[PATCH 4.19 74/81] x86/unwind/orc: Fix unwind_get_return_address_ptr() for inactive tasks

From: Greg Kroah-Hartman
Date: Tue May 26 2020 - 15:05:43 EST

From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

commit 187b96db5ca79423618dfa29a05c438c34f9e1f0 upstream.

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

Fixes: f1d9a2abff66 ("x86/unwind/orc: Don't skip the first frame for inactive tasks")
Reported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200522135435.vbxs7umku5pyrdbk@treble
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

arch/x86/kernel/unwind_orc.c | 7 +++++++
1 file changed, 7 insertions(+)

--- a/arch/x86/kernel/unwind_orc.c
+++ b/arch/x86/kernel/unwind_orc.c
@@ -300,12 +300,19 @@ EXPORT_SYMBOL_GPL(unwind_get_return_addr

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;