x86/boot: Simplify boot stack setup

Define the symbol __top_init_kernel_stack instead of duplicating
the offset from __end_init_task in multiple places.
Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Uros Bizjak <ubizjak@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20240321180506.89030-1-brgerst@gmail.com

arch/x86/include/asm/processor.h

@@ -636,12 +636,10 @@ static __always_inline void prefetchw(const void *x)
 #define KSTK_ESP(task)		(task_pt_regs(task)->sp)
 
 #else
-extern unsigned long __end_init_task[];
+extern unsigned long __top_init_kernel_stack[];
 
 #define INIT_THREAD {							\
-	.sp	= (unsigned long)&__end_init_task -			\
-		  TOP_OF_KERNEL_STACK_PADDING -				\
-		  sizeof(struct pt_regs),				\
+	.sp	= (unsigned long)&__top_init_kernel_stack,		\
 }
 
 extern unsigned long KSTK_ESP(struct task_struct *task);

arch/x86/kernel/head_32.S

@@ -44,9 +44,6 @@
 #define X86_CAPABILITY	new_cpu_data+CPUINFO_x86_capability
 #define X86_VENDOR_ID	new_cpu_data+CPUINFO_x86_vendor_id
 
-
-#define SIZEOF_PTREGS 17*4
-
 /*
  * Worst-case size of the kernel mapping we need to make:
  * a relocatable kernel can live anywhere in lowmem, so we need to be able
@@ -488,13 +485,7 @@ SYM_DATA_END(initial_page_table)
 
 .data
 .balign 4
-/*
- * The SIZEOF_PTREGS gap is a convention which helps the in-kernel unwinder
- * reliably detect the end of the stack.
- */
-SYM_DATA(initial_stack,
-		.long init_thread_union + THREAD_SIZE -
-		SIZEOF_PTREGS - TOP_OF_KERNEL_STACK_PADDING)
+SYM_DATA(initial_stack, .long __top_init_kernel_stack)
 
 __INITRODATA
 int_msg:

arch/x86/kernel/head_64.S

@@ -66,7 +66,7 @@ SYM_CODE_START_NOALIGN(startup_64)
 	mov	%rsi, %r15
 
 	/* Set up the stack for verify_cpu() */
-	leaq	(__end_init_task - TOP_OF_KERNEL_STACK_PADDING - PTREGS_SIZE)(%rip), %rsp
+	leaq	__top_init_kernel_stack(%rip), %rsp
 
 	/* Setup GSBASE to allow stack canary access for C code */
 	movl	$MSR_GS_BASE, %ecx

arch/x86/kernel/vmlinux.lds.S

@@ -172,6 +172,9 @@ SECTIONS
 		/* init_task */
 		INIT_TASK_DATA(THREAD_SIZE)
 
+		/* equivalent to task_pt_regs(&init_task) */
+		__top_init_kernel_stack = __end_init_task - TOP_OF_KERNEL_STACK_PADDING - PTREGS_SIZE;
+
 #ifdef CONFIG_X86_32
 		/* 32 bit has nosave before _edata */
 		NOSAVE_DATA

arch/x86/xen/xen-head.S

@@ -49,7 +49,7 @@ SYM_CODE_START(startup_xen)
 	ANNOTATE_NOENDBR
 	cld
 
-	leaq	(__end_init_task - TOP_OF_KERNEL_STACK_PADDING - PTREGS_SIZE)(%rip), %rsp
+	leaq	__top_init_kernel_stack(%rip), %rsp
 
 	/* Set up %gs.
 	 *