[RFC PATCH v2 1/1] arm64: Implement stack trace termination record

[madvenka@linux.microsoft.com]

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

Reliable stacktracing requires that we identify when a stacktrace is
terminated early. We can do this by ensuring all tasks have a final
frame record at a known location on their task stack, and checking
that this is the final frame record in the chain.

Kernel Tasks
============

All tasks except the idle task have a pt_regs structure right after the
task stack. This is called the task pt_regs. The pt_regs structure has a
special stackframe field. Make this stackframe field the final frame in the
task stack. This needs to be done in copy_thread() which initializes a new
task's pt_regs and initial CPU context.

For the idle task, there is no task pt_regs. For our purpose, we need one.
So, create a pt_regs just like other kernel tasks and make
pt_regs->stackframe the final frame in the idle task stack. This needs to be
done at two places:

	- On the primary CPU, the boot task runs. It calls start_kernel()
	  and eventually becomes the idle task for the primary CPU. Just
	  before start_kernel() is called, set up the final frame.

	- On each secondary CPU, a startup task runs that calls
	  secondary_startup_kernel() and eventually becomes the idle task
	  on the secondary CPU. Just before secondary_start_kernel() is
	  called, set up the final frame.

User Tasks
==========

User tasks are initially set up like kernel tasks when they are created.
Then, they return to userland after fork via ret_from_fork(). After that,
they enter the kernel only on an EL0 exception. (In arm64, system calls are
also EL0 exceptions). The EL0 exception handler stores state in the task
pt_regs and calls different functions based on the type of exception. The
stack trace for an EL0 exception must end at the task pt_regs. So, make
task pt_regs->stackframe as the final frame in the EL0 exception stack.

In summary, task pt_regs->stackframe is where a successful stack trace ends.

Stack trace termination
=======================

In the unwinder, terminate the stack trace successfully when
task_pt_regs(task)->stackframe is reached. For stack traces in the kernel,
this will correctly terminate the stack trace at the right place.

However, debuggers terminate the stack trace when FP == 0. In the
pt_regs->stackframe, the PC is 0 as well. So, stack traces taken in the
debugger may print an extra record 0x0 at the end. While this is not
pretty, this does not do any harm. This is a small price to pay for
having reliable stack trace termination in the kernel.

Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
---
 arch/arm64/kernel/entry.S      |  8 +++++---
 arch/arm64/kernel/head.S       | 29 +++++++++++++++++++++++------
 arch/arm64/kernel/process.c    |  5 +++++
 arch/arm64/kernel/stacktrace.c | 10 +++++-----
 4 files changed, 38 insertions(+), 14 deletions(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index a31a0a713c85..e2dc2e998934 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -261,16 +261,18 @@ alternative_else_nop_endif
 	stp	lr, x21, [sp, #S_LR]
 
 	/*
-	 * For exceptions from EL0, terminate the callchain here.
+	 * For exceptions from EL0, terminate the callchain here at
+	 * task_pt_regs(current)->stackframe.
+	 *
 	 * For exceptions from EL1, create a synthetic frame record so the
 	 * interrupted code shows up in the backtrace.
 	 */
 	.if \el == 0
-	mov	x29, xzr
+	stp	xzr, xzr, [sp, #S_STACKFRAME]
 	.else
 	stp	x29, x22, [sp, #S_STACKFRAME]
-	add	x29, sp, #S_STACKFRAME
 	.endif
+	add	x29, sp, #S_STACKFRAME
 
 #ifdef CONFIG_ARM64_SW_TTBR0_PAN
 alternative_if_not ARM64_HAS_PAN
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 840bda1869e9..743c019a42c7 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -393,6 +393,23 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
 	ret	x28
 SYM_FUNC_END(__create_page_tables)
 
+	/*
+	 * The boot task becomes the idle task for the primary CPU. The
+	 * CPU startup task on each secondary CPU becomes the idle task
+	 * for the secondary CPU.
+	 *
+	 * The idle task does not require pt_regs. But create a dummy
+	 * pt_regs so that task_pt_regs(idle_task)->stackframe can be
+	 * set up to be the final frame on the idle task stack just like
+	 * all the other kernel tasks. This helps the unwinder to
+	 * terminate the stack trace at a well-known stack offset.
+	 */
+	.macro setup_final_frame
+	sub	sp, sp, #PT_REGS_SIZE
+	stp	xzr, xzr, [sp, #S_STACKFRAME]
+	add	x29, sp, #S_STACKFRAME
+	.endm
+
 /*
  * The following fragment of code is executed with the MMU enabled.
  *
@@ -447,9 +464,9 @@ SYM_FUNC_START_LOCAL(__primary_switched)
 #endif
 	bl	switch_to_vhe			// Prefer VHE if possible
 	add	sp, sp, #16
-	mov	x29, #0
-	mov	x30, #0
-	b	start_kernel
+	setup_final_frame
+	bl	start_kernel
+	nop
 SYM_FUNC_END(__primary_switched)
 
 	.pushsection ".rodata", "a"
@@ -606,14 +623,14 @@ SYM_FUNC_START_LOCAL(__secondary_switched)
 	cbz	x2, __secondary_too_slow
 	msr	sp_el0, x2
 	scs_load x2, x3
-	mov	x29, #0
-	mov	x30, #0
+	setup_final_frame
 
 #ifdef CONFIG_ARM64_PTR_AUTH
 	ptrauth_keys_init_cpu x2, x3, x4, x5
 #endif
 
-	b	secondary_start_kernel
+	bl	secondary_start_kernel
+	nop
 SYM_FUNC_END(__secondary_switched)
 
 SYM_FUNC_START_LOCAL(__secondary_too_slow)
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 325c83b1a24d..906baa232a89 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -437,6 +437,11 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
 	}
 	p->thread.cpu_context.pc = (unsigned long)ret_from_fork;
 	p->thread.cpu_context.sp = (unsigned long)childregs;
+	/*
+	 * For the benefit of the unwinder, set up childregs->stackframe
+	 * as the final frame for the new task.
+	 */
+	p->thread.cpu_context.fp = (unsigned long)childregs->stackframe;
 
 	ptrace_hw_copy_thread(p);
 
diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index ad20981dfda4..72f5af8c69dc 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -44,16 +44,16 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
 	unsigned long fp = frame->fp;
 	struct stack_info info;
 
-	/* Terminal record; nothing to unwind */
-	if (!fp)
+	if (!tsk)
+		tsk = current;
+
+	/* Final frame; nothing to unwind */
+	if (fp == (unsigned long) task_pt_regs(tsk)->stackframe)
 		return -ENOENT;
 
 	if (fp & 0xf)
 		return -EINVAL;
 
-	if (!tsk)
-		tsk = current;
-
 	if (!on_accessible_stack(tsk, fp, &info))
 		return -EINVAL;
 
-- 
2.25.1