sched/x86: Add 'struct inactive_task_frame' to better document the sleeping task stack frame

Add 'struct inactive_task_frame', which defines the layout of the stack for
a sleeping process.  For now, the only defined field is the BP register
(frame pointer).

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1471106302-10159-4-git-send-email-brgerst@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/stacktrace.h

@@ -8,6 +8,7 @@
 
 #include <linux/uaccess.h>
 #include <linux/ptrace.h>
+#include <asm/switch_to.h>
 
 extern int kstack_depth_to_print;
 
@@ -70,8 +71,7 @@ stack_frame(struct task_struct *task, struct pt_regs *regs)
 		return bp;
 	}
 
-	/* bp is the last reg pushed by switch_to */
-	return *(unsigned long *)task->thread.sp;
+	return ((struct inactive_task_frame *)task->thread.sp)->bp;
 }
 #else
 static inline unsigned long

arch/x86/include/asm/switch_to.h

@@ -30,6 +30,11 @@ static inline void prepare_switch_to(struct task_struct *prev,
 #endif
 }
 
+/* data that is pointed to by thread.sp */
+struct inactive_task_frame {
+	unsigned long bp;
+};
+
 #ifdef CONFIG_X86_32
 
 #ifdef CONFIG_CC_STACKPROTECTOR

arch/x86/kernel/kgdb.c

@@ -50,6 +50,7 @@
 #include <asm/apicdef.h>
 #include <asm/apic.h>
 #include <asm/nmi.h>
+#include <asm/switch_to.h>
 
 struct dbg_reg_def_t dbg_reg_def[DBG_MAX_REG_NUM] =
 {
@@ -166,7 +167,7 @@ void sleeping_thread_to_gdb_regs(unsigned long *gdb_regs, struct task_struct *p)
 	gdb_regs[GDB_DX]	= 0;
 	gdb_regs[GDB_SI]	= 0;
 	gdb_regs[GDB_DI]	= 0;
-	gdb_regs[GDB_BP]	= *(unsigned long *)p->thread.sp;
+	gdb_regs[GDB_BP]	= ((struct inactive_task_frame *)p->thread.sp)->bp;
 #ifdef CONFIG_X86_32
 	gdb_regs[GDB_DS]	= __KERNEL_DS;
 	gdb_regs[GDB_ES]	= __KERNEL_DS;

arch/x86/kernel/process.c

@@ -32,6 +32,7 @@
 #include <asm/tlbflush.h>
 #include <asm/mce.h>
 #include <asm/vm86.h>
+#include <asm/switch_to.h>
 
 /*
  * per-CPU TSS segments. Threads are completely 'soft' on Linux,
@@ -556,7 +557,7 @@ unsigned long get_wchan(struct task_struct *p)
 	if (sp < bottom || sp > top)
 		return 0;
 
-	fp = READ_ONCE_NOCHECK(*(unsigned long *)sp);
+	fp = READ_ONCE_NOCHECK(((struct inactive_task_frame *)sp)->bp);
 	do {
 		if (fp < bottom || fp > top)
 			return 0;