live-patching.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Mark Rutland <mark.rutland@arm.com>
To: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
Cc: broonie@kernel.org, jpoimboe@redhat.com, jthierry@redhat.com,
	catalin.marinas@arm.com, will@kernel.org,
	linux-arm-kernel@lists.infradead.org,
	live-patching@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v2 5/8] arm64: Detect an FTRACE frame and mark a stack trace unreliable
Date: Tue, 23 Mar 2021 18:30:53 +0000	[thread overview]
Message-ID: <20210323183053.GH98545@C02TD0UTHF1T.local> (raw)
In-Reply-To: <bc450f09-1881-9a9c-bfbc-5bb31c01d8ce@linux.microsoft.com>

On Tue, Mar 23, 2021 at 12:23:34PM -0500, Madhavan T. Venkataraman wrote:
> On 3/23/21 12:02 PM, Mark Rutland wrote:

[...]

> I think that I did a bad job of explaining what I wanted to do. It is not
> for any additional protection at all.
> 
> So, let us say we create a field in the task structure:
> 
> 	u64		unreliable_stack;
> 
> Whenever an EL1 exception is entered or FTRACE is entered and pt_regs get
> set up and pt_regs->stackframe gets chained, increment unreliable_stack.
> On exiting the above, decrement unreliable_stack.
> 
> In arch_stack_walk_reliable(), simply do this check upfront:
> 
> 	if (task->unreliable_stack)
> 		return -EINVAL;
> 
> This way, the function does not even bother unwinding the stack to find
> exception frames or checking for different return addresses or anything.
> We also don't have to worry about code being reorganized, functions
> being renamed, etc. It also may help in debugging to know if a task is
> experiencing an exception and the level of nesting, etc.

As in my other reply, since this is an optimization that is not
necessary for functional correctness, I would prefer to avoid this for
now. We can reconsider that in future if we encounter performance
problems.

Even with this there will be cases where we have to identify
non-unwindable functions explicitly (e.g. the patchable-function-entry
trampolines, where the real return address is in x9), and I'd prefer
that we use one mechanism consistently.

I suspect that in the future we'll need to unwind across exception
boundaries using metadata, and we can treat the non-unwindable metadata
in the same way.

[...]

> > 3. Figure out exception boundary handling. I'm currently working to
> >    simplify the entry assembly down to a uniform set of stubs, and I'd
> >    prefer to get that sorted before we teach the unwinder about
> >    exception boundaries, as it'll be significantly simpler to reason
> >    about and won't end up clashing with the rework.
> 
> So, here is where I still have a question. Is it necessary for the unwinder
> to know the exception boundaries? Is it not enough if it knows if there are
> exceptions present? For instance, using something like num_special_frames
> I suggested above?

I agree that it would be legitimate to bail out early if we knew there
was going to be an exception somewhere in the trace. Regardless, I think
it's simpler overall to identify non-unwindability during the trace, and
doing that during the trace aligns more closely with the structure that
we'll need to permit unwinding across these boundaries in future, so I'd
prefer we do that rather than trying to optimize for early returns
today.

Thanks,
Mark.

  parent reply	other threads:[~2021-03-23 18:31 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <5997dfe8d261a3a543667b83c902883c1e4bd270>
2021-03-15 16:57 ` [RFC PATCH v2 0/8] arm64: Implement reliable stack trace madvenka
2021-03-15 16:57   ` [RFC PATCH v2 1/8] arm64: Implement stack trace termination record madvenka
2021-03-18 15:09     ` Mark Brown
2021-03-18 20:26       ` Madhavan T. Venkataraman
2021-03-19 12:30         ` Mark Brown
2021-03-19 14:29           ` Madhavan T. Venkataraman
2021-03-19 18:19             ` Madhavan T. Venkataraman
2021-03-19 22:03               ` Madhavan T. Venkataraman
2021-03-23 10:24                 ` Mark Rutland
2021-03-23 12:39                   ` Madhavan T. Venkataraman
2021-03-15 16:57   ` [RFC PATCH v2 2/8] arm64: Implement frame types madvenka
2021-03-18 17:40     ` Mark Brown
2021-03-18 22:22       ` Madhavan T. Venkataraman
2021-03-19 13:22         ` Mark Brown
2021-03-19 14:40           ` Madhavan T. Venkataraman
2021-03-19 15:02             ` Madhavan T. Venkataraman
2021-03-19 16:20               ` Mark Brown
2021-03-19 16:27                 ` Madhavan T. Venkataraman
2021-03-23 10:34     ` Mark Rutland
2021-03-15 16:57   ` [RFC PATCH v2 3/8] arm64: Terminate the stack trace at TASK_FRAME and EL0_FRAME madvenka
2021-03-18 18:26     ` Mark Brown
2021-03-18 20:29       ` Madhavan T. Venkataraman
2021-03-23 10:36         ` Mark Rutland
2021-03-23 12:40           ` Madhavan T. Venkataraman
2021-03-15 16:57   ` [RFC PATCH v2 4/8] arm64: Detect an EL1 exception frame and mark a stack trace unreliable madvenka
2021-03-23 10:42     ` Mark Rutland
2021-03-23 12:46       ` Madhavan T. Venkataraman
2021-03-23 13:04         ` Mark Rutland
2021-03-23 13:31           ` Madhavan T. Venkataraman
2021-03-23 14:33             ` Mark Rutland
2021-03-23 15:22               ` Madhavan T. Venkataraman
2021-03-15 16:57   ` [RFC PATCH v2 5/8] arm64: Detect an FTRACE " madvenka
2021-03-23 10:51     ` Mark Rutland
2021-03-23 12:56       ` Madhavan T. Venkataraman
2021-03-23 13:36         ` Mark Rutland
2021-03-23 13:38           ` Madhavan T. Venkataraman
2021-03-23 14:15             ` Madhavan T. Venkataraman
2021-03-23 14:57               ` Mark Rutland
2021-03-23 15:26                 ` Madhavan T. Venkataraman
2021-03-23 16:20                   ` Madhavan T. Venkataraman
2021-03-23 17:02                     ` Mark Rutland
2021-03-23 17:23                       ` Madhavan T. Venkataraman
2021-03-23 17:27                         ` Madhavan T. Venkataraman
2021-03-23 18:27                         ` Mark Brown
2021-03-23 20:23                           ` Madhavan T. Venkataraman
2021-03-23 18:30                         ` Mark Rutland [this message]
2021-03-23 20:24                           ` Madhavan T. Venkataraman
2021-03-23 21:04                             ` Madhavan T. Venkataraman
2021-03-23 16:48                   ` Mark Rutland
2021-03-23 16:53                     ` Madhavan T. Venkataraman
2021-03-23 17:09                       ` Mark Rutland
2021-03-15 16:57   ` [RFC PATCH v2 6/8] arm64: Check the return PC of every stack frame madvenka
2021-03-15 16:57   ` [RFC PATCH v2 7/8] arm64: Detect kretprobed functions in stack trace madvenka
2021-03-15 16:58   ` [RFC PATCH v2 8/8] arm64: Implement arch_stack_walk_reliable() madvenka
2021-03-15 19:01   ` [RFC PATCH v2 0/8] arm64: Implement reliable stack trace 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=20210323183053.GH98545@C02TD0UTHF1T.local \
    --to=mark.rutland@arm.com \
    --cc=broonie@kernel.org \
    --cc=catalin.marinas@arm.com \
    --cc=jpoimboe@redhat.com \
    --cc=jthierry@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=will@kernel.org \
    /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
Be sure your reply has a Subject: header at the top and a blank line before the message body.
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).