x86: Prepare removal of previous_esp from i386 thread_info structure

The i386 thread_info contains a previous_esp field that is used
to daisy chain the different stacks for dump_stack()
(ie. irq, softirq, thread stacks).

The goal is to eventual make i386 handling of thread_info the same
as x86_64, which means that the thread_info will not be in the stack
but as a per_cpu variable. We will no longer depend on thread_info
being able to daisy chain different stacks as it will only exist
in one location (the thread stack).

By moving previous_esp to the end of thread_info and referencing
it as an offset instead of using a thread_info field, this becomes
a stepping stone to moving the thread_info.

The offset to get to the previous stack is rather ugly in this
patch, but this is only temporary and the prev_esp will be changed
in the next commit. This commit is more for sanity checks of the
change.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Robert Richter <rric@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/20110806012353.891757693@goodmis.org
Link: http://lkml.kernel.org/r/20140206144321.608754481@goodmis.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/include/asm/thread_info.h

@@ -32,13 +32,14 @@ struct thread_info {
 	mm_segment_t		addr_limit;
 	struct restart_block    restart_block;
 	void __user		*sysenter_return;
+	unsigned int		sig_on_uaccess_error:1;
+	unsigned int		uaccess_err:1;	/* uaccess failed */
 #ifdef CONFIG_X86_32
 	unsigned long           previous_esp;   /* ESP of the previous stack in
 						   case of nested (IRQ) stacks
+						   (Moved to end, to be removed soon)
 						*/
 #endif
-	unsigned int		sig_on_uaccess_error:1;
-	unsigned int		uaccess_err:1;	/* uaccess failed */
 };
 
 #define INIT_THREAD_INFO(tsk)			\

arch/x86/kernel/dumpstack_32.c

@@ -22,6 +22,7 @@ void dump_trace(struct task_struct *task, struct pt_regs *regs,
 		const struct stacktrace_ops *ops, void *data)
 {
 	int graph = 0;
+	u32 *prev_esp;
 
 	if (!task)
 		task = current;
@@ -44,9 +45,17 @@ void dump_trace(struct task_struct *task, struct pt_regs *regs,
 			((unsigned long)stack & (~(THREAD_SIZE - 1)));
 		bp = ops->walk_stack(context, stack, bp, ops, data, NULL, &graph);
 
-		stack = (unsigned long *)context->previous_esp;
+		/* Stop if not on irq stack */
+		if (task_stack_page(task) == context)
+			break;
+
+		/* The previous esp is just above the context */
+		prev_esp = (u32 *) ((char *)context + sizeof(struct thread_info) -
+				    sizeof(long));
+		stack = (unsigned long *)*prev_esp;
 		if (!stack)
 			break;
+
 		if (ops->stack(data, "IRQ") < 0)
 			break;
 		touch_nmi_watchdog();

arch/x86/kernel/irq_32.c

@@ -81,7 +81,7 @@ static inline int
 execute_on_irq_stack(int overflow, struct irq_desc *desc, int irq)
 {
 	union irq_ctx *curctx, *irqctx;
-	u32 *isp, arg1, arg2;
+	u32 *isp, *prev_esp, arg1, arg2;
 
 	curctx = (union irq_ctx *) current_thread_info();
 	irqctx = __this_cpu_read(hardirq_ctx);
@@ -98,7 +98,10 @@ execute_on_irq_stack(int overflow, struct irq_desc *desc, int irq)
 	/* build the stack frame on the IRQ stack */
 	isp = (u32 *) ((char *)irqctx + sizeof(*irqctx));
 	irqctx->tinfo.task = curctx->tinfo.task;
-	irqctx->tinfo.previous_esp = current_stack_pointer;
+	/* Save the next esp after thread_info */
+	prev_esp = (u32 *) ((char *)irqctx + sizeof(struct thread_info) -
+			    sizeof(long));
+	*prev_esp = current_stack_pointer;
 
 	if (unlikely(overflow))
 		call_on_stack(print_stack_overflow, isp);
@@ -149,16 +152,20 @@ void do_softirq_own_stack(void)
 {
 	struct thread_info *curctx;
 	union irq_ctx *irqctx;
-	u32 *isp;
+	u32 *isp, *prev_esp;
 
 	curctx = current_thread_info();
 	irqctx = __this_cpu_read(softirq_ctx);
 	irqctx->tinfo.task = curctx->task;
-	irqctx->tinfo.previous_esp = current_stack_pointer;
 
 	/* build the stack frame on the softirq stack */
 	isp = (u32 *) ((char *)irqctx + sizeof(*irqctx));
 
+	/* Push the previous esp onto the stack */
+	prev_esp = (u32 *) ((char *)irqctx + sizeof(struct thread_info) -
+			    sizeof(long));
+	*prev_esp = current_stack_pointer;
+
 	call_on_stack(__do_softirq, isp);
 }
 

arch/x86/kernel/ptrace.c

@@ -184,14 +184,14 @@ unsigned long kernel_stack_pointer(struct pt_regs *regs)
 {
 	unsigned long context = (unsigned long)regs & ~(THREAD_SIZE - 1);
 	unsigned long sp = (unsigned long)&regs->sp;
-	struct thread_info *tinfo;
+	u32 *prev_esp;
 
 	if (context == (sp & ~(THREAD_SIZE - 1)))
 		return sp;
 
-	tinfo = (struct thread_info *)context;
+	prev_esp = (u32 *)(context + sizeof(struct thread_info) - sizeof(long));
-	if (tinfo->previous_esp)
+	if (prev_esp)
-		return tinfo->previous_esp;
+		return (unsigned long)prev_esp;
 
 	return (unsigned long)regs;
 }