[RFC PATCH v2 8/8] arm64: Implement arch_stack_walk_reliable()

[madvenka@linux.microsoft.com]

From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>

unwind_frame() already sets the reliable flag in the stack frame during
a stack walk to indicate whether the stack trace is reliable or not.

Implement arch_stack_walk_reliable() like arch_stack_walk() but abort
the stack walk as soon as the reliable flag is set to false for any
reason.

Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
---
 arch/arm64/Kconfig             |  1 +
 arch/arm64/kernel/stacktrace.c | 35 ++++++++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 1f212b47a48a..954f60c35b26 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -167,6 +167,7 @@ config ARM64
 		if $(cc-option,-fpatchable-function-entry=2)
 	select FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY \
 		if DYNAMIC_FTRACE_WITH_REGS
+	select HAVE_RELIABLE_STACKTRACE
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS
 	select HAVE_FAST_GUP
 	select HAVE_FTRACE_MCOUNT_RECORD
diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index 752b77f11c61..5d15c111f3aa 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -361,4 +361,39 @@ void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
 	walk_stackframe(task, &frame, consume_entry, cookie);
 }
 
+/*
+ * Walk the stack like arch_stack_walk() but stop the walk as soon as
+ * some unreliability is detected in the stack.
+ */
+int arch_stack_walk_reliable(stack_trace_consume_fn consume_entry,
+			      void *cookie, struct task_struct *task)
+{
+	struct stackframe frame;
+	int ret = 0;
+
+	if (task == current) {
+		start_backtrace(&frame,
+				(unsigned long)__builtin_frame_address(0),
+				(unsigned long)arch_stack_walk_reliable);
+	} else {
+		/*
+		 * The task must not be running anywhere for the duration of
+		 * arch_stack_walk_reliable(). The caller must guarantee
+		 * this.
+		 */
+		start_backtrace(&frame, thread_saved_fp(task),
+				thread_saved_pc(task));
+	}
+
+	while (!ret) {
+		if (!frame.reliable)
+			return -EINVAL;
+		if (!consume_entry(cookie, frame.pc))
+			return -EINVAL;
+		ret = unwind_frame(task, &frame);
+	}
+
+	return ret == -ENOENT ? 0 : -EINVAL;
+}
+
 #endif
-- 
2.25.1