Re: [PATCH v11 1/5] arm64: Call stack_backtrace() only from within walk_stackframe()
From: Madhavan T. Venkataraman
Date: Tue Nov 30 2021 - 15:30:14 EST
On 11/30/21 12:29 PM, Mark Rutland wrote:
> On Tue, Nov 30, 2021 at 11:13:28AM -0600, Madhavan T. Venkataraman wrote:
>> On 11/30/21 9:05 AM, Mark Rutland wrote:
>>> On Tue, Nov 23, 2021 at 01:37:19PM -0600, madvenka@xxxxxxxxxxxxxxxxxxx wrote:
>>>> From: "Madhavan T. Venkataraman" <madvenka@xxxxxxxxxxxxxxxxxxx>
>>>>
>>>> Currently, arch_stack_walk() calls start_backtrace() and walk_stackframe()
>>>> separately. There is no need to do that. Instead, call start_backtrace()
>>>> from within walk_stackframe(). In other words, walk_stackframe() is the only
>>>> unwind function a consumer needs to call.
>
>>>> @@ -143,15 +140,19 @@ static int notrace unwind_frame(struct task_struct *tsk,
>>>> NOKPROBE_SYMBOL(unwind_frame);
>>>>
>>>> static void notrace walk_stackframe(struct task_struct *tsk,
>>>> - struct stackframe *frame,
>>>> + unsigned long fp, unsigned long pc,
>>>> bool (*fn)(void *, unsigned long), void *data)
>>>> {
>>>> + struct stackframe frame;
>>>> +
>>>> + start_backtrace(&frame, fp, pc);
>>>> +
>>>> while (1) {
>>>> int ret;
>>>>
>>>> - if (!fn(data, frame->pc))
>>>> + if (!fn(data, frame.pc))
>>>> break;
>>>> - ret = unwind_frame(tsk, frame);
>>>> + ret = unwind_frame(tsk, &frame);
>>>> if (ret < 0)
>>>> break;
>>>> }
>>>> @@ -195,17 +196,19 @@ noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
>>>> void *cookie, struct task_struct *task,
>>>> struct pt_regs *regs)
>>>> {
>>>> - struct stackframe frame;
>>>> -
>>>> - if (regs)
>>>> - start_backtrace(&frame, regs->regs[29], regs->pc);
>>>> - else if (task == current)
>>>> - start_backtrace(&frame,
>>>> - (unsigned long)__builtin_frame_address(1),
>>>> - (unsigned long)__builtin_return_address(0));
>>>> - else
>>>> - start_backtrace(&frame, thread_saved_fp(task),
>>>> - thread_saved_pc(task));
>>>> -
>>>> - walk_stackframe(task, &frame, consume_entry, cookie);
>>>> + unsigned long fp, pc;
>>>> +
>>>> + if (regs) {
>>>> + fp = regs->regs[29];
>>>> + pc = regs->pc;
>>>> + } else if (task == current) {
>>>> + /* Skip arch_stack_walk() in the stack trace. */
>>>> + fp = (unsigned long)__builtin_frame_address(1);
>>>> + pc = (unsigned long)__builtin_return_address(0);
>>>> + } else {
>>>> + /* Caller guarantees that the task is not running. */
>>>> + fp = thread_saved_fp(task);
>>>> + pc = thread_saved_pc(task);
>>>> + }
>>>> + walk_stackframe(task, fp, pc, consume_entry, cookie);
>>>
>>> I'd prefer to leave this as-is. The new and old structure are largely
>>> equivalent, so we haven't made this any simpler, but we have added more
>>> arguments to walk_stackframe().
>>>
>>
>> This is just to simplify things when we eventually add arch_stack_walk_reliable().
>> That is all. All of the unwinding is done by a single unwinding function and
>> there are two consumers of that unwinding function - arch_stack_walk() and
>> arch_stack_walk_reliable().
>
> I understand the theory, but I don't think that moving the start_backtrace()
> call actually simplifies this in a meaningful way, and I think it'll make it
> harder for us to make more meaningful simplifications later on.
