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SubjectRe: [RFC PATCH v1 1/1] arm64: Unwinder enhancements for reliable stack trace
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On 2/23/21 1:02 PM, Mark Brown wrote:
> On Tue, Feb 23, 2021 at 12:12:43PM -0600, madvenka@linux.microsoft.com wrote:
>> From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
>>
>> Unwinder changes
>> ================
>
> This is making several different changes so should be split into a patch
> series - for example the change to terminate on a specific function
> pointer rather than NULL and the changes to the exception/interupt
> detection should be split. Please see submitting-patches.rst for some
> discussion about how to split things up. In general if you've got a
> changelog enumerating a number of different changes in a patch that's a
> warning sign that it might be good split things up.
>

Will do.

> You should also copy the architecture maintainers (Catalin and Will) on
> any arch/arm64 submissions.
>

Will do when I resubmit.

>> Unwinder return value
>> =====================
>>
>> Currently, the unwinder returns -EINVAL for stack trace termination
>> as well as stack trace error. Return -ENOENT for stack trace
>> termination and -EINVAL for error to disambiguate. This idea has
>> been borrowed from Mark Brown.
>
> You could just include my patch for this in your series.
>

OK.

>> Reliable stack trace function
>> =============================
>>
>> Implement arch_stack_walk_reliable(). This function walks the stack like
>> the existing stack trace functions with a couple of additional checks:
>>
>> Return address check
>> --------------------
>>
>> For each frame, check the return address to see if it is a
>> proper kernel text address. If not, return -EINVAL.
>>
>> Exception frame check
>> ---------------------
>>
>> Check each frame to see if it is an EL1 exception frame. If it is,
>> return -EINVAL.
>
> Again, this should be at least one separate patch. How does this ensure
> that we don't have any issues with any of the various probe mechanisms?
> If there's no need to explicitly check anything that should be called
> out in the changelog.
>

I am trying to do this in an incremental fashion. I have to study the probe
mechanisms a little bit more before I can come up with a solution. But
if you want to see that addressed in this patch set, I could do that.
It will take a little bit of time. That is all.

> Since all these changes are mixed up this is a fairly superficial
> review of the actual code.
>

Understood. I will split things up and we can take it from there.

>> +static notrace struct pt_regs *get_frame_regs(struct task_struct *task,
>> + struct stackframe *frame)
>> +{
>> + unsigned long stackframe, regs_start, regs_end;
>> + struct stack_info info;
>> +
>> + stackframe = frame->prev_fp;
>> + if (!stackframe)
>> + return NULL;
>> +
>> + (void) on_accessible_stack(task, stackframe, &info);
>
> Shouldn't we return NULL if we are not on an accessible stack?
>

The prev_fp has already been checked by the unwinder in the previous
frame. That is why I don't check the return value. If that is acceptable,
I will add a comment.

>> +static notrace int update_frame(struct task_struct *task,
>> + struct stackframe *frame)
>
> This function really needs some documentation, the function is just
> called update_frame() which doesn't say what sort of updates it's
> supposed to do and most of the checks aren't explained, not all of them
> are super obvious.
>

I will add the documentation as well as try think of a better name.

>> +{
>> + unsigned long lsb = frame->fp & 0xf;
>> + unsigned long fp = frame->fp & ~lsb;
>> + unsigned long pc = frame->pc;
>> + struct pt_regs *regs;
>> +
>> + frame->exception_frame = false;
>> +
>> + if (fp == (unsigned long) arm64_last_frame &&
>> + pc == (unsigned long) arm64_last_func)
>> + return -ENOENT;
>> +
>> + if (!lsb)
>> + return 0;
>> + if (lsb != 1)
>> + return -EINVAL;
>> +
>> + /*
>> + * This looks like an EL1 exception frame.
>
> For clarity it would be good to spell out the properties of an EL1
> exception frame. It is not clear to me why we don't reference the frame
> type information the unwinder already records as part of these checks.
>
> In general, especially for the bits specific to reliable stack trace, I
> think we want to err on the side of verbosity here so that it is crystal
> clear what all the checks are supposed to be doing and it's that much
> easier to tie everything through to the requirements document.

OK. I will improve the documentation.

Madhavan

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