From: Mark Rutland <mark.rutland@arm.com>
To: Torsten Duwe <duwe@lst.de>
Cc: Julien Thierry <julien.thierry@arm.com>,
Arnd Bergmann <arnd@arndb.de>,
Ard Biesheuvel <ard.biesheuvel@linaro.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Jiri Kosina <jikos@kernel.org>, Will Deacon <will.deacon@arm.com>,
linux-kernel@vger.kernel.org,
Steven Rostedt <rostedt@goodmis.org>,
AKASHI Takahiro <takahiro.akashi@linaro.org>,
Ingo Molnar <mingo@redhat.com>,
Ruslan Bilovol <ruslan.bilovol@gmail.com>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Amit Daniel Kachhap <amit.kachhap@arm.com>,
live-patching@vger.kernel.org,
linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v8 0/5] arm64: ftrace with regs
Date: Mon, 21 Oct 2019 12:37:25 +0100 [thread overview]
Message-ID: <20191021113724.GA56589@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <20191019130135.10de9324@blackhole.lan>
On Sat, Oct 19, 2019 at 01:01:35PM +0200, Torsten Duwe wrote:
> Hi Mark!
Hi Torsten!
> On Fri, 18 Oct 2019 18:41:02 +0100 Mark Rutland
> <mark.rutland@arm.com> wrote:
>
> > In the process of reworking this I spotted some issues that will get
> > in the way of livepatching. Notably:
> >
> > * When modules can be loaded far away from the kernel, we'll
> > potentially need a PLT for each function within a module, if each can
> > be patched to a unique function. Currently we have a fixed number,
> > which is only sufficient for the two ftrace entry trampolines.
> >
> > IIUC, the new code being patched in is itself a module, in which
> > case we'd need a PLT for each function in the main kernel image.
>
> When no live patching is involved, obviously all cases need to have
> been handled so far. And when a live patching module comes in, there
> are calls in and out of the new patch code:
>
> Calls going into the live patch are not aware of this. They are caught
> by an active ftrace intercept, and the actual call into the LP module
> is done in klp_arch_set_pc, by manipulating the intercept (call site)
> return address (in case thread lives in the "new world", for
> completeness' sake). This is an unsigned long write in C.
I was under the impression that (at some point) the patch site would be
patched to call the LP code directly. From the above I understand that's
not the case, and it will always be directed via the regular ftrace
entry code -- have I got that right?
Assuming that is the case, that sounds fine to me, and sorry for the
noise.
> All calls going _out_ from the KLP module are newly generated, as part
> of the KLP module building process, and are thus aware of them being
> "extern" -- a PLT entry should be generated and accounted for in the
> KLP module.
Yup; understood.
> > We have a few options here, e.g. changing which memory size model we
> > use, or reserving space for a PLT before each function using
> > -f patchable-function-entry=N,M.
>
> Nonetheless I'm happy I once added the ,M option here. You never know :)
Yup; we may have other reasons to need this in future (and I see parisc
uses this today).
> > * There are windows where backtracing will miss the callsite's caller,
> > as its address is not live in the LR or existing chain of frame
> > records. Thus we cannot claim to have a reliable stacktrace.
> >
> > I suspect we'll have to teach the stacktrace code to handle this as
> > a special-case.
>
> Yes, that's where I had to step back. The unwinder needs to stop where
> the chain is even questionable. In _all_ cases. Missing only one race
> condition means a lurking inconsistency.
Sure. I'm calling this out now so that we don't miss this in future.
I've added comments to the ftrace entry asm to this effect for now.
> OTOH it's not a problem to report "not reliable" when in doubt; the
> thread in question will then get woken up and unwind itself.
> It is only an optimisation to let all kernel threads which are
> guaranteed to not contain any patched functions sleep on.
I just want to make it clear that some care will be needed if/when
adding CONFIG_HAVE_RELIABLE_STACKTRACE so that we handle this case
correctly.
> > I'll try to write these up, as similar probably applies to other
> > architectures with a link register.
>
> I thought I'd quickly give you my feedback upfront here.
Thanks; it's much appreciated!
Mark.
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2019-10-21 11:37 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-08 15:08 [PATCH v8 0/5] arm64: ftrace with regs Torsten Duwe
2019-02-08 15:10 ` [PATCH v8 1/5] arm64: replace -pg with CC_FLAGS_FTRACE in arm64 Makefiles Torsten Duwe
2019-02-11 11:53 ` Mark Rutland
2019-02-08 15:10 ` [PATCH v8 2/5] arm64: replace -pg with CC_FLAGS_FTRACE in efi Makefiles Torsten Duwe
2019-02-11 11:59 ` Mark Rutland
2019-04-08 15:23 ` Mark Rutland
2019-04-08 20:49 ` Ard Biesheuvel
2019-02-08 15:10 ` [PATCH v8 3/5] arm64: replace -pg with CC_FLAGS_FTRACE in mm/kasan Makefile Torsten Duwe
2019-02-11 11:55 ` Mark Rutland
2019-02-11 11:56 ` Mark Rutland
2019-04-08 15:25 ` Mark Rutland
2019-04-08 15:35 ` Andrey Ryabinin
2019-02-08 15:10 ` [PATCH v8 4/5] arm64: implement ftrace with regs Torsten Duwe
2019-02-13 10:30 ` Julien Thierry
2019-02-08 15:10 ` [PATCH v8 5/5] arm64: use -fpatchable-function-entry if available Torsten Duwe
2019-02-13 11:11 ` [PATCH v8 0/5] arm64: ftrace with regs Julien Thierry
2019-03-11 11:49 ` Torsten Duwe
2019-03-11 12:18 ` Mark Rutland
2019-03-29 10:18 ` Torsten Duwe
2019-04-08 15:36 ` Mark Rutland
2019-04-09 17:52 ` Will Deacon
2019-07-10 12:27 ` Ruslan Bilovol
2019-07-24 16:15 ` Mark Rutland
2019-10-16 11:42 ` Jiri Kosina
2019-10-16 17:58 ` Mark Rutland
2019-10-18 17:41 ` Mark Rutland
2019-10-19 11:01 ` Torsten Duwe
2019-10-21 11:37 ` Mark Rutland [this message]
2019-10-21 13:20 ` Josh Poimboeuf
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=20191021113724.GA56589@lakrids.cambridge.arm.com \
--to=mark.rutland@arm.com \
--cc=amit.kachhap@arm.com \
--cc=ard.biesheuvel@linaro.org \
--cc=arnd@arndb.de \
--cc=catalin.marinas@arm.com \
--cc=duwe@lst.de \
--cc=jikos@kernel.org \
--cc=jpoimboe@redhat.com \
--cc=julien.thierry@arm.com \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=live-patching@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=rostedt@goodmis.org \
--cc=ruslan.bilovol@gmail.com \
--cc=takahiro.akashi@linaro.org \
--cc=will.deacon@arm.com \
/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).