x86/unwind: Read stack return address in update_stack_state()

Instead of reading the return address when unwind_get_return_address()
is called, read it from update_stack_state() and store it in the unwind
state.  This enables the next patch to check the return address from
unwind_next_frame() so it can detect an entry code frame.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Dave Jones <davej@codemonkey.org.uk>
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>
Link: http://lkml.kernel.org/r/af0c5e4560c49c0343dca486ea26c4fa92bc4e35.1492020577.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/unwind.h

@@ -14,6 +14,7 @@ struct unwind_state {
 #ifdef CONFIG_FRAME_POINTER
 	unsigned long *bp, *orig_sp;
 	struct pt_regs *regs;
+	unsigned long ip;
 #else
 	unsigned long *sp;
 #endif

arch/x86/kernel/unwind_frame.c

@@ -56,20 +56,10 @@ static void unwind_dump(struct unwind_state *state, unsigned long *sp)
 
 unsigned long unwind_get_return_address(struct unwind_state *state)
 {
-	unsigned long addr;
-	unsigned long *addr_p = unwind_get_return_address_ptr(state);
-
 	if (unwind_done(state))
 		return 0;
 
-	if (state->regs && user_mode(state->regs))
-		return 0;
-
-	addr = READ_ONCE_TASK_STACK(state->task, *addr_p);
-	addr = ftrace_graph_ret_addr(state->task, &state->graph_idx, addr,
-				     addr_p);
-
-	return __kernel_text_address(addr) ? addr : 0;
+	return __kernel_text_address(state->ip) ? state->ip : 0;
 }
 EXPORT_SYMBOL_GPL(unwind_get_return_address);
 
@@ -141,7 +131,7 @@ static bool update_stack_state(struct unwind_state *state,
 	struct stack_info *info = &state->stack_info;
 	enum stack_type prev_type = info->type;
 	struct pt_regs *regs;
-	unsigned long *frame, *prev_frame_end;
+	unsigned long *frame, *prev_frame_end, *addr_p, addr;
 	size_t len;
 
 	if (state->regs)
@@ -185,6 +175,16 @@ static bool update_stack_state(struct unwind_state *state,
 		state->regs = NULL;
 	}
 
+	/* Save the return address: */
+	if (state->regs && user_mode(state->regs))
+		state->ip = 0;
+	else {
+		addr_p = unwind_get_return_address_ptr(state);
+		addr = READ_ONCE_TASK_STACK(state->task, *addr_p);
+		state->ip = ftrace_graph_ret_addr(state->task, &state->graph_idx,
+						  addr, addr_p);
+	}
+
 	/* Save the original stack pointer for unwind_dump(): */
 	if (!state->orig_sp || info->type != prev_type)
 		state->orig_sp = frame;
@@ -228,6 +228,7 @@ bool unwind_next_frame(struct unwind_state *state)
 		 */
 		state->regs = regs;
 		state->bp = NULL;
+		state->ip = 0;
 		return true;
 	}