linux-kernel - Re: [PATCH RFC] kprobes: Remove MODULES dependency

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Message-ID: <20200710104910.GG4800@hirez.programming.kicks-ass.net>
Date:   Fri, 10 Jul 2020 12:49:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Will Deacon <will@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>, Alexandre Ghiti <alex@...ti.fr>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Peter Collingbourne <pcc@...gle.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Jiri Olsa <jolsa@...hat.com>
Subject: Re: [PATCH RFC] kprobes: Remove MODULES dependency

On Fri, Jul 10, 2020 at 01:36:38PM +0300, Jarkko Sakkinen wrote:
> Just so that I know (and learn), what did exactly disable optprobes?

So regular, old-skool style kprobe is:

  - copy original instruction out
  - replace instruction with breakpoint (int3 on x86)
  - have exception handler return to the copied instruction with
    single-step on
  - have single step exception handler return to the original
    instruction stream

which is 2 exceptions.

optprobes avoid the single-step by not only writing a single
instruction, but additionally placing a JMP instruction behind it such
that it will automagically continue in the original instruction stream.

This brings the requirement that the copied instruction is placed
within the JMP displacement of the regular kernel text (s32 on x86).

module_alloc() ensures the memory provided is within that range.