x86/boot: Fix the end of the stack for idle tasks

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

However, the boot CPU's idle "swapper" task doesn't follow that
convention.  Fix that by starting its stack at a sizeof(pt_regs) offset
from the end of the stack page.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nilay Vaish <nilayvaish@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/81aee3beb6ed88e44f1bea6986bb7b65c368f77a.1474480779.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/head_32.S

@@ -63,6 +63,8 @@
 #define PAGE_TABLE_SIZE(pages) ((pages) / PTRS_PER_PGD)
 #endif
 
+#define SIZEOF_PTREGS 17*4
+
 /*
  * Number of possible pages in the lowmem region.
  *
@@ -708,7 +710,12 @@ ENTRY(initial_page_table)
 .data
 .balign 4
 ENTRY(initial_stack)
-	.long init_thread_union+THREAD_SIZE
+	/*
+	 * The SIZEOF_PTREGS gap is a convention which helps the in-kernel
+	 * unwinder reliably detect the end of the stack.
+	 */
+	.long init_thread_union + THREAD_SIZE - SIZEOF_PTREGS - \
+	      TOP_OF_KERNEL_STACK_PADDING;
 
 __INITRODATA
 int_msg:

arch/x86/kernel/head_64.S

@@ -66,13 +66,8 @@ startup_64:
 	 * tables and then reload them.
 	 */
 
-	/*
-	 * Setup stack for verify_cpu(). "-8" because initial_stack is defined
-	 * this way, see below. Our best guess is a NULL ptr for stack
-	 * termination heuristics and we don't want to break anything which
-	 * might depend on it (kgdb, ...).
-	 */
-	leaq	(__end_init_task - 8)(%rip), %rsp
+	/* Set up the stack for verify_cpu(), similar to initial_stack below */
+	leaq	(__end_init_task - SIZEOF_PTREGS)(%rip), %rsp
 
 	/* Sanitize CPU configuration */
 	call verify_cpu
@@ -329,7 +324,11 @@ ENDPROC(start_cpu0)
 	GLOBAL(initial_gs)
 	.quad	INIT_PER_CPU_VAR(irq_stack_union)
 	GLOBAL(initial_stack)
-	.quad  init_thread_union+THREAD_SIZE-8
+	/*
+	 * The SIZEOF_PTREGS gap is a convention which helps the in-kernel
+	 * unwinder reliably detect the end of the stack.
+	 */
+	.quad  init_thread_union + THREAD_SIZE - SIZEOF_PTREGS
 	__FINITDATA
 
 bad_address: