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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 2 Sep 2022 09:43:45 -0700
Message-ID: <CAHk-=wig7_=CpkvZXrbcM97pBGk5MCbVkA0yBGP2moiho-XS_Q@mail.gmail.com>
Subject: Re: [PATCH v2 37/59] x86/putuser: Provide room for padding
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        x86@kernel.org, Tim Chen <tim.c.chen@linux.intel.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Andrew Cooper <Andrew.Cooper3@citrix.com>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Johannes Wikner <kwikner@ethz.ch>,
        Alyssa Milburn <alyssa.milburn@linux.intel.com>,
        Jann Horn <jannh@google.com>, "H.J. Lu" <hjl.tools@gmail.com>,
        Joao Moreira <joao.moreira@intel.com>,
        Joseph Nuzman <joseph.nuzman@intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Juergen Gross <jgross@suse.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        K Prateek Nayak <kprateek.nayak@amd.com>,
        Eric Dumazet <edumazet@google.com>
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Precedence: bulk

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