[RFC PATCH v1.9 03/14] x86/asm/head: standardize the bottom of the stack for idle tasks

[Josh Poimboeuf]

Thanks to all the recent x86 entry code refactoring, most tasks' kernel
stacks start at the same offset right above their saved pt_regs,
regardless of which syscall was used to enter the kernel.  That creates
a useful convention which makes it straightforward to identify the
"bottom" of the stack, which can be useful for stack walking code which
needs to verify the stack is sane.

However there are still a few types of tasks which don't yet follow that
convention:

1) CPU idle tasks, aka the "swapper" tasks

2) freshly forked tasks which haven't run yet and have TIF_FORK set

Make the idle tasks conform to the new stack bottom convention by
starting their stack at a sizeof(pt_regs) offset from the end of the
stack page.

Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 arch/x86/kernel/head_64.S | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index f2deae7..255fe54 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -20,6 +20,7 @@
 #include <asm/processor-flags.h>
 #include <asm/percpu.h>
 #include <asm/nops.h>
+#include "../entry/calling.h"
 
 #ifdef CONFIG_PARAVIRT
 #include <asm/asm-offsets.h>
@@ -287,8 +288,9 @@ 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
@@ -316,7 +318,7 @@ ENDPROC(start_cpu0)
 	GLOBAL(initial_gs)
 	.quad	INIT_PER_CPU_VAR(irq_stack_union)
 	GLOBAL(initial_stack)
-	.quad  init_thread_union+THREAD_SIZE-8
+	.quad  init_thread_union + THREAD_SIZE - SIZEOF_PTREGS
 	__FINITDATA
 
 bad_address:
-- 
2.4.3