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Date:   Thu, 9 Jun 2022 08:37:54 +0300
From:   Jarkko Sakkinen <jarkko@kernel.org>
To:     Ard Biesheuvel <ardb@kernel.org>
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Subject: Re: [PATCH] kprobes: Enable tracing for mololithic kernel images
Message-ID: <YqGHMs4ha7JMvODf@iki.fi>
References: <20220608000014.3054333-1-jarkko@profian.com>
 <CAMj1kXFsdEq6XZ6eOuf8Ks-F4qgneVxFeLYNN_S4JaPy8koEyw@mail.gmail.com>
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On Wed, Jun 08, 2022 at 06:27:51PM +0200, Ard Biesheuvel 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.

OK, I apologize for my ignorance. It's been a while.

> 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.
> 
> 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()?

All arch's, except nios2 use vmalloc() in the end for module_alloc().
nios2 uses kmalloc() for the reasons that I'm not aware of, but it does
not support kprobes in the first place.

Based on this, I think that could work out just fine.

I could cope with that.

BR, Jarkko