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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Thu, 9 Jun 2022 15:23:16 +0200
Message-ID: <CAMj1kXGGyO-DL9hjKYKR2sp87s4KExiQybES8pp4JgqJcHkfLA@mail.gmail.com>
Subject: Re: [PATCH] kprobes: Enable tracing for mololithic kernel images
To:     Jarkko Sakkinen <jarkko@kernel.org>
Cc:     Song Liu <song@kernel.org>, Masami Hiramatsu <mhiramat@kernel.org>,
        Guo Ren <guoren@kernel.org>,
        Jarkko Sakkinen <jarkko@profian.com>,
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Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Thu, 9 Jun 2022 at 15:14, Jarkko Sakkinen <jarkko@kernel.org> wrote:
>
> On Wed, Jun 08, 2022 at 09:12:34AM -0700, Song Liu wrote:
> > On Wed, Jun 8, 2022 at 7:21 AM Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > >
> > > Hi Jarkko,
> > >
> > > On Wed, 8 Jun 2022 08:25:38 +0300
> > > Jarkko Sakkinen <jarkko@kernel.org> wrote:
> > >
> > > > On Wed, Jun 08, 2022 at 10:35:42AM +0800, Guo Ren wrote:
> > > > > .
> > > > >
> > > > > On Wed, Jun 8, 2022 at 8:02 AM Jarkko Sakkinen <jarkko@profian.com> wrote:
> > > > > >
> > > > > > Tracing with kprobes while running a monolithic kernel is currently
> > > > > > impossible because CONFIG_KPROBES is dependent of CONFIG_MODULES.  This
> > > > > > dependency is a result of kprobes code using the module allocator for the
> > > > > > trampoline code.
> > > > > >
> > > > > > Detaching kprobes from modules helps to squeeze down the user space,
> > > > > > e.g. when developing new core kernel features, while still having all
> > > > > > the nice tracing capabilities.
> > > > > >
> > > > > > For kernel/ and arch/*, move module_alloc() and module_memfree() to
> > > > > > module_alloc.c, and compile as part of vmlinux when either CONFIG_MODULES
> > > > > > or CONFIG_KPROBES is enabled.  In addition, flag kernel module specific
> > > > > > code with CONFIG_MODULES.
> > > > > >
> > > > > > As the result, kprobes can be used with a monolithic kernel.
> > > > > It's strange when MODULES is n, but vmlinux still obtains module_alloc.
> > > > >
> > > > > Maybe we need a kprobe_alloc, right?
> > > >
> > > > Perhaps not the best name but at least it documents the fact that
> > > > they use the same allocator.
> > > >
> > > > Few years ago I carved up something "half-way there" for kprobes,
> > > > and I used the name text_alloc() [*].
> > > >
> > > > [*] https://lore.kernel.org/all/20200724050553.1724168-1-jarkko.sakkinen@linux.intel.com/
> > >
> > > Yeah, I remember that. Thank you for updating your patch!
> > > I think the idea (split module_alloc() from CONFIG_MODULE) is good to me.
> > > If module support maintainers think this name is not good, you may be
> > > able to rename it as text_alloc() and make the module_alloc() as a
> > > wrapper of it.
> >
> > IIUC, most users of module_alloc() use it to allocate memory for text, except
> > that module code uses it for both text and data. Therefore, I guess calling it
> > text_alloc() is not 100% accurate until we change the module code (to use
> > a different API to allocate memory for data).
>
> After reading the feedback, I'd stay on using module_alloc() because
> it has arch-specific quirks baked in. Easier to deal with them in one
> place.
>

In that case, please ensure that you enable this only on architectures
where it is needed. arm64 implements alloc_insn_page() without relying
on module_alloc() so I would not expect to see any changes there.