[tip:x86/asm] x86/boot/64: Put a real return address on the idle task stack

[tip-bot for Josh Poimboeuf]

Commit-ID:  595c1e645d9fd8561104b5680931f68a429aaa1c
Gitweb:     http://git.kernel.org/tip/595c1e645d9fd8561104b5680931f68a429aaa1c
Author:     Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
AuthorDate: Wed, 21 Sep 2016 16:04:05 -0500
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Thu, 20 Oct 2016 09:15:23 +0200

x86/boot/64: Put a real return address on the idle task stack

The frame at the end of each idle task stack has a zeroed return
address.  This is inconsistent with real task stacks, which have a real
return address at that spot.  This inconsistency can be confusing for
stack unwinders.  It also hides useful information about what asm code
was involved in calling into C.

Make it a real address by using the side effect of a call instruction to
push the instruction pointer on the stack.

Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Nilay Vaish <nilayvaish@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/f59593ae7b15d5126f872b0a23143173d28aa32d.1474480779.git.jpoimboe@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/kernel/head_64.S | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 3631777..1c5e5db 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -299,8 +299,9 @@ ENTRY(start_cpu)
 	 *	REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,
 	 *		address given in m16:64.
 	 */
-	movq	initial_code(%rip),%rax
-	pushq	$0		# fake return address to stop unwinder
+	call	1f		# put return address on stack for unwinder
+1:	xorq	%rbp, %rbp	# clear frame pointer
+	movq	initial_code(%rip), %rax
 	pushq	$__KERNEL_CS	# set correct cs
 	pushq	%rax		# target address in negative space
 	lretq