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From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
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
Subject: Re: [PATCH v10 05/11] arm64: Make dump_stacktrace() use arch_stack_walk()
Date: Wed, 27 Oct 2021 11:09:13 -0500	[thread overview]
Message-ID: <a6991ee4-835d-f8b8-84a5-00755bbfe56e@linux.microsoft.com> (raw)
In-Reply-To: <20211026120516.GA34073@C02TD0UTHF1T.local>



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

  reply	other threads:[~2021-10-27 16:09 UTC|newest]

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <c05ce30dcc9be1bd6b5e24a2ca8fe1d66246980b>
2021-10-15  2:34 ` [PATCH v9 00/11] arm64: Reorganize the unwinder and implement stack trace reliability checks madvenka
2021-10-15  2:34   ` [PATCH v9 01/11] arm64: Select STACKTRACE in arch/arm64/Kconfig madvenka
2021-10-15  2:34   ` [PATCH v9 10/11] arm64: Introduce stack trace reliability checks in the unwinder madvenka
2021-10-15  2:34   ` [PATCH v9 11/11] arm64: Create a list of SYM_CODE functions, check return PC against list madvenka
2021-10-15  2:34   ` [PATCH v9 02/11] arm64: Make perf_callchain_kernel() use arch_stack_walk() madvenka
2021-10-15  2:34   ` [PATCH v9 03/11] arm64: Make get_wchan() " madvenka
2021-10-15  2:34   ` [PATCH v9 04/11] arm64: Make return_address() " madvenka
2021-10-15  2:34   ` [PATCH v9 05/11] arm64: Make dump_stacktrace() " madvenka
2021-10-15  2:34   ` [PATCH v9 06/11] arm64: Make profile_pc() " madvenka
2021-10-15  2:34   ` [PATCH v9 07/11] arm64: Call stack_backtrace() only from within walk_stackframe() madvenka
2021-10-15  2:34   ` [PATCH v9 08/11] arm64: Rename unwinder functions, prevent them from being traced and kprobed madvenka
2021-10-15  2:34   ` [PATCH v9 09/11] arm64: Make the unwind loop in unwind() similar to other architectures madvenka
2021-10-15  2:53   ` [PATCH v9 00/11] arm64: Reorganize the unwinder and implement stack trace reliability checks Madhavan T. Venkataraman
2021-10-15  2:58 ` [PATCH v10 " madvenka
2021-10-15  2:58   ` [PATCH v10 01/11] arm64: Select STACKTRACE in arch/arm64/Kconfig madvenka
2021-10-15 18:28     ` Mark Brown
2021-10-21 12:28       ` Madhavan T. Venkataraman
2021-10-22 18:02     ` Mark Rutland
2021-11-12 17:44       ` Mark Rutland
2021-11-14 16:15         ` Madhavan T. Venkataraman
2021-10-15  2:58   ` [PATCH v10 02/11] arm64: Make perf_callchain_kernel() use arch_stack_walk() madvenka
2021-10-20 14:59     ` Mark Brown
2021-10-21 12:28       ` Madhavan T. Venkataraman
2021-10-22 18:11     ` Mark Rutland
2021-10-23 12:49       ` Madhavan T. Venkataraman
2021-10-15  2:58   ` [PATCH v10 03/11] arm64: Make get_wchan() " madvenka
2021-10-20 16:10     ` Mark Brown
2021-10-21 12:30       ` Madhavan T. Venkataraman
2021-10-15  2:58   ` [PATCH v10 04/11] arm64: Make return_address() " madvenka
2021-10-20 15:03     ` Mark Brown
2021-10-21 12:29       ` Madhavan T. Venkataraman
2021-10-22 18:51     ` Mark Rutland
2021-10-23 12:51       ` Madhavan T. Venkataraman
2021-10-15  2:58   ` [PATCH v10 05/11] arm64: Make dump_stacktrace() " madvenka
2021-10-25 16:49     ` Mark Rutland
2021-10-26 12:05       ` Mark Rutland
2021-10-27 16:09         ` Madhavan T. Venkataraman [this message]
2021-10-15  2:58   ` [PATCH v10 06/11] arm64: Make profile_pc() " madvenka
2021-10-25  2:18     ` nobuta.keiya
2021-10-27 16:10       ` Madhavan T. Venkataraman
2021-10-27 13:32     ` Mark Rutland
2021-10-27 16:15       ` Madhavan T. Venkataraman
2021-10-15  2:58   ` [PATCH v10 07/11] arm64: Call stack_backtrace() only from within walk_stackframe() madvenka
2021-10-15  2:58   ` [PATCH v10 08/11] arm64: Rename unwinder functions, prevent them from being traced and kprobed madvenka
2021-10-27 17:53     ` Mark Rutland
2021-10-27 20:07       ` Madhavan T. Venkataraman
2021-10-15  2:58   ` [PATCH v10 09/11] arm64: Make the unwind loop in unwind() similar to other architectures madvenka
2021-10-15  2:58   ` [PATCH v10 10/11] arm64: Introduce stack trace reliability checks in the unwinder madvenka
2021-11-04 12:39     ` nobuta.keiya
2021-11-10  3:13       ` Madhavan T. Venkataraman
2021-10-15  2:58   ` [PATCH v10 11/11] arm64: Create a list of SYM_CODE functions, check return PC against list madvenka
2021-10-15 17:00   ` [PATCH v10 00/11] arm64: Reorganize the unwinder and implement stack trace reliability checks Madhavan T. Venkataraman

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