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

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Message-ID: <CAHk-=wiOGOm54vmHJx5t7P3MjWSsda9pLKdSzKziA+TrQcLb_Q@mail.gmail.com>
Date:   Mon, 18 Jul 2022 15:34:51 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.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>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>,
        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 Mon, Jul 18, 2022 at 2:43 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> FWIW, when I was poking at this last week, I found that -falign-function
> only seems to apply to the normal .text section and not to random other
> sections with text we create.
>
> Or rather, I was seeind a lot of unaligned functions that all had custom
> sections despite explicitly using the (what I thought was a global)
> function alignment toggle.

Hmm. This triggers a memory..

I think we may have two different issues at play.

One is that I think our linker script only aligns code sections to 8
bytes by default. Grep for ALIGN_FUNCTION.

And I think that any .align directive (or .p2align) only aligns
relative to that section, so if the section itself wasn't aligned, it
doesn't help to have some alignment within the section.

I may be wrong.

But I can definitely see gcc not aligning functions too, and doing a

    nm vmlinux | grep ' t ' | grep -v '0 t ' | grep -v '\.cold$' | sort

shows a _lot_ of them for me.

I think the main cause is that the ACPI code builds with

    ccflags-y                       := -Os -D_LINUX -DBUILDING_ACPICA

and that '-Os' will disable all function alignment. I think there's a
few other places that do that too.

I don't see the same effect in my clang build, so I think that -Os
behavior is likely gcc-specific.

In my clang build, I do see a few unaligned function symbols, but they
seem to be all our own assembler ones (eg "nested_nmi") and they seem
to be intentional (ie that "nested_nmi" thing is in the middle of the
"asm_exc_nmi" function, which is the real thing and which _is_
aligned).

                Linus