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Subject: Re: [PATCH 1/4] x86/thread_info: introduce ->ftrace_int3_stack member
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20190427100639.15074-2-nstange@suse.de>
Date:   Sun, 28 Apr 2019 10:41:10 -0700
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, 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>,
        Andy Lutomirski <luto@kernel.org>,
        Joerg Roedel <jroedel@suse.de>, linux-kernel@vger.kernel.org,
        live-patching@vger.kernel.org, linux-kselftest@vger.kernel.org
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References: <20190427100639.15074-1-nstange@suse.de> <20190427100639.15074-2-nstange@suse.de>
To:     Nicolai Stange <nstange@suse.de>
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> On Apr 27, 2019, at 3:06 AM, Nicolai Stange <nstange@suse.de> wrote:
>=20
> Before actually rewriting an insn, x86' DYNAMIC_FTRACE  implementation
> places an int3 breakpoint on it. Currently, ftrace_int3_handler() simply
> treats the insn in question as nop and advances %rip past it.

How does this not crash all the time?

> An upcoming
> patch will improve this by making the int3 trap handler emulate the call
> insn. To this end, ftrace_int3_handler() will be made to change its iret
> frame's ->ip to some stub which will then mimic the function call in the
> original context.
>=20
> Somehow the trapping ->ip address will have to get communicated from
> ftrace_int3_handler() to these stubs though.

Cute.  But this should get =E2=80=9Cftrace=E2=80=9D removed from the name, s=
ince it=E2=80=99s potentially more generally useful.  Josh wanted something l=
ike this for static_call.

> Note that at any given point
> in time, there can be at most four such call insn emulations pending:
> namely at most one per "process", "irq", "softirq" and "nmi" context.
>=20

That=E2=80=99s quite an assumption. I think your list should also contain ex=
ception, exceptions nested inside that exception, and machine check, at the v=
ery least.  I=E2=80=99m also wondering why irq and softirq are treated separ=
ately.

All this makes me think that one of the other solutions we came up with last=
 time we discussed this might be better.