Re: [RFC PATCH v8 1/4] arm64: Make all stack walking functions use arch_stack_walk()

Thanks for the review. Responses inline...

On 8/24/21 8:13 AM, Mark Rutland wrote:

On Thu, Aug 12, 2021 at 02:06:00PM -0500, madvenka@linux.microsoft.com wrote:

quoted

From: "Madhavan T. Venkataraman" <redacted>

Currently, there are multiple functions in ARM64 code that walk the
stack using start_backtrace() and unwind_frame(). Convert all of
them to use arch_stack_walk(). This makes maintenance easier.

It would be good to split this into a series of patches as Mark Brown
suggested in v7.

Will do.

quoted

Here is the list of functions:

	perf_callchain_kernel()
	get_wchan()
	return_address()
	dump_backtrace()
	profile_pc()

Note that arch_stack_walk() depends on CONFIG_STACKTRACE (which is not in
defconfig), so we'll need to reorganise things such that it's always defined,
or factor out the core of that function and add a wrapper such that we
can always use it.

I will include CONFIG_STACKTRACE in defconfig, if that is OK with you and
Mark Brown.

quoted

Signed-off-by: Madhavan T. Venkataraman <redacted>
---
 arch/arm64/include/asm/stacktrace.h |  3 ---
 arch/arm64/kernel/perf_callchain.c  |  5 +---
 arch/arm64/kernel/process.c         | 39 ++++++++++++++++++-----------
 arch/arm64/kernel/return_address.c  |  6 +----
 arch/arm64/kernel/stacktrace.c      | 38 +++-------------------------
 arch/arm64/kernel/time.c            | 22 +++++++++-------
 6 files changed, 43 insertions(+), 70 deletions(-)

diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
index 8aebc00c1718..e43dea1c6b41 100644
--- a/arch/arm64/include/asm/stacktrace.h
+++ b/arch/arm64/include/asm/stacktrace.h

@@ -61,9 +61,6 @@ struct stackframe {
 #endif
 };
 
-extern int unwind_frame(struct task_struct *tsk, struct stackframe *frame);
-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);
 

diff --git a/arch/arm64/kernel/perf_callchain.c b/arch/arm64/kernel/perf_callchain.c
index 4a72c2727309..2f289013c9c9 100644
--- a/arch/arm64/kernel/perf_callchain.c
+++ b/arch/arm64/kernel/perf_callchain.c

@@ -147,15 +147,12 @@ static bool callchain_trace(void *data, unsigned long pc)
 void perf_callchain_kernel(struct perf_callchain_entry_ctx *entry,
 			   struct pt_regs *regs)
 {
-	struct stackframe frame;
-
 	if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
 		/* We don't support guest os callchain now */
 		return;
 	}
 
-	start_backtrace(&frame, regs->regs[29], regs->pc);
-	walk_stackframe(current, &frame, callchain_trace, entry);
+	arch_stack_walk(callchain_trace, entry, current, regs);
 }

We can also update callchain_trace take the return value of
perf_callchain_store into acount, e.g.

| static bool callchain_trace(void *data, unsigned long pc) 
| {
| 	struct perf_callchain_entry_ctx *entry = data;
| 	return perf_callchain_store(entry, pc) == 0;
| }

OK.

quoted

 
 unsigned long perf_instruction_pointer(struct pt_regs *regs)

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index c8989b999250..52c12fd26407 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c

@@ -544,11 +544,28 @@ __notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev,
 	return last;
 }
 
+struct wchan_info {
+	unsigned long	pc;
+	int		count;
+};
+
+static bool get_wchan_cb(void *arg, unsigned long pc)
+{
+	struct wchan_info *wchan_info = arg;
+
+	if (!in_sched_functions(pc)) {
+		wchan_info->pc = pc;
+		return false;
+	}
+	wchan_info->count--;
+	return !!wchan_info->count;
+}

This will terminate one entry earlier than the old logic since we used
to use a post-increment (testing the prior value), and now we're
effectively using a pre-decrement (testing the new value).

I don't think that matters all that much in practice, but it might be
best to keep the original logic, e.g. initialize `count` to 0 and here
do:

	return wchan_info->count++ < 16;

The reason I did it this way is that with the old logic the actual limit
implemented is 17 instead of 16. That seemed odd. But I could do it the
way you have suggested.

quoted

+
 unsigned long get_wchan(struct task_struct *p)
 {
-	struct stackframe frame;
-	unsigned long stack_page, ret = 0;
-	int count = 0;
+	unsigned long stack_page;
+	struct wchan_info wchan_info;
+
 	if (!p || p == current || task_is_running(p))
 		return 0;
 

@@ -556,20 +573,12 @@ unsigned long get_wchan(struct task_struct *p)
 	if (!stack_page)
 		return 0;
 
-	start_backtrace(&frame, thread_saved_fp(p), thread_saved_pc(p));
+	wchan_info.pc = 0;
+	wchan_info.count = 16;
+	arch_stack_walk(get_wchan_cb, &wchan_info, p, NULL);
 
-	do {
-		if (unwind_frame(p, &frame))
-			goto out;
-		if (!in_sched_functions(frame.pc)) {
-			ret = frame.pc;
-			goto out;
-		}
-	} while (count++ < 16);
-
-out:
 	put_task_stack(p);
-	return ret;
+	return wchan_info.pc;
 }

Other than the comment above, this looks good to me.

