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From:   Song Liu <song@kernel.org>
Date:   Wed, 8 Jun 2022 11:19:19 -0700
Message-ID: <CAPhsuW7NZXepczZGyV2Ti4hNSupLBoKgA64K=zJN4uVBTxN37g@mail.gmail.com>
Subject: Re: [PATCH] kprobes: Enable tracing for mololithic kernel images
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Jarkko Sakkinen <jarkko@profian.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Nathaniel McCallum <nathaniel@profian.com>,
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Precedence: bulk

On Wed, Jun 8, 2022 at 9:28 AM Ard Biesheuvel <ardb@kernel.org> wrote:
>
> Hello Jarkko,
>
> On Wed, 8 Jun 2022 at 02:02, 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.
>
> I think I may have mentioned this the previous time as well, but I
> don't think this is the right approach.
>
> Kprobes uses alloc_insn_page() to allocate executable memory, but the
> requirements for this memory are radically different compared to
> loadable modules, which need to be within an arch-specific distance of
> the core kernel, need KASAN backing etc etc.

I think the distance of core kernel requirement is the same for kprobe
alloc_insn_page and modules, no?

Thanks,
Song

>
> This is why arm64, for instance, does not implement alloc_insn_page()
> in terms of module_alloc() [and likely does not belong in this patch
> for that reason]


>
> Is there any reason kprobes cannot simply use vmalloc()?
>