live-patching.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Mark Rutland <mark.rutland@arm.com>
To: madvenka@linux.microsoft.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: Tue, 26 Oct 2021 13:05:16 +0100	[thread overview]
Message-ID: <20211026120516.GA34073@C02TD0UTHF1T.local> (raw)
In-Reply-To: <20211025164925.GB2001@C02TD0UTHF1T.local>

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.

Thanks,
Mark.

  reply	other threads:[~2021-10-26 12:05 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 [this message]
2021-10-27 16:09         ` Madhavan T. Venkataraman
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20211026120516.GA34073@C02TD0UTHF1T.local \
    --to=mark.rutland@arm.com \
    --cc=ardb@kernel.org \
    --cc=broonie@kernel.org \
    --cc=catalin.marinas@arm.com \
    --cc=jmorris@namei.org \
    --cc=jpoimboe@redhat.com \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=live-patching@vger.kernel.org \
    --cc=madvenka@linux.microsoft.com \
    --cc=nobuta.keiya@fujitsu.com \
    --cc=sjitindarsingh@gmail.com \
    --cc=will@kernel.org \
    --subject='Re: [PATCH v10 05/11] arm64: Make dump_stacktrace() use arch_stack_walk()' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).