[PATCH 3/4] x86/unwind: include __schedule() in stack traces

Josh Poimboeuf
2017-01-09T18:01:46Z

In the following commit:

0100301bfdf5 ("sched/x86: Rewrite the switch_to() code")

... the layout of the 'inactive_task_frame' struct was designed to have
a frame pointer header embedded in it, so that the unwinder could use
the 'bp' and 'ret_addr' fields to report __schedule() on the stack (or
ret_from_fork() for newly forked tasks which haven't actually run yet).

Finish the job by changing get_frame_pointer() to return a pointer to
inactive_task_frame's 'bp' field rather than 'bp' itself. This allows
the unwinder to start one frame higher on the stack, so that it properly
reports __schedule().

Reported-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
arch/x86/include/asm/stacktrace.h | 5 +----
arch/x86/include/asm/switch_to.h | 10 +++++++++-
2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/stacktrace.h b/arch/x86/include/asm/stacktrace.h
index 20ce3db..2e41c50 100644
--- a/arch/x86/include/asm/stacktrace.h
+++ b/arch/x86/include/asm/stacktrace.h
@@ -52,16 +52,13 @@ static inline bool on_stack(struct stack_info *info, void *addr, size_t len)
static inline unsigned long *
get_frame_pointer(struct task_struct *task, struct pt_regs *regs)
{
- struct inactive_task_frame *frame;
-
if (regs)
return (unsigned long *)regs->bp;

if (task == current)
return __builtin_frame_address(0);

- frame = (struct inactive_task_frame *)task->thread.sp;
- return (unsigned long *)READ_ONCE_NOCHECK(frame->bp);
+ return &((struct inactive_task_frame *)task->thread.sp)->bp;
}
#else
static inline unsigned long *
diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
index 5cb436a..fcc5cd3 100644
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -36,7 +36,10 @@ static inline void prepare_switch_to(struct task_struct *prev,

asmlinkage void ret_from_fork(void);

-/* data that is pointed to by thread.sp */
+/*
+ * This is the structure pointed to by thread.sp for an inactive task. The
+ * order of the fields must match the code in __switch_to_asm().
+ */
struct inactive_task_frame {
#ifdef CONFIG_X86_64
unsigned long r15;
@@ -48,6 +51,11 @@ struct inactive_task_frame {
unsigned long di;
#endif
unsigned long bx;
+
+ /*
+ * These two fields must be together. They form a stack frame header,
+ * needed by get_frame_pointer().
+ */
unsigned long bp;
unsigned long ret_addr;
};
--
2.7.4