linux-kernel - Re: [PATCH v2 15/39] x86/ibt,kprobes: Fix more +0 assumptions

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Message-Id: <1646153789.geynpzwbid.naveen@linux.ibm.com>
Date:   Tue, 01 Mar 2022 22:33:52 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     alexei.starovoitov@...il.com, alyssa.milburn@...el.com,
        andrew.cooper3@...rix.com, hjl.tools@...il.com,
        joao@...rdrivepizza.com, jpoimboe@...hat.com,
        keescook@...omium.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, mbenes@...e.cz, ndesaulniers@...gle.com,
        rostedt@...dmis.org, samitolvanen@...gle.com, x86@...nel.org
Subject: Re: [PATCH v2 15/39] x86/ibt,kprobes: Fix more +0 assumptions

Hi Peter,

Peter Zijlstra wrote:
> On Mon, Feb 28, 2022 at 03:07:05PM +0900, Masami Hiramatsu wrote:
>> Hi Peter,
>> 
>> So, instead of this change, can you try below?
>> This introduce the arch_adjust_kprobe_addr() and use it in the kprobe_addr()
>> so that it can handle the case that user passed the probe address in 
>> _text+OFFSET format.
> 
> It works a little... at the very least it still needs
> arch_kprobe_on_func_entry() allowing offset 4.
> 
> But looking at this, we've got:
> 
> kprobe_on_func_entry(addr, sym, offset)
>   _kprobe_addr(addr, sym, offset)
>     if (sym)
>       addr = kprobe_lookup_name()
>            = kallsyms_lookup_name()
>     arch_adjust_kprobe_addr(addr+offset)
>       skip_endbr()
>         kallsyms_loopup_size_offset(addr, ...)
>   kallsyms_lookup_size_offset(addr, NULL, &offset)
>   arch_kprobe_on_func_entry(offset)
> 
> Which is _3_ kallsyms lookups and 3 weak/arch hooks.
> 
> Surely we can make this a little more streamlined? The below seems to
> work.
> 
> I think with a little care and testing it should be possible to fold all
> the magic of PowerPC's kprobe_lookup_name() into this one hook as well,
> meaning we can get rid of kprobe_lookup_name() entirely.  Naveen?

This is timely. I've been looking at addressing a similar set of issues 
on powerpc:
http://lkml.kernel.org/r/cover.1645096227.git.naveen.n.rao@linux.vnet.ibm.com

> 
> This then gets us down to a 1 kallsyms call and 1 arch hook. Hmm?

I was going to propose making _kprobe_addr() into a weak function in 
place of kprobe_lookup_name() in response to Masami in the other thread, 
but this is looking better.

> 
> ---
>  arch/powerpc/kernel/kprobes.c  |   34 +++++++++++++++---------
>  arch/x86/kernel/kprobes/core.c |   17 ++++++++++++
>  include/linux/kprobes.h        |    3 +-
>  kernel/kprobes.c               |   56 ++++++++++++++++++++++++++++++++++-------
>  4 files changed, 87 insertions(+), 23 deletions(-)

I will take a closer look at this tomorrow and revert.


Thanks,
- Naveen