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

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Date:   Mon, 24 Oct 2022 17:11:13 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.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 4:03 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> This required some updates to fork and the maple_tree code to allow it to
> be called with enabling interrupts in the time when interrupts must remain
> disabled.

Yeah, moving special cases from one place to another doesn't really
help. Particularly to something as core as dup_mm().

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.

At this point, there is no actual user VM to even copy, so 99% of
everything that duip_mm() does is not just pointless, but actively
wrong, like the mmap_write_lock_nested() when we're in early boot.

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.

Yes, I see that commit 4fc19708b165 ("x86/alternatives: Initialize
temporary mm for patching"), and no, none of that makes any sense to
me. It seems just (mis-)designed to fail.

                 Linus