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

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Message-Id: <20200714205204.210a558f0560d8feae19e55d@kernel.org>
Date:   Tue, 14 Jul 2020 20:52:04 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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 Mon, 13 Jul 2020 08:05:49 +0300
Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com> wrote:

> On Fri, Jul 10, 2020 at 12:49:10PM +0200, Peter Zijlstra wrote:
> > 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.
> 
> Out of pure interest, how does it handle a jump (as the original
> opcode), given that it single steps a copy?

Yes, the jump will be executed with a single-step on the copy buffer
and kprobes (on x86) fixes up the result, this means we modifies
the regs->ip. Also, there are some architectures which emulate the
jump instead of single-stepping.

> 
> > 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.
> 
> Right, a relative jump is placed instead of 0xcc to the breakpoint?

Yes, a relative (far) jump is used. So the target address (copied buffer)
must be in +-2GB range from the jump.

Thank you,

-- 
Masami Hiramatsu <mhiramat@...nel.org>