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>

4 files changed, 4 insertions, 3 deletions

diff --git a/arch/x86/include/asm/processor-flags.h b/arch/x86/include/asm/processor-flags.h
index 2dddb317bb39..f8ab3eaad128 100644
--- a/arch/x86/include/asm/processor-flags.h
+++ b/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 */
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index cfad7fce6163..a20e1cb9dc87 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/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*/
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index ee5d4fbd53b4..15763af7bfe3 100644
--- a/arch/x86/kernel/process.c
+++ b/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);
diff --git a/drivers/lguest/x86/core.c b/drivers/lguest/x86/core.c
index 65af42f2d593..39809035320a 100644
--- a/drivers/lguest/x86/core.c
+++ b/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