Re: [PATCH v4 05/15] mm: introduce execmem_alloc() and execmem_free()

Mark Rutland
2024-04-15T17:36:55Z

On Mon, Apr 15, 2024 at 09:52:41AM +0200, Peter Zijlstra wrote:
> On Thu, Apr 11, 2024 at 07:00:41PM +0300, Mike Rapoport wrote:
> > +/**
> > + * enum execmem_type - types of executable memory ranges
> > + *
> > + * There are several subsystems that allocate executable memory.
> > + * Architectures define different restrictions on placement,
> > + * permissions, alignment and other parameters for memory that can be used
> > + * by these subsystems.
> > + * Types in this enum identify subsystems that allocate executable memory
> > + * and let architectures define parameters for ranges suitable for
> > + * allocations by each subsystem.
> > + *
> > + * @EXECMEM_DEFAULT: default parameters that would be used for types that
> > + * are not explcitly defined.
> > + * @EXECMEM_MODULE_TEXT: parameters for module text sections
> > + * @EXECMEM_KPROBES: parameters for kprobes
> > + * @EXECMEM_FTRACE: parameters for ftrace
> > + * @EXECMEM_BPF: parameters for BPF
> > + * @EXECMEM_TYPE_MAX:
> > + */
> > +enum execmem_type {
> > + EXECMEM_DEFAULT,
> > + EXECMEM_MODULE_TEXT = EXECMEM_DEFAULT,
> > + EXECMEM_KPROBES,
> > + EXECMEM_FTRACE,
> > + EXECMEM_BPF,
> > + EXECMEM_TYPE_MAX,
> > +};
>
> Can we please get a break-down of how all these types are actually
> different from one another?
>
> I'm thinking some platforms have a tiny immediate space (arm64 comes to
> mind) and has less strict placement constraints for some of them?

Yeah, and really I'd *much* rather deal with that in arch code, as I have said
several times.

For arm64 we have two bsaic restrictions:

1) Direct branches can go +/-128M
We can expand this range by having direct branches go to PLTs, at a
performance cost.

2) PREL32 relocations can go +/-2G
We cannot expand this further.

* We don't need to allocate memory for ftrace. We do not use trampolines.

* Kprobes XOL areas don't care about either of those; we don't place any
PC-relative instructions in those. Maybe we want to in future.

* Modules care about both; we'd *prefer* to place them within +/-128M of all
other kernel/module code, but if there's no space we can use PLTs and expand
that to +/-2G. Since modules can refreence other modules, that ends up
actually being halved, and modules have to fit within some 2G window that
also covers the kernel.

* I'm not sure about BPF's requirements; it seems happy doing the same as
modules.

So if we *must* use a common execmem allocator, what we'd reall want is our own
types, e.g.

EXECMEM_ANYWHERE
EXECMEM_NOPLT
EXECMEM_PREL32

... and then we use those in arch code to implement module_alloc() and friends.

Mark.