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* [PATCH] x86/unwind/orc: Fix unwind_get_return_address_ptr() for inactive tasks
       [not found]           ` <alpine.LSU.2.21.2005220940070.18061@pobox.suse.cz>
@ 2020-05-22 13:54             ` Josh Poimboeuf
  0 siblings, 0 replies; only message in thread
From: Josh Poimboeuf @ 2020-05-22 13:54 UTC (permalink / raw)
  To: Miroslav Benes
  Cc: Tetsuo Handa, Ingo Molnar, x86, Dmitry Vyukov, Linus Torvalds,
	Andy Lutomirski, Dave Jones, Jann Horn, Peter Zijlstra,
	Thomas Gleixner, Vince Weaver

Normally, show_trace_log_lvl() scans the stack, looking for text
addresses to print.  In parallel, it unwinds the stack with
unwind_next_frame().  If the stack address matches the pointer returned
by unwind_get_return_address_ptr() for the current frame, the text
address is printed normally without a question mark.  Otherwise it's
considered a breadcrumb (potentially from a previous call path) and it's
printed with a question mark to indicate that the address is unreliable
and typically can be ignored.

Since the following commit:

  f1d9a2abff66 ("x86/unwind/orc: Don't skip the first frame for inactive tasks")

... for inactive tasks, show_trace_log_lvl() prints *only* unreliable
addresses (prepended with '?').

That happens because, for the first frame of an inactive task,
unwind_get_return_address_ptr() returns the wrong return address
pointer: one word *below* the task stack pointer.  show_trace_log_lvl()
starts scanning at the stack pointer itself, so it never finds the first
'reliable' address, causing only guesses to being printed.

The first frame of an inactive task isn't a normal stack frame.  It's
actually just an instance of 'struct inactive_task_frame' which is left
behind by __switch_to_asm().  Now that this inactive frame is actually
exposed to callers, fix unwind_get_return_address_ptr() to interpret it
properly.

Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Fixes: f1d9a2abff66 ("x86/unwind/orc: Don't skip the first frame for inactive tasks")
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
 arch/x86/kernel/unwind_orc.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/unwind_orc.c b/arch/x86/kernel/unwind_orc.c
index fa79e4227d3d..7f969b2d240f 100644
--- a/arch/x86/kernel/unwind_orc.c
+++ b/arch/x86/kernel/unwind_orc.c
@@ -320,12 +320,19 @@ EXPORT_SYMBOL_GPL(unwind_get_return_address);
 
 unsigned long *unwind_get_return_address_ptr(struct unwind_state *state)
 {
+	struct task_struct *task = state->task;
+
 	if (unwind_done(state))
 		return NULL;
 
 	if (state->regs)
 		return &state->regs->ip;
 
+	if (task != current && state->sp == task->thread.sp) {
+		struct inactive_task_frame *frame = (void *)task->thread.sp;
+		return &frame->ret_addr;
+	}
+
 	if (state->sp)
 		return (unsigned long *)state->sp - 1;
 
-- 
2.21.1


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-- links below jump to the message on this page --
2020-05-20 20:53 INFO: task hung in locks_remove_posix syzbot
2020-05-21 14:09 ` Tetsuo Handa
2020-05-21 14:21   ` Dmitry Vyukov
2020-05-21 23:34     ` [5.7-rc5 x86 regression] ORC unwinder generates unreliable traces Tetsuo Handa
     [not found]       ` <54652cf1-ca04-c3ec-a2fe-d0f47484fb5f@i-love.sakura.ne.jp>
     [not found]         ` <20200522043037.fny37n7kjbfyrxo4@treble>
     [not found]           ` <alpine.LSU.2.21.2005220940070.18061@pobox.suse.cz>
2020-05-22 13:54             ` [PATCH] x86/unwind/orc: Fix unwind_get_return_address_ptr() for inactive tasks Josh Poimboeuf

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