[PATCH 1/3] module: Rename module_alloc() to text_alloc() and move to kernel proper

[Ard Biesheuvel]

On Tue, 14 Jul 2020 at 05:04, Steven Rostedt <rostedt at goodmis.org> wrote:
>
> On Mon, 13 Jul 2020 22:49:48 +0300
> Ard Biesheuvel <ardb at kernel.org> wrote:
>
> > On arm64, we no longer use module_alloc for bpf or kprobes, to avoid
> > wasting va space on code that does not need to be loaded close to the
> > kernel. Also, module_alloc() allocates kasan shadow, which is
> > unnecessary for kprobes or bpf programs, which don't have kasan
> > instrumentation.
> >
> > This patch suggests that there are other reasons why conflating
> > allocation of module space and allocating  text pages for other uses
> > is a bad idea, but switching all users to text_alloc() is a step in
> > the wrong direction. It would be better to stop using module_alloc()
> > in core code except in the module loader, and have a generic
> > text_alloc() that can be overridden by the arch if necessary. Note
> > that x86  and s390 are the only architectures that use module_alloc()
> > in ftrace code.
> >
> > Please have a look at alloc_insn_page() or bpf_jit_alloc_exec() in the
> > arm64 tree to see what I mean.
>
> Hmm, so you have another method for allocating memory for trampolines?
> (I haven't looked at those functions you pointed out, out of sheer
> laziness ;-)
>
> It would be nice to implement the trampoline optimization in arm, which
> x86 has (see arch_ftrace_update_trampoline() and
> arch_ftrace_trampoline_func()).
>
> It helps when you have two different callbacks for different functions
> (like having live patching enabled and function tracing enabled, or
> kprobes using ftrace). Each callback will get its own allocated
> trampoline to jump to instead of jumping to the a trampoline that calls
> a looping function that tests to see which callback wants to be called
> by the traced function.
>

So in what sense are ftrace trampolines like kernel modules, apart
from the fact that they are executable pages that live in the vmalloc
space?