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

On Tue, 14 Jul 2020 at 05:04, Steven Rostedt [off-list ref] wrote:

On Mon, 13 Jul 2020 22:49:48 +0300
Ard Biesheuvel [off-list ref] wrote:

quoted

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?