linux-kernel - Re: [PATCH v2 37/59] x86/putuser: Provide room for padding

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Date:   Fri, 2 Sep 2022 09:43:45 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        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>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH v2 37/59] x86/putuser: Provide room for padding

So I don't hate this patch and it's probably good for consistency, but
I really think that the retbleed tracking could perhaps be improved to
let this be all unnecessary.

The whole return stack depth counting is already not 100% exact, and I
think we could just make the rule be that we don't track leaf
functions.

Why? It's just a off-by-one in the already not exact tracking. And -
perhaps equally importantly - leaf functions are very very common
dynamically, and I suspect it's trivial to see them.

Yes, yes, you could make objtool even smarter and actually do some
kind of function flow graph thing (and I think some people were
talking about that with the whole ret counting long long ago), but the
leaf function thing is the really simple low-hanging fruit case of
that.

            Linus