[PATCH v3 4/4] arm64: reliable stacktraces

[Torsten Duwe]

Make unwind_frame() report whether it had to stop normally or due to
an error condition; walk_stackframe() will pass that info.
__save_stack_trace() is used to check the validity of a frame;
save_stack_trace_tsk_reliable() can now trivially be implemented.
Modify arch/arm64/kernel/time.c for the new semantics.

Signed-off-by: Torsten Duwe <d...@suse.de>

--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -127,8 +127,9 @@ config ARM64
        select HAVE_PERF_EVENTS
        select HAVE_PERF_REGS
        select HAVE_PERF_USER_STACK_DUMP
-       select HAVE_REGS_AND_STACK_ACCESS_API
        select HAVE_RCU_TABLE_FREE
+       select HAVE_REGS_AND_STACK_ACCESS_API
+       select HAVE_RELIABLE_STACKTRACE
        select HAVE_STACKPROTECTOR
        select HAVE_SYSCALL_TRACEPOINTS
        select HAVE_KPROBES
--- a/arch/arm64/include/asm/stacktrace.h
+++ b/arch/arm64/include/asm/stacktrace.h
@@ -33,7 +33,7 @@ struct stackframe {
 };
 
 extern int unwind_frame(struct task_struct *tsk, struct stackframe *frame);
-extern void walk_stackframe(struct task_struct *tsk, struct stackframe *frame,
+extern int walk_stackframe(struct task_struct *tsk, struct stackframe *frame,
                            int (*fn)(struct stackframe *, void *), void *data);
 extern void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk);
 
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -40,6 +40,13 @@
  *     ldp     x29, x30, [sp]
  *     add     sp, sp, #0x10
  */
+
+/*
+ * unwind_frame -- unwind a single stack frame.
+ * Returns 0 when there are more frames to go.
+ * 1 means reached end of stack; negative (error)
+ * means stopped because information is not reliable.
+ */
 int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
 {
        unsigned long fp = frame->fp;
@@ -81,23 +88,27 @@ int notrace unwind_frame(struct task_str
         * both are NULL.
         */
        if (!frame->fp && !frame->pc)
-               return -EINVAL;
+               return 1;
 
        return 0;
 }
 
-void notrace walk_stackframe(struct task_struct *tsk, struct stackframe *frame,
+int notrace walk_stackframe(struct task_struct *tsk, struct stackframe *frame,
                     int (*fn)(struct stackframe *, void *), void *data)
 {
        while (1) {
                int ret;
 
-               if (fn(frame, data))
-                       break;
+               ret = fn(frame, data);
+               if (ret)
+                       return ret;
                ret = unwind_frame(tsk, frame);
                if (ret < 0)
+                       return ret;
+               if (ret > 0)
                        break;
        }
+       return 0;
 }
 
 #ifdef CONFIG_STACKTRACE
@@ -145,14 +156,15 @@ void save_stack_trace_regs(struct pt_reg
                trace->entries[trace->nr_entries++] = ULONG_MAX;
 }
 
-static noinline void __save_stack_trace(struct task_struct *tsk,
+static noinline int __save_stack_trace(struct task_struct *tsk,
        struct stack_trace *trace, unsigned int nosched)
 {
        struct stack_trace_data data;
        struct stackframe frame;
+       int ret;
 
        if (!try_get_task_stack(tsk))
-               return;
+               return -EBUSY;
 
        data.trace = trace;
        data.skip = trace->skip;
@@ -171,11 +183,12 @@ static noinline void __save_stack_trace(
        frame.graph = tsk->curr_ret_stack;
 #endif
 
-       walk_stackframe(tsk, &frame, save_trace, &data);
+       ret = walk_stackframe(tsk, &frame, save_trace, &data);
        if (trace->nr_entries < trace->max_entries)
                trace->entries[trace->nr_entries++] = ULONG_MAX;
 
        put_task_stack(tsk);
+       return ret;
 }
 EXPORT_SYMBOL_GPL(save_stack_trace_tsk);
 
@@ -190,4 +203,12 @@ void save_stack_trace(struct stack_trace
 }
 
 EXPORT_SYMBOL_GPL(save_stack_trace);
+
+int save_stack_trace_tsk_reliable(struct task_struct *tsk,
+                                 struct stack_trace *trace)
+{
+       return __save_stack_trace(tsk, trace, 1);
+}
+EXPORT_SYMBOL_GPL(save_stack_trace_tsk_reliable);
+
 #endif
--- a/arch/arm64/kernel/time.c
+++ b/arch/arm64/kernel/time.c
@@ -56,7 +56,7 @@ unsigned long profile_pc(struct pt_regs
 #endif
        do {
                int ret = unwind_frame(NULL, &frame);
-               if (ret < 0)
+               if (ret)
                        return 0;
        } while (in_lock_functions(frame.pc));