OK.

quoted

 unsigned long arch_align_stack(unsigned long sp)

diff --git a/arch/arm64/kernel/return_address.c b/arch/arm64/kernel/return_address.c
index a6d18755652f..92a0f4d434e4 100644
--- a/arch/arm64/kernel/return_address.c
+++ b/arch/arm64/kernel/return_address.c

@@ -35,15 +35,11 @@ NOKPROBE_SYMBOL(save_return_addr);
 void *return_address(unsigned int level)
 {
 	struct return_address_data data;
-	struct stackframe frame;
 
 	data.level = level + 2;
 	data.addr = NULL;
 
-	start_backtrace(&frame,
-			(unsigned long)__builtin_frame_address(0),
-			(unsigned long)return_address);
-	walk_stackframe(current, &frame, save_return_addr, &data);
+	arch_stack_walk(save_return_addr, &data, current, NULL);
 
 	if (!data.level)
 		return data.addr;

Nor that arch_stack_walk() will start with it's caller, so
return_address() will be included in the trace where it wasn't
previously, which implies we need to skip an additional level.

You are correct. I will fix this. Thanks for catching this.

That said, I'm not entirely sure why we need to skip 2 levels today; it
might be worth checking that's correct.

AFAICT, return_address() acts like builtin_return_address(). That is, it
returns the address of the caller. If func() calls return_address(),
func() wants its caller's address. So, return_address() and func() need to
be skipped.

I will change it to skip 3 levels instead of 2.

We should also mark return_address() as noinline to avoid surprises with
LTO.

Will do.

quoted

diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index 8982a2b78acf..1800310f92be 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c

@@ -151,23 +151,21 @@ void notrace walk_stackframe(struct task_struct *tsk, struct stackframe *frame,
 }
 NOKPROBE_SYMBOL(walk_stackframe);
 
-static void dump_backtrace_entry(unsigned long where, const char *loglvl)
+static bool dump_backtrace_entry(void *arg, unsigned long where)
 {
+	char *loglvl = arg;
 	printk("%s %pSb\n", loglvl, (void *)where);
+	return true;
 }
 
 void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
 		    const char *loglvl)
 {
-	struct stackframe frame;
-	int skip = 0;
-
 	pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk);
 
 	if (regs) {
 		if (user_mode(regs))
 			return;
-		skip = 1;
 	}

We can simplifiy this to:

	if (regs && user_mode(regs))
		return;

OK.

quoted

 
 	if (!tsk)

@@ -176,36 +174,8 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
 	if (!try_get_task_stack(tsk))
 		return;
 
-	if (tsk == current) {
-		start_backtrace(&frame,
-				(unsigned long)__builtin_frame_address(0),
-				(unsigned long)dump_backtrace);
-	} else {
-		/*
-		 * task blocked in __switch_to
-		 */
-		start_backtrace(&frame,
-				thread_saved_fp(tsk),
-				thread_saved_pc(tsk));
-	}
-
 	printk("%sCall trace:\n", loglvl);
-	do {
-		/* skip until specified stack frame */
-		if (!skip) {
-			dump_backtrace_entry(frame.pc, loglvl);
-		} 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, loglvl);
-		}
-	} while (!unwind_frame(tsk, &frame));
+	arch_stack_walk(dump_backtrace_entry, (void *)loglvl, tsk, regs);

It turns out we currently need this skipping to get the balance the
ftrace call stack, and arch_stack_walk() doesn't currently do the right
thing when starting from regs. That balancing isn't quite right, and
will be wrong in some case when unwinding across exception boundaries;
we could implement HAVE_FUNCTION_GRAPH_RET_ADDR_PTR using the FP to
solve that.

I am not sure that I completely understand. So, I will study this and get back
to you with any questions.

quoted

 
 	put_task_stack(tsk);
 }

diff --git a/arch/arm64/kernel/time.c b/arch/arm64/kernel/time.c
index eebbc8d7123e..671b3038a772 100644
--- a/arch/arm64/kernel/time.c
+++ b/arch/arm64/kernel/time.c

@@ -32,22 +32,26 @@
 #include <asm/stacktrace.h>
 #include <asm/paravirt.h>
 
+static bool profile_pc_cb(void *arg, unsigned long pc)
+{
+	unsigned long *prof_pc = arg;
+
+	if (in_lock_functions(pc))
+		return true;
+	*prof_pc = pc;
+	return false;
+}
+
 unsigned long profile_pc(struct pt_regs *regs)
 {
-	struct stackframe frame;
+	unsigned long prof_pc = 0;
 
 	if (!in_lock_functions(regs->pc))
 		return regs->pc;
 
-	start_backtrace(&frame, regs->regs[29], regs->pc);
-
-	do {
-		int ret = unwind_frame(NULL, &frame);
-		if (ret < 0)
-			return 0;
-	} while (in_lock_functions(frame.pc));
+	arch_stack_walk(profile_pc_cb, &prof_pc, current, regs);
 
-	return frame.pc;
+	return prof_pc;
 }
 EXPORT_SYMBOL(profile_pc);

Mdoulo the problem above w.r.t. unwinding from regs, this looks good.

Great. Thanks.

Madhavan