Re: [PATCH v10 05/11] arm64: Make dump_stacktrace() use arch_stack_walk()

Madhavan T. Venkataraman
2021-10-27T16:09:17Z

On 10/26/21 7:05 AM, Mark Rutland wrote:
> On Mon, Oct 25, 2021 at 05:49:25PM +0100, Mark Rutland wrote:
>> From f3e66ca75aff3474355839f72d123276028204e1 Mon Sep 17 00:00:00 2001
>> From: Mark Rutland <mark.rutland@arm.com>
>> Date: Mon, 25 Oct 2021 13:23:11 +0100
>> Subject: [PATCH] arm64: ftrace: use HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
>>
>> When CONFIG_FUNCTION_GRAPH_TRACER is selected, and the function graph:
>> tracer is in use, unwind_frame() may erroneously asscociate a traced
>> function with an incorrect return address. This can happen when starting
>> an unwind from a pt_regs, or when unwinding across an exception
>> boundary.
>>
>> The underlying problem is that ftrace_graph_get_ret_stack() takes an
>> index offset from the most recent entry added to the fgraph return
>> stack. We start an unwind at offset 0, and increment the offset each
>> time we encounter `return_to_handler`, which indicates a rewritten
>> return address. This is broken in two cases:
>>
>> * Between creating a pt_regs and starting the unwind, function calls may
>> place entries on the stack, leaving an abitrary offset which we can
>> only determine by performing a full unwind from the caller of the
>> unwind code. While this initial unwind is open-coded in
>> dump_backtrace(), this is not performed for other unwinders such as
>> perf_callchain_kernel().
>>
>> * When unwinding across an exception boundary (whether continuing an
>> unwind or starting a new unwind from regs), we always consume the LR
>> of the interrupted context, though this may not have been live at the
>> time of the exception. Where the LR was not live but happened to
>> contain `return_to_handler`, we'll recover an address from the graph
>> return stack and increment the current offset, leaving subsequent
>> entries off-by-one.
>>
>> Where the LR was not live and did not contain `return_to_handler`, we
>> will still report an erroneous address, but subsequent entries will be
>> unaffected.
>
> It turns out I had this backwards, and we currently always *skip* the LR
> when unwinding across regs, because:
>
> * The entry assembly creates a synthetic frame record with the original
> FP and the ELR_EL1 value (i.e. the PC at the point of the exception),
> skipping the LR.
>
> * In arch_stack_walk() we start the walk from regs->pc, and continue
> with the frame record, skipping the LR.
>
> * In the existing dump_backtrace, we skip until we hit a frame record
> whose FP value matches the FP in the regs (i.e. the synthetic frame
> record created by the entry assembly). That'll dump the ELR_EL1 value,
> then continue to the next frame record, skipping the LR.
>
> So case two is bogus, and only case one can happen today. This cleanup
> shouldn't trigger the WARN_ON_ONCE() in unwind_frame(), and we can fix
> the missing LR entry in a subsequent cleanup.
>

OK. Thanks.

Madhavan