EFLAGS_IF /* eflags - interrupts on */
+	pushq_cfi $(X86_EFLAGS_IF|X86_EFLAGS_BIT1) /* eflags - interrupts on */
 	/*CFI_REL_OFFSET	rflags,0*/
 	pushq_cfi $__KERNEL_CS /* cs */
 	/*CFI_REL_OFFSET	cs,0*/

arch/x86/kernel/process.c

@@ -293,7 +293,7 @@ int kernel_thread(int (*fn)(void *), void *arg, unsigned long flags)
 	regs.orig_ax = -1;
 	regs.ip = (unsigned long) kernel_thread_helper;
 	regs.cs = __KERNEL_CS | get_kernel_rpl();
-	regs.flags = X86_EFLAGS_IF | 0x2;
+	regs.flags = X86_EFLAGS_IF | X86_EFLAGS_BIT1;
 
 	/* Ok, create the new process.. */
 	return do_fork(flags | CLONE_VM | CLONE_UNTRACED, 0, &regs, 0, NULL, NULL);

drivers/lguest/x86/core.c

@@ -697,7 +697,7 @@ void lguest_arch_setup_regs(struct lg_cpu *cpu, unsigned long start)
 	 * interrupts are enabled.  We always leave interrupts enabled while
 	 * running the Guest.
 	 */
-	regs->eflags = X86_EFLAGS_IF | 0x2;
+	regs->eflags = X86_EFLAGS_IF | X86_EFLAGS_BIT1;
 
 	/*
 	 * The "Extended Instruction Pointer" register says where the Guest is