Re: [PATCH v2 1/3] module: Rename module_alloc() to text_alloc() and move to kernel proper

From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will@kernel.org>
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Subject: Re: [PATCH v2 1/3] module: Rename module_alloc() to text_alloc() and move to kernel proper
Date: Tue, 14 Jul 2020 13:29:27 +0200	[thread overview]
Message-ID: <20200714112927.GV10769@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200714102826.GB4756@willie-the-truck>

On Tue, Jul 14, 2020 at 11:28:27AM +0100, Will Deacon wrote:

> As Ard says, module_alloc() _is_ special, in the sense that the virtual
> memory it allocates wants to be close to the kernel text, whereas the
> concept of allocating executable memory is broader and doesn't have these
> restrictions. So, while I'm in favour of having a text_alloc() interface
> that can be used by callers which only require an executable mapping, I'd
> much prefer for the module_alloc() code to remain for, err, modules.

So on x86 all those things (kprobes, bpf, ftrace) require that same
closeness.

An interface like the late vmalloc_exec() will simply not work for us.

We recently talked about arm64-kprobes and how you're not doing any of
the optimizations and fully rely on the exception return. And I see
you're one of the few archs that has bpf_jit_alloc_exec() (also,
shouldn't you be using PAGE_KERNEL_EXEC there?). But the BPF core seems
to use module_alloc() as a default means of allocating text.

So what should this look like? Have a text_alloc() with an argument that
indicates where? But then I suppose we also need a means to manage PLT
entries. Otherwise I don't exactly see how you're going to call BPF
code, or how that BPF stuff is going to call back into its helpers.

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