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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 25 Mar 2020 11:26:38 -0700
Message-ID: <CAHk-=wiPrOd_yM5NGHQDE3AFtNWqV=MDFno2xgZOYV3jwGQU4w@mail.gmail.com>
Subject: Re: [RESEND][PATCH v3 14/17] static_call: Add static_cond_call()
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Jason Baron <jbaron@akamai.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Nadav Amit <namit@vmware.com>,
        Peter Anvin <hpa@zytor.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>
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On Wed, Mar 25, 2020 at 11:13 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> To clarify; the problem is a task getting preempted with its RIP at the
> RET. Then when we rewrite the text to be a CALL/JMP.d32 it will read
> garbage (1 byte into the displacement of the instruction) instead of a
> RET when it resumes.

Yeah, I realized it when you mentioned it. I was trying to come up
with some clever way to avoid it, but can't see anything.

I can come up with insane models - you could replace the xor;ret
sequence with a jump to a trampoline where you've aligned the target
trampoline so that the third byte in the "jmp xxx" remains a 'ret'
instruction. Then replace _that_ one with a "call_rcu()" callback.
Wild handwaving of "I'm sure this could be made to work".

But nothing remotely sane comes to mind.

              Linus