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Date:   Tue, 14 Jun 2022 15:30:14 +0300
From:   Jarkko Sakkinen <jarkko@kernel.org>
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Song Liu <song@kernel.org>, Ard Biesheuvel <ardb@kernel.org>,
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Subject: Re: [PATCH] kprobes: Enable tracing for mololithic kernel images
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References: <20220608000014.3054333-1-jarkko@profian.com>
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On Sun, Jun 12, 2022 at 09:30:41PM +0900, Masami Hiramatsu wrote:
> On Wed, 8 Jun 2022 11:19:19 -0700
> Song Liu <song@kernel.org> wrote:
> 
> > 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?
> 
> This strongly depends on how kprobes (software breakpoint and
> single-step) is implemented on the arch. For example, x86 implements
> the so-called "kprobe-booster" which jumps back from the single
> stepping trampoline buffer. Then the buffer address must be within
> the range where it can jump to the original address.
> However, if the arch implements single-step as an instruction
> emulation, it has no such limitation. As far as I know, arm64
> will do emulation for the instructions which change PC register
> and will do direct execution with another software breakpoint
> for other instructions.
> 
> Why I'm using module_alloc() for a generic function, is that
> can cover the limitation most widely.
> Thus, if we have CONFIG_ARCH_HAVE_ALLOC_INSN_PAGE flag and
> kprobes can check it instead of using __weak function, the
> kprobes may not need to depend on module_alloc() in general.

OK, I guess this is what Luis meant. 

I'll try to carve up something based on this.

BR, Jarkko