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Subject: Re: [PATCH v10 05/11] arm64: Make dump_stacktrace() use
 arch_stack_walk()
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     broonie@kernel.org, jpoimboe@redhat.com, ardb@kernel.org,
        nobuta.keiya@fujitsu.com, sjitindarsingh@gmail.com,
        catalin.marinas@arm.com, will@kernel.org, jmorris@namei.org,
        linux-arm-kernel@lists.infradead.org,
        live-patching@vger.kernel.org, linux-kernel@vger.kernel.org
References: <c05ce30dcc9be1bd6b5e24a2ca8fe1d66246980b>
 <20211015025847.17694-1-madvenka@linux.microsoft.com>
 <20211015025847.17694-6-madvenka@linux.microsoft.com>
 <20211025164925.GB2001@C02TD0UTHF1T.local>
 <20211026120516.GA34073@C02TD0UTHF1T.local>
From:   "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
Message-ID: <a6991ee4-835d-f8b8-84a5-00755bbfe56e@linux.microsoft.com>
Date:   Wed, 27 Oct 2021 11:09:13 -0500
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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