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>

4 files changed, 11 insertions, 4 deletions

diff --git a/arch/x86/include/asm/stacktrace.h b/arch/x86/include/asm/stacktrace.h
index 0944218af9e2..7646fb2772f8 100644
--- a/arch/x86/include/asm/stacktrace.h
+++ b/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
diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
index 14e4b20f0aaf..ec689c62c01f 100644
--- a/arch/x86/include/asm/switch_to.h
+++ b/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
diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
index 5e3f294ce264..8e36f249646e 100644
--- a/arch/x86/kernel/kgdb.c
+++ b/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;
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 62c0b0ea2ce4..0115a4a4db96 100644
--- a/arch/x86/kernel/process.c
+++ b/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;