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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 7 May 2019 07:57:22 -0700
Message-ID: <CAHk-=wjMFRZ7bX4NFc1+=Sy5NDHh=8rAvK-JBqB+HySM1W-Nbg@mail.gmail.com>
Subject: Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Nicolai Stange <nstange@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jiri Kosina <jikos@kernel.org>,
        Miroslav Benes <mbenes@suse.cz>,
        Petr Mladek <pmladek@suse.com>,
        Joe Lawrence <joe.lawrence@redhat.com>,
        Shuah Khan <shuah@kernel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Mimi Zohar <zohar@linux.ibm.com>,
        Juergen Gross <jgross@suse.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nayna Jain <nayna@linux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Joerg Roedel <jroedel@suse.de>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>, stable <stable@vger.kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>
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On Tue, May 7, 2019 at 7:48 AM Andy Lutomirski <luto@amacapital.net> wrote:
>
> IOW I think your trick only works if the old and new states are CALL, but=
 we don=E2=80=99t know that until we=E2=80=99ve looked up the record, at wh=
ich point we can just use the result of the lookup.

It would indeed only work for call instructions. I was thinking we'd
know that because we only ever batch up call instructions, though.

But it doesn't matter. I was looking at the ftrace code because I
thought there was some subtle timing bug or race or similar. But it
turned out my "memmove()" was the problem. See the patch I just sent
out.

               Linus