arm64: ftrace: fix a stack tracer's output under function graph tracer

Function graph tracer modifies a return address (LR) in a stack frame
to hook a function return. This will result in many useless entries
(return_to_handler) showing up in
 a) a stack tracer's output
 b) perf call graph (with perf record -g)
 c) dump_backtrace (at panic et al.)

For example, in case of a),
  $ echo function_graph > /sys/kernel/debug/tracing/current_tracer
  $ echo 1 > /proc/sys/kernel/stack_trace_enabled
  $ cat /sys/kernel/debug/tracing/stack_trace
        Depth    Size   Location    (54 entries)
        -----    ----   --------
  0)     4504      16   gic_raise_softirq+0x28/0x150
  1)     4488      80   smp_cross_call+0x38/0xb8
  2)     4408      48   return_to_handler+0x0/0x40
  3)     4360      32   return_to_handler+0x0/0x40
  ...

In case of b),
  $ echo function_graph > /sys/kernel/debug/tracing/current_tracer
  $ perf record -e mem:XXX:x -ag -- sleep 10
  $ perf report
                  ...
                  |          |          |--0.22%-- 0x550f8
                  |          |          |          0x10888
                  |          |          |          el0_svc_naked
                  |          |          |          sys_openat
                  |          |          |          return_to_handler
                  |          |          |          return_to_handler
                  ...

In case of c),
  $ echo function_graph > /sys/kernel/debug/tracing/current_tracer
  $ echo c > /proc/sysrq-trigger
  ...
  Call trace:
  [<ffffffc00044d3ac>] sysrq_handle_crash+0x24/0x30
  [<ffffffc000092250>] return_to_handler+0x0/0x40
  [<ffffffc000092250>] return_to_handler+0x0/0x40
  ...

This patch replaces such entries with real addresses preserved in
current->ret_stack[] at unwind_frame(). This way, we can cover all
the cases.

Reviewed-by: Jungseok Lee <jungseoklee85@gmail.com>
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
[will: fixed minor context changes conflicting with irq stack bits]
Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 20 insertions, 6 deletions

diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index 937008523fa5..bdc293f6adc4 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -147,17 +147,14 @@ static void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
 {
         struct stackframe frame;
         unsigned long irq_stack_ptr = IRQ_STACK_PTR(smp_processor_id());
+        int skip;
 
         pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk);
 
         if (!tsk)
                 tsk = current;
 
-        if (regs) {
-                frame.fp = regs->regs[29];
-                frame.sp = regs->sp;
-                frame.pc = regs->pc;
-        } else if (tsk == current) {
+        if (tsk == current) {
                 frame.fp = (unsigned long)__builtin_frame_address(0);
                 frame.sp = current_stack_pointer;
                 frame.pc = (unsigned long)dump_backtrace;
@@ -169,14 +166,31 @@ static void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
                 frame.sp = thread_saved_sp(tsk);
                 frame.pc = thread_saved_pc(tsk);
         }
+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
+        frame.graph = tsk->curr_ret_stack;
+#endif
 
+        skip = !!regs;
         pr_emerg("Call trace:\n");
         while (1) {
                 unsigned long where = frame.pc;
                 unsigned long stack;
                 int ret;
 
-                dump_backtrace_entry(where);
+                /* skip until specified stack frame */
+                if (!skip) {
+                        dump_backtrace_entry(where);
+                } else if (frame.fp == regs->regs[29]) {
+                        skip = 0;
+                        /*
+                         * Mostly, this is the case where this function is
+                         * called in panic/abort. As exception handler's
+                         * stack frame does not contain the corresponding pc
+                         * at which an exception has taken place, use regs->pc
+                         * instead.
+                         */
+                        dump_backtrace_entry(regs->pc);
+                }
                 ret = unwind_frame(tsk, &frame);
                 if (ret < 0)
                         break;