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Subject: Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <CAHk-=wjGNx8xcwg=7nE_0-nLQ_d4UALHvJ8O+TurbA25n8MyNg@mail.gmail.com>
Date:   Sat, 4 May 2019 13:12:09 -0700
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>
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References: <20190501202830.347656894@goodmis.org> <20190501203152.397154664@goodmis.org> <20190501232412.1196ef18@oasis.local.home> <20190502162133.GX2623@hirez.programming.kicks-ass.net> <CAHk-=wijZ-MD4g3zMJ9W2r=h8LUWneiu29OWuxZEoSfAF=0bhQ@mail.gmail.com> <20190502181811.GY2623@hirez.programming.kicks-ass.net> <CAHk-=wi6A9tgw=kkPh5Ywqt687VvsVEjYXVkAnq0jpt0u0tk6g@mail.gmail.com> <20190502202146.GZ2623@hirez.programming.kicks-ass.net> <CAHk-=wh8bi5c_GkyjPtDAiaXaZRqtmhWs30usUvs4qK_F+c9tg@mail.gmail.com> <20190503152405.2d741af8@gandalf.local.home> <CAHk-=wiA-WbrFrDs-kOfJZMXy4zMo9-SZfk=7B-GfmBJ866naw@mail.gmail.com> <20190503184919.2b7ef242@gandalf.local.home> <CAHk-=wh2vPLvsGBi6JtmEYeqHxB5UpTzHDjY5JsWG=YR0Lypzw@mail.gmail.com> <20190504001756.17fad840@oasis.local.home> <CAHk-=wiuSFbv_rELND-BLWcP0GSZ0yF=xOAEcf61GE3bU9d=yg@mail.gmail.com> <CAHk-=wjGNx8xcwg=7nE_0-nLQ_d4UALHvJ8O+TurbA25n8MyNg@mail.gmail.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On May 4, 2019, at 11:59 AM, Linus Torvalds <torvalds@linux-foundation.org=
> wrote:
>=20
> On Fri, May 3, 2019 at 10:08 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>=20
>> I'll look at it tomorrow, but I think this actually makes unnecessary cha=
nges.
>>=20
>> In particular, I think we could keep the existing entry code almost uncha=
nged with this whole approach.
>=20
> So here's what I *think* should work. Note that I also removed your
> test-case code, because it really didn't have a chance in hell of
> working. Doing that
>=20
>        int3_emulate_call(regs, (unsigned long)&int3_magic);
>=20
> inside of int3_exception_notify() could not possibly be valid, since
> int3_emulate_call() returns the new pt_regs that need to be used, and
> throwing it away is clearly wrong.
>=20
> So you can't use a register_die_notifier() to try to intercept the
> 'int3' error and then do it manually, it needs to be done by the
> ftrace_int3_handler() code that actually returns the new regs, and
> where do_kernel_int3() will then return it to the low-level handler.

I hate register_die_notifier(), so I consider this a plus. I=E2=80=99ve occa=
sionally considered removing the ability for the notifiers to skip normal pr=
ocessing, because, as it stands, figuring out what actually happens in the t=
rap handlers is almost impossible.

It generally looks sane to me.

As an aside, is it even *possible* to get #BP from v8086 mode?  On a quick S=
DM read, the INT3 instruction causes #GP if VM=3D1 and IOPL<3.  And, if we a=
llow vm86() to have IOPL=3D3, we should just remove that ability. It=E2=80=99=
s nuts.

(We should maybe consider a config option for iopl() that defaults off. We=E2=
=80=99ve supported ioperm() for a long, long time.)=