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Date:   Tue, 01 Mar 2022 22:33:52 +0530
From:   "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Subject: Re: [PATCH v2 15/39] x86/ibt,kprobes: Fix more +0 assumptions
To:     Masami Hiramatsu <mhiramat@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     alexei.starovoitov@gmail.com, alyssa.milburn@intel.com,
        andrew.cooper3@citrix.com, hjl.tools@gmail.com,
        joao@overdrivepizza.com, jpoimboe@redhat.com,
        keescook@chromium.org, linux-kernel@vger.kernel.org,
        mark.rutland@arm.com, mbenes@suse.cz, ndesaulniers@google.com,
        rostedt@goodmis.org, samitolvanen@google.com, x86@kernel.org
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        <20220224151322.892372059@infradead.org>
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Hi Peter,

Peter Zijlstra wrote:
> On Mon, Feb 28, 2022 at 03:07:05PM +0900, Masami Hiramatsu wrote:
>> Hi Peter,
>>=20
>> So, instead of this change, can you try below?
>> This introduce the arch_adjust_kprobe_addr() and use it in the kprobe_ad=
dr()
>> so that it can handle the case that user passed the probe address in=20
>> _text+OFFSET format.
>=20
> It works a little... at the very least it still needs
> arch_kprobe_on_func_entry() allowing offset 4.
>=20
> But looking at this, we've got:
>=20
> kprobe_on_func_entry(addr, sym, offset)
>   _kprobe_addr(addr, sym, offset)
>     if (sym)
>       addr =3D kprobe_lookup_name()
>            =3D 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)
>=20
> Which is _3_ kallsyms lookups and 3 weak/arch hooks.
>=20
> Surely we can make this a little more streamlined? The below seems to
> work.
>=20
> 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=20
on powerpc:
http://lkml.kernel.org/r/cover.1645096227.git.naveen.n.rao@linux.vnet.ibm.c=
om

>=20
> 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=20
place of kprobe_lookup_name() in response to Masami in the other thread,=20
but this is looking better.

>=20
> ---
>  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