Re: [PATCH] kprobes: Enable tracing for mololithic kernel images

[Jarkko Sakkinen <jarkko@kernel.org>]

Yamada <masahiroy@kernel.org>, Jarkko Sakkinen <jarkko@profian.com>, Sami Tolvanen <samitolvanen@google.com>, "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>, Marco Elver <elver@google.com>, Kees Cook <keescook@chromium.org>, Steven Rostedt <rostedt@goodmis.org>, Nathan Chancellor <nathan@kernel.org>, "Russell King \(Oracle\)" <rmk+kernel@armlinux.org.uk>, Mark Brown <broonie@kernel.org>, Borislav Petkov <bp@alien8.de>, Alexander Egorenkov <egorenar@linux.ibm.com>, Thomas Bogendoerfer <tsbogend@alpha.franken.de>, linux-parisc@vger.kernel.org, Nathaniel McCallum <nathaniel@profian.com>, Dmitry Torokhov <dmitry.torokhov@gmail.com>, "David S. Miller" <davem@davemloft.net>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Tobias Huschle <huschle@linux.ibm.com>, "Peter Zijlstra \(Intel\)" <peterz@infradead.org>, "H. Peter Anvin" <hpa@zytor.com>, sparclinux@vger.kernel.org, Tiezhu Yang <yangtiezhu@loongson.cn>, Miroslav Benes <mbenes@suse.cz>, Chen Zhongjin <chenzhongjin@huawei.com>, l
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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