[PATCH v2] arch/riscv: Enable kprobes when CONFIG_MODULES=n

[Mark Rutland]

Hi Masami,

On Mon, Mar 25, 2024 at 11:56:32AM +0900, Masami Hiramatsu wrote:
> Hi Jarkko,
> 
> On Sun, 24 Mar 2024 01:29:08 +0200
> Jarkko Sakkinen <jarkko at kernel.org> wrote:
> 
> > Tracing with kprobes while running a monolithic kernel is currently
> > impossible due the kernel module allocator dependency.
> > 
> > Address the issue by allowing architectures to implement module_alloc()
> > and module_memfree() independent of the module subsystem. An arch tree
> > can signal this by setting HAVE_KPROBES_ALLOC in its Kconfig file.
> > 
> > Realize the feature on RISC-V by separating allocator to module_alloc.c
> > and implementing module_memfree().
> 
> Even though, this involves changes in arch-independent part. So it should
> be solved by generic way. Did you checked Calvin's thread?
> 
> https://lore.kernel.org/all/cover.1709676663.git.jcalvinowens@gmail.com/
> 
> 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?

Thanks,
Mark.