[RFC PATCH v6 2/3] arm64: Introduce stack trace reliability checks in the unwinder

[madvenka@linux.microsoft.com]

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

The unwinder should check for the presence of various features and
conditions that can render the stack trace unreliable. Introduce a
function unwind_check_frame() for this purpose.

Introduce the first reliability check in unwind_check_frame() - If
a return PC is not a valid kernel text address, consider the stack
trace unreliable. It could be some generated code.

Other reliability checks will be added in the future.

If a reliability check fails, it is a non-fatal error. Introduce a new
return code, UNWIND_CONTINUE_WITH_RISK, for non-fatal errors.

Call unwind_check_frame() from unwind_frame(). Also, call it from
start_backtrace() to remove the current assumption that the starting
frame is reliable.

Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
---
 arch/arm64/include/asm/stacktrace.h |  4 +++-
 arch/arm64/kernel/stacktrace.c      | 17 ++++++++++++++++-
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
index 6fcd58553fb1..d1625d55b980 100644
--- a/arch/arm64/include/asm/stacktrace.h
+++ b/arch/arm64/include/asm/stacktrace.h
@@ -32,6 +32,7 @@ struct stack_info {
 
 enum unwind_rc {
 	UNWIND_CONTINUE,		/* No errors encountered */
+	UNWIND_CONTINUE_WITH_RISK,	/* Non-fatal errors encountered */
 	UNWIND_ABORT,			/* Fatal errors encountered */
 	UNWIND_FINISH,			/* End of stack reached successfully */
 };
@@ -73,6 +74,7 @@ extern void walk_stackframe(struct task_struct *tsk, struct stackframe *frame,
 			    bool (*fn)(void *, unsigned long), void *data);
 extern void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
 			   const char *loglvl);
+extern enum unwind_rc unwind_check_frame(struct stackframe *frame);
 
 DECLARE_PER_CPU(unsigned long *, irq_stack_ptr);
 
@@ -176,7 +178,7 @@ static inline enum unwind_rc start_backtrace(struct stackframe *frame,
 	bitmap_zero(frame->stacks_done, __NR_STACK_TYPES);
 	frame->prev_fp = 0;
 	frame->prev_type = STACK_TYPE_UNKNOWN;
-	return UNWIND_CONTINUE;
+	return unwind_check_frame(frame);
 }
 
 #endif	/* __ASM_STACKTRACE_H */
diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index e9c2c1fa9dde..ba7b97b119e4 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -18,6 +18,21 @@
 #include <asm/stack_pointer.h>
 #include <asm/stacktrace.h>
 
+/*
+ * Check the stack frame for conditions that make unwinding unreliable.
+ */
+enum unwind_rc unwind_check_frame(struct stackframe *frame)
+{
+	/*
+	 * If the PC is not a known kernel text address, then we cannot
+	 * be sure that a subsequent unwind will be reliable, as we
+	 * don't know that the code follows our unwind requirements.
+	 */
+	if (!__kernel_text_address(frame->pc))
+		return UNWIND_CONTINUE_WITH_RISK;
+	return UNWIND_CONTINUE;
+}
+
 /*
  * AArch64 PCS assigns the frame pointer to x29.
  *
@@ -109,7 +124,7 @@ enum unwind_rc notrace unwind_frame(struct task_struct *tsk,
 
 	frame->pc = ptrauth_strip_insn_pac(frame->pc);
 
-	return UNWIND_CONTINUE;
+	return unwind_check_frame(frame);
 }
 NOKPROBE_SYMBOL(unwind_frame);
 
-- 
2.25.1