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Date: Tue, 14 Jul 2020 13:45:20 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Subject: Re: [PATCH 1/3] module: Rename module_alloc() to text_alloc() and
 move to kernel proper
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On Tue, Jul 14, 2020 at 11:33:33AM +0100, Russell King - ARM Linux admin wrote:
> For 32-bit ARM, our bpf code uses "blx/bx" (or equivalent code
> sequences) rather than encoding a "bl" or "b", so BPF there doesn't
> care where the executable memory is mapped, and doesn't need any
> PLTs.  Given that, should bpf always allocate from the vmalloc()
> region to preserve the module space for modules?

Ah, okay, then I suspect arm64 does something similar there. Thanks!

> I'm more concerned about ftrace though, but only because I don't
> have the understanding of that subsystem to really say whether there
> are any side effects from having the allocations potentially be out
> of range of a "bl" or "b" instruction.
> 
> If ftrace jumps both to and from the allocated page using a "load
> address to register, branch to register" approach like BPF does, then
> ftrace should be safe - and again, raises the issue that maybe it
> should always come from vmalloc() space.

I think the problem with ftrace is patching multiple instruction;
because it sounds like you'd need something to load the absolute address
in a register and then jump to that. And where it's relatively easy to
replace a single instruction, replace multiple instructions gets real
tricky real quick.

Which then leads to you being stuck with that 26bit displacement, IIRC.