Re: [RFC] kreplace: Rebootless kernel updates

[Nikanth Karthikesan]

On Friday 21 November 2008 19:08:00 Ananth N Mavinakayanahalli wrote:
> On Fri, Nov 21, 2008 at 05:20:25PM +0530, Nikanth Karthikesan wrote:
> > This RFC patch adds support for limited form of rebootless kernel
> > patching even without building the entire kernel.
> >
> > When looking for a shortcut to avoid the rebuild/reboot cycle when
> > hacking the kernel - the ksplice[1] was posted. This patch extends
> > kprobes to do something similar, which would require even lesser time to
> > _experiment_ with the running kernel.
>
> There have been other implementations of this feature, I am sure quite a
> few people would have objections to having this as part of the kernel :-)
>

I think the few would be quiet large ;)

> > This small patch extends jprobes so that the jprobe's handler is executed
> > but skips executing the actual function. But this has its own limitations
> > such as Cannot access symbols not exported for modules (ofcourse hacks
> > like pointers[2] can be used.), problems related to return values[3],
> > etc... This is currently a x86_64 only _hack_.
>
> There are many other issues too... How do you enforce correct usage of this
> infrastrucutre? What prevents people from overriding core-kernel
> functions with their own?
>

I agree, this is incomplete.

> Kprobes themselves provide enough ammunition to users to shoot themselves
> in the foot, but this is way more dangerous than that.
> ...
>

Yes. 

> > The kernel patch for kreplace, an extension to kprobes to do hot
> > patching. Only on x86_64. Do not try this on any other platforms without
> > modifying.
> >
> > Signed-off-by: Nikanth Karthikesan <knikanth@xxxxxxx>
> >
> > ---
> >  arch/x86/kernel/kprobes.c |   18 ++++++++++++++----
> >  include/linux/kprobes.h   |    5 ++++-
> >  kernel/kprobes.c          |   37 ++++++++++++++++++++++++++++++++-----
> >  3 files changed, 50 insertions(+), 10 deletions(-)
> >
> > diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c
> > index 6c27679..9e2ea2b 100644
> > --- a/arch/x86/kernel/kprobes.c
> > +++ b/arch/x86/kernel/kprobes.c
> > @@ -340,9 +340,13 @@ static void __kprobes fix_riprel(struct kprobe *p)
> >  #endif
> >  }
> >
> > -static void __kprobes arch_copy_kprobe(struct kprobe *p)
> > +static void __kprobes arch_copy_kprobe(struct kprobe *p, int replace)
> >  {
> > -	memcpy(p->ainsn.insn, p->addr, MAX_INSN_SIZE *
> > sizeof(kprobe_opcode_t)); +	if (replace)
> > +		memcpy(p->ainsn.insn, ((unsigned char []){0xc3}), 1);
> > +	else
> > +		memcpy(p->ainsn.insn, p->addr,
> > +				MAX_INSN_SIZE * sizeof(kprobe_opcode_t));
>
> This is limiting - especially since we allow multiple probes at the same
> address. You modify the instruction underneath to always be a ret.
>
> It also breaks existing functionality -- especially aggregate probes and
> return probes.
>

Oh, yeah. And it is possible to implement this correctly in other ways! 

> ...
>
> > diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
> > index 497b1d1..91e83fb 100644
> > --- a/include/linux/kprobes.h
> > +++ b/include/linux/kprobes.h
> > @@ -202,7 +202,7 @@ static inline int init_test_probes(void)
> >  #endif /* CONFIG_KPROBES_SANITY_TEST */
> >
> >  extern struct mutex kprobe_mutex;
> > -extern int arch_prepare_kprobe(struct kprobe *p);
> > +extern int arch_prepare_kprobe(struct kprobe *p, int replace);
> >  extern void arch_arm_kprobe(struct kprobe *p);
> >  extern void arch_disarm_kprobe(struct kprobe *p);
> >  extern int arch_init_kprobes(void);
> > @@ -240,11 +240,14 @@ int register_kprobes(struct kprobe **kps, int num);
> >  void unregister_kprobes(struct kprobe **kps, int num);
> >  int setjmp_pre_handler(struct kprobe *, struct pt_regs *);
> >  int longjmp_break_handler(struct kprobe *, struct pt_regs *);
> > +int register_kreplace(struct jprobe *p);
> > +void unregister_kreplace(struct jprobe *p);
> >  int register_jprobe(struct jprobe *p);
> >  void unregister_jprobe(struct jprobe *p);
> >  int register_jprobes(struct jprobe **jps, int num);
> >  void unregister_jprobes(struct jprobe **jps, int num);
> >  void jprobe_return(void);
> > +void set_ax(unsigned long);
>
> Please choose a better arch agnostic naming scheme -- set_ret()?
>

I did write more helpers to return values of different sizes, but only their 
function names look good. So, I didnt post them as I wanted to know the 
comments for the idea first.

Thanks a lot for your comments. 

Thanks
Nikanth





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