linux-kernel - Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation

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Date:   Wed, 20 Jul 2022 10:09:37 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        Joao Moreira <joao.moreira@...el.com>,
        Joseph Nuzman <joseph.nuzman@...el.com>,
        Juergen Gross <jgross@...e.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation

On Wed, Jul 20, 2022 at 9:57 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> [    2.488712] WARNING: CPU: 0 PID: 0 at arch/x86/kernel/alternative.c:558 apply_returns+0xa3/0x1ec

That warning is kind of annoying, in how it doesn't actually give any
information about where the problem is.

I do note that we only fix up JMP32_INSN_OPCODE, and I wonder if we
have a "jmp __x86_return_thunk" that is close enough to the return
thunk that it actually uses a byte offset?

But that WARN_ON_ONCE() should probably be changed to actually give
some information about where the problem is.

The silly thing is, there's even debug output in that function that
you could enable, but it will enable output for the *normal* case, not
for the WARN_ON_ONCE() case or the "we didn't do anything" case. That
seems a bit backwards.

               Linus