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

[Mark Rutland]

On Tue, Mar 26, 2024 at 09:15:14AM -0700, 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:
> > > 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.

I'd be very happy with eBPF following the same approach, with BPF-specific
alloc/free functions that we can implement in arch code.

IIUC eBPF code *does* want to be within range of the core kernel image, so for
arm64 we'd want to factor some common logic out of module_alloc() and into
something that module_alloc() and "bpf_alloc()" (or whatever it would be
called) could use. So I don't think we'd necessarily save on touching
module_alloc(), but I think the resulting split would be better.

Thanks,
Mark.