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Date: Tue, 26 Mar 2024 16:38:07 +0000
From: Mark Rutland <mark.rutland@arm.com>
To: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Jarkko Sakkinen <jarkko@kernel.org>, linux-riscv@lists.infradead.org,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>, linux-kernel@vger.kernel.org,
	"Naveen N . Rao" <naveen.n.rao@linux.ibm.com>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
	"David S . Miller" <davem@davemloft.net>,
	linux-trace-kernel@vger.kernel.org,
	Calvin Owens <jcalvinowens@gmail.com>
Subject: Re: [PATCH v2] arch/riscv: Enable kprobes when CONFIG_MODULES=n
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On Wed, Mar 27, 2024 at 12:24:03AM +0900, Masami Hiramatsu wrote:
> On Tue, 26 Mar 2024 14:46:10 +0000
> Mark Rutland <mark.rutland@arm.com> wrote:
> > 
> > On Mon, Mar 25, 2024 at 11:56:32AM +0900, Masami Hiramatsu wrote:
> > > I think, we'd better to introduce `alloc_execmem()`,
> > > CONFIG_HAVE_ALLOC_EXECMEM and CONFIG_ALLOC_EXECMEM at first
> > > 
> > >   config HAVE_ALLOC_EXECMEM
> > > 	bool
> > > 
> > >   config ALLOC_EXECMEM
> > > 	bool "Executable trampline memory allocation"
> > > 	depends on MODULES || HAVE_ALLOC_EXECMEM
> > > 
> > > And define fallback macro to module_alloc() like this.
> > > 
> > > #ifndef CONFIG_HAVE_ALLOC_EXECMEM
> > > #define alloc_execmem(size, gfp)	module_alloc(size)
> > > #endif
> > 
> > Please can we *not* do this? I think this is abstracting at the wrong level (as
> > I mentioned on the prior execmem proposals).
> > 
> > Different exectuable allocations can have different requirements. For example,
> > on arm64 modules need to be within 2G of the kernel image, but the kprobes XOL
> > areas can be anywhere in the kernel VA space.
> > 
> > Forcing those behind the same interface makes things *harder* for architectures
> > and/or makes the common code more complicated (if that ends up having to track
> > all those different requirements). From my PoV it'd be much better to have
> > separate kprobes_alloc_*() functions for kprobes which an architecture can then
> > choose to implement using a common library if it wants to.
> > 
> > I took a look at doing that using the core ifdeffery fixups from Jarkko's v6,
> > and it looks pretty clean to me (and works in testing on arm64):
> > 
> >   https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=kprobes/without-modules
> > 
> > Could we please start with that approach, with kprobe-specific alloc/free code
> > provided by the architecture?
> 
> OK, as far as I can read the code, this method also works and neat! 
> (and minimum intrusion). I actually found that exposing CONFIG_ALLOC_EXECMEM
> to user does not help, it should be an internal change. So hiding this change
> from user is better choice. Then there is no reason to introduce the new
> alloc_execmem, but just expand kprobe_alloc_insn_page() is reasonable.
> 
> Mark, can you send this series here, so that others can review/test it?

I've written up a cover letter and sent that out:
  
  https://lore.kernel.org/lkml/20240326163624.3253157-1-mark.rutland@arm.com/

Mark.