x86/unwind: Silence entry-related warnings

A few people have reported unwinder warnings like the following:

  WARNING: kernel stack frame pointer at ffffc90000fe7ff0 in rsync:1157 has bad value           (null)
  unwind stack type:0 next_sp:          (null) mask:2 graph_idx:0
  ffffc90000fe7f98: ffffc90000fe7ff0 (0xffffc90000fe7ff0)
  ffffc90000fe7fa0: ffffffffb7000f56 (trace_hardirqs_off_thunk+0x1a/0x1c)
  ffffc90000fe7fa8: 0000000000000246 (0x246)
  ffffc90000fe7fb0: 0000000000000000 ...
  ffffc90000fe7fc0: 00007ffe3af639bc (0x7ffe3af639bc)
  ffffc90000fe7fc8: 0000000000000006 (0x6)
  ffffc90000fe7fd0: 00007f80af433fc5 (0x7f80af433fc5)
  ffffc90000fe7fd8: 00007ffe3af638e0 (0x7ffe3af638e0)
  ffffc90000fe7fe0: 00007ffe3af638e0 (0x7ffe3af638e0)
  ffffc90000fe7fe8: 00007ffe3af63970 (0x7ffe3af63970)
  ffffc90000fe7ff0: 0000000000000000 ...
  ffffc90000fe7ff8: ffffffffb7b74b9a (entry_SYSCALL_64_after_swapgs+0x17/0x4f)

This warning can happen when unwinding a code path where an interrupt
occurred in x86 entry code before it set up the first stack frame.
Silently ignore any warnings for this case.

Reported-by: Daniel Borkmann <daniel@iogearbox.net>
Reported-by: Dave Jones <davej@codemonkey.org.uk>
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: 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>
Fixes: c32c47c68a0a ("x86/unwind: Warn on bad frame pointer")
Link: http://lkml.kernel.org/r/dbd6838826466a60dc23a52098185bc973ce2f1e.1492020577.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/unwind.h

@@ -12,6 +12,7 @@ struct unwind_state {
 	struct task_struct *task;
 	int graph_idx;
 #ifdef CONFIG_FRAME_POINTER
+	bool got_irq;
 	unsigned long *bp, *orig_sp;
 	struct pt_regs *regs;
 	unsigned long ip;

arch/x86/kernel/unwind_frame.c

@@ -1,6 +1,8 @@
 #include <linux/sched.h>
 #include <linux/sched/task.h>
 #include <linux/sched/task_stack.h>
+#include <linux/interrupt.h>
+#include <asm/sections.h>
 #include <asm/ptrace.h>
 #include <asm/bitops.h>
 #include <asm/stacktrace.h>
@@ -72,6 +74,21 @@ static size_t regs_size(struct pt_regs *regs)
 	return sizeof(*regs);
 }
 
+static bool in_entry_code(unsigned long ip)
+{
+	char *addr = (char *)ip;
+
+	if (addr >= __entry_text_start && addr < __entry_text_end)
+		return true;
+
+#if defined(CONFIG_FUNCTION_GRAPH_TRACER) || defined(CONFIG_KASAN)
+	if (addr >= __irqentry_text_start && addr < __irqentry_text_end)
+		return true;
+#endif
+
+	return false;
+}
+
 #ifdef CONFIG_X86_32
 #define GCC_REALIGN_WORDS 3
 #else
@@ -144,6 +161,7 @@ static bool update_stack_state(struct unwind_state *state,
 	if (regs) {
 		frame = (unsigned long *)regs;
 		len = regs_size(regs);
+		state->got_irq = true;
 	} else {
 		frame = next_bp;
 		len = FRAME_HEADER_SIZE;
@@ -239,16 +257,8 @@ bool unwind_next_frame(struct unwind_state *state)
 		next_bp = (unsigned long *)READ_ONCE_TASK_STACK(state->task, *state->bp);
 
 	/* Move to the next frame if it's safe: */
-	if (!update_stack_state(state, next_bp)) {
-		/*
-		 * Don't warn on bad regs->bp.  An interrupt in entry code
-		 * might cause a false positive warning.
-		 */
-		if (state->regs)
-			goto the_end;
-
+	if (!update_stack_state(state, next_bp))
 		goto bad_address;
-	}
 
 	return true;
 
@@ -263,6 +273,13 @@ bad_address:
 	if (state->task != current)
 		goto the_end;
 
+	/*
+	 * Don't warn if the unwinder got lost due to an interrupt in entry
+	 * code before the stack was set up:
+	 */
+	if (state->got_irq && in_entry_code(state->ip))
+		goto the_end;
+
 	if (state->regs) {
 		printk_deferred_once(KERN_WARNING
 			"WARNING: kernel stack regs at %p in %s:%d has bad 'bp' value %p\n",
@@ -289,6 +306,7 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
 
 	memset(state, 0, sizeof(*state));
 	state->task = task;
+	state->got_irq = (regs);
 
 	/* Don't even attempt to start from user mode regs: */
 	if (regs && user_mode(regs)) {