linux-kernel - Re: [RFC PATCH] text_poke/ftrace/x86: Allow text

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Date:   Tue, 25 Oct 2022 12:28:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-arch@...r.kernel.org
Subject: Re: [RFC PATCH] text_poke/ftrace/x86: Allow text_poke() to be called
 in early boot

On Mon, Oct 24, 2022 at 05:11:13PM -0700, Linus Torvalds wrote:

> All of this comes from "poking_init()" being a steaming pile of bovine
> excrement, doing random odd things, and having that special
> "copy_init_mm()" helper that just makes things even worse. Nothing
> else uses that, and it shouldn't have called "dup_mm()" in the first
> place.

Agreed; dup_mm() makes no sense and it is easily removed, see my earlier
patch. Perhaps it can be simplified further to:

	__poking_mm = init_mm

omitting the mm_init() I retained, but I need to stare harder at all
that.

> I'm not even sure why "poking_mm" exists at all, and why it has
> created a whole new copy of "init_mm", and why this code isn't just
> using '&init_mm' like everything else that wants to just walk the
> kernel page tables.

Because it instantiates user-space page-tables in it, you really don't
want those in init_mm.

The whole (and sole) purpose of poking_mm is to contain the writable
aliases. Only the CPU that has the poking_mm active has access to them.