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Message-ID: <ed143443-a406-a83e-bf7c-18083b98e503@linux.microsoft.com>
Date: Thu, 9 Dec 2021 22:13:50 -0600
From: "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To: Mark Rutland <mark.rutland@....com>
Cc: broonie@...nel.org, jpoimboe@...hat.com, ardb@...nel.org,
nobuta.keiya@...itsu.com, sjitindarsingh@...il.com,
catalin.marinas@....com, will@...nel.org, jmorris@...ei.org,
linux-arm-kernel@...ts.infradead.org,
live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 1/5] arm64: Call stack_backtrace() only from within
walk_stackframe()
Hey Mark,
Do you have any comments on the rest of the series? I am working on the next version of the patchset.
If you have any other comments, I will wait.
Thanks.
Madhavan
On 11/30/21 2:29 PM, Madhavan T. Venkataraman wrote:
>
>
> 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@...ux.microsoft.com wrote:
>>>>> From: "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
>>>>>
>>>>> 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
>
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