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

[Jarkko Sakkinen]

On Tue Mar 26, 2024 at 6:15 PM EET, Calvin Owens wrote:
> On Wednesday 03/27 at 00:24 +0900, Masami Hiramatsu wrote:
> > On Tue, 26 Mar 2024 14:46:10 +0000
> > Mark Rutland <mark.rutland at arm.com> wrote:
> > 
> > > 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?
>
> Heh, I also noticed that dead !RWX branch in arm64 patch_map(), I was
> about to send a patch to remove it.
>
> > 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.
>
> I'm happy with this, it solves the first half of my problem. But I want
> eBPF to work in the !MODULES case too.
>
> I think Mark's approach can work for bpf as well, without needing to
> touch module_alloc() at all? So I might be able to drop that first patch
> entirely.

Yeah, I think we're aligned. Later on, if/when you send the bpf series
please also cc me and I might possibly test those patches too.

BR, Jarkko