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x86/unwind/orc: Fix unwind_get_return_address_ptr() for inactive tasks

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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:

  f1d9a2abff ("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.

Fixes: f1d9a2abff ("x86/unwind/orc: Don't skip the first frame for inactive tasks")
Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200522135435.vbxs7umku5pyrdbk@treble
This commit is contained in:
Josh Poimboeuf 2020-05-22 08:54:35 -05:00 committed by Peter Zijlstra
parent d7110a26e5
commit 187b96db5c
1 changed files with 7 additions and 0 deletions
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7
arch/x86/kernel/unwind_orc.c
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@ -320,12 +320,19 @@ EXPORT_SYMBOL_GPL(unwind_get_return_address);
unsigned long *unwind_get_return_address_ptr(struct unwind_state *state) unsigned long *unwind_get_return_address_ptr(struct unwind_state *state)
{ {
struct task_struct *task = state->task;
if (unwind_done(state)) if (unwind_done(state))
return NULL; return NULL;
if (state->regs) if (state->regs)
return &state->regs->ip; 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) if (state->sp)
return (unsigned long *)state->sp - 1; return (unsigned long *)state->sp - 1;