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

[Naveen N. Rao]

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@xxxxxxxxxxxxxxxxxx

> 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