>
> As of patch 4 of this series, we'll have:
>
> | noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
> | void *cookie, struct task_struct *task,
> | struct pt_regs *regs)
> | {
> | unsigned long fp, pc;
> |
> | if (regs) {
> | fp = regs->regs[29];
> | pc = regs->pc;
> | } else if (task == current) {
> | /* Skip arch_stack_walk() in the stack trace. */
> | fp = (unsigned long)__builtin_frame_address(1);
> | pc = (unsigned long)__builtin_return_address(0);
> | } else {
> | /* Caller guarantees that the task is not running. */
> | fp = thread_saved_fp(task);
> | pc = thread_saved_pc(task);
> | }
> | walk_stackframe(task, fp, pc, consume_entry, cookie);
> | }
> |
> | noinline int notrace arch_stack_walk_reliable(stack_trace_consume_fn consume_fn,
> | void *cookie,
> | struct task_struct *task)
> | {
> | unsigned long fp, pc;
> |
> | if (task == current) {
> | /* Skip arch_stack_walk_reliable() in the stack trace. */
> | fp = (unsigned long)__builtin_frame_address(1);
> | pc = (unsigned long)__builtin_return_address(0);
> | } else {
> | /* Caller guarantees that the task is not running. */
> | fp = thread_saved_fp(task);
> | pc = thread_saved_pc(task);
> | }
> | if (unwind(task, fp, pc, consume_fn, cookie))
> | return 0;
> | return -EINVAL;
> | }
>
> Which I do not think is substantially simpler than the naive extrapolation from
> what we currently have, e.g.
>
> | noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
> | void *cookie, struct task_struct *task,
> | struct pt_regs *regs)
> | {
> | struct stackframe frame;
> |
> | if (regs) {
> | unwind_init(&frame, regs->regs[29], regs->pc)
> | } else if (task == current) {
> | unwind_init(&frame, __builtin_frame_address(1),
> | __builtin_return_address(0));
> | } else {
> | unwind_init(&frame, thread_saved_fp(task),
> | thread_saved_pc(task);
> | }
> | walk_stackframe(task, &frame, consume_entry, cookie);
> | }
> |
> | noinline int notrace arch_stack_walk_reliable(stack_trace_consume_fn consume_fn,
> | void *cookie,
> | struct task_struct *task)
> | {
> | struct stackframe frame;
> |
> | if (task == current) {
> | unwind_init(&frame, __builtin_frame_address(1),
> | __builtin_return_address(0));
> | } else {
> | unwind_init(&frame, thread_saved_fp(task),
> | thread_saved_pc(task);
> | }
> | if (unwind(task, &frame, consume_fn, cookie))
> | return 0;
> | return -EINVAL;
> | }
>
> Further, I think we can factor this in a different way to reduce the
> duplication:
>
> | /*
> | * TODO: document requirements here
> | */
> | static inline void unwind_init_from_current_regs(struct stackframe *frame,
> | struct pt_regs *regs)
> | {
> | unwind_init(frame, regs->regs[29], regs->pc);
> | }
> |
> | /*
> | * TODO: document requirements here
> | */
> | static inline void unwind_init_from_blocked_task(struct stackframe *frame,
> | struct task_struct *tsk)
> | {
> | unwind_init(&frame, thread_saved_fp(task),
> | thread_saved_pc(task));
> | }
> |
> | /*
> | * TODO: document requirements here
> | *
> | * Note: this is always inlined, and we expect our caller to be a noinline
> | * function, such that this starts from our caller's caller.
> | */
> | static __always_inline void unwind_init_from_caller(struct stackframe *frame)
> | {
> | unwind_init(frame, __builtin_frame_address(1),
> | __builtin_return_address(0));
> | }
> |
> | noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
> | void *cookie, struct task_struct *task,
> | struct pt_regs *regs)
> | {
> | struct stackframe frame;
> |
> | if (regs)
> | unwind_init_current_regs(&frame, regs);
> | else if (task == current)
> | unwind_init_from_caller(&frame);
> | else
> | unwind_init_blocked_task(&frame, task);
> |
> | unwind(task, &frame, consume_entry, cookie);
> | }
> |
> | noinline int notrace arch_stack_walk_reliable(stack_trace_consume_fn consume_fn,
> | void *cookie,
> | struct task_struct *task)
> | {
> | struct stackframe frame;
> |
> | if (task == current)
> | unwind_init_from_caller(&frame);
> | else
> | unwind_init_from_blocked_task(&frame, task);
> |
> | if (unwind(task, &frame, consume_fn, cookie))
> | return 0;
> | return -EINVAL;
> | }
>
> ... which minimizes the duplication and allows us to add specialized
> initialization for each case if necessary, which I believe we will need in
> future to make unwinding across exception boundaries (such as when starting
> with regs) more useful.
>
> Thanks,
> Mark.
>
OK. I don't mind doing it this way.
Thanks.
Madhavan