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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 3 May 2019 11:57:22 -0700
Message-ID: <CAHk-=wjrOLqBG1qe9C3T=fLN0m=78FgNOGOEL22gU=+Pw6Mu9Q@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>,
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        Mimi Zohar <zohar@linux.ibm.com>,
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        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>
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On Fri, May 3, 2019 at 9:21 AM Andy Lutomirski <luto@amacapital.net> wrote:
>
> So here=E2=80=99s a somewhat nutty suggestion: how about we tweak the 32-=
bit entry code to emulate the sane 64-bit frame, not just for int3 but alwa=
ys?

What would the code actually end up looking like? I don't necessarily
object, since that kernel_stack_pointer() thing certainly looks
horrible, but honestly, my suggestion to just pass in the 'struct
pt_regs' and let the call emulation fix it up would have also worked,
and avoided that bug (and who knows what else might be hiding).

I really think that you're now hitting all the special case magic
low-level crap that I wanted to avoid.

                   Linus