linux-kernel - Re: [PATCH v2 2/2] arm64/module: Use text-poke API for late relocations.

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Message-ID: <CADBMgpzrM7PMePtWaZLaaNXt6z++V0rX2VXtWK4vzdTCS=BpQA@mail.gmail.com>
Date: Tue, 22 Apr 2025 17:27:20 -0700
From: Dylan Hatch <dylanbhatch@...gle.com>
To: Song Liu <song@...nel.org>
Cc: Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Puranjay Mohan <puranjay@...nel.org>, 
	Xu Kuohai <xukuohai@...weicloud.com>, Philippe Mathieu-Daudé <philmd@...aro.org>, 
	Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	"Mike Rapoport (Microsoft)" <rppt@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	Geert Uytterhoeven <geert@...ux-m68k.org>, Luis Chamberlain <mcgrof@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Ard Biesheuvel <ardb@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, Roman Gushchin <roman.gushchin@...ux.dev>
Subject: Re: [PATCH v2 2/2] arm64/module: Use text-poke API for late relocations.

On Mon, Apr 21, 2025 at 11:25 PM Song Liu <song@...nel.org> wrote:
>
> On Mon, Apr 21, 2025 at 5:35 PM Song Liu <song@...nel.org> wrote:
> >
> > On Fri, Apr 11, 2025 at 6:10 PM Dylan Hatch <dylanbhatch@...gle.com> wrote:
> > >
> > > To enable late module patching, livepatch modules need to be able to
> > > apply some of their relocations well after being loaded. In this
> > > scenario, use the text-poking API to allow this, even with
> > > STRICT_MODULE_RWX.
> > >
> > > This patch is largely based off commit 88fc078a7a8f6 ("x86/module: Use
> > > text_poke() for late relocations").
> > >
> > > Signed-off-by: Dylan Hatch <dylanbhatch@...gle.com>
>
> Could you please share how you test this?
>

For context, we enable livepatch for arm64 by porting this RFC series
(along with other internal patches) into our kernel:
https://lore.kernel.org/all/20230202074036.507249-1-madvenka@linux.microsoft.com/.

The way I tested this patch is: with STRICT_MODULE_RWX, load a module
and a livepatch that touches that module (in either order), and
confirm the kernel doesn't crash.

Without this patch, a crash is caused in apply_relocate_add() if both
a module and a livepatch that touches the module are both loaded. This
happens through one of two code paths:

  1. If the module is already loaded when the livepatch is applied,
through the module_init() callback.
  2. If the module is loaded after the livepatch is applied, through
prepare_coming_module().

In both scenarios, the livepatch module's text is already RX-only.

Thanks,
Dylan