Hitachi djprobe mechanism

Thu Jul 28 23:53:00 GMT 2005

There are more efficient ways of implementing a jmp type hook - see the
kernel hooks package, where we evloved past this string of 5 no-ops
implementation Here we moved an immediate value - 1 byte - into a reg and
jumped on the reg being non-zero. To spring the hook we stored the one
immediate byte in the mov instruction. This technique works quite well on
IA64 where one can use a predicate register for the same purpose.
- -
Richard J Moore
IBM Advanced Linux Response Team - Linux Technology Centre
MOBEX: 264807; Mobile (+44) (0)7739-875237
Office: (+44) (0)1962-817072

             Keshavamurthy                                                 
             Anil S                                                        
             <anil.s.keshava                                            To 
             murthy@intel.co         Mathieu Desnoyers                     
             m>                      <compudj@krystal.dyndns.org>          
                                                                        cc 
             28/07/2005              Keshavamurthy Anil S                  
             21:34                   <anil.s.keshavamurthy@intel.com>,     
                                     Karim Yaghmour <karim@opersys.com>,   
                                     Masami Hiramatsu                      
             Please respond          <masami.hiramatsu@gmail.com>, Masami  
                   to                Hiramatsu                             
              Keshavamurthy          <hiramatu@sdl.hitachi.co.jp>, Roland  
                 Anil S              McGrath <roland@redhat.com>, Richard  
                                     J Moore/UK/IBM@IBMGB,                 
                                     systemtap@sources.redhat.com,         
                                     sugita@sdl.hitachi.co.jp, Satoshi     
                                     Oshima <soshima@redhat.com>,          
                                     michel.dagenais@polymtl.ca            
                                                                       bcc 

                                                                   Subject 
                                     Re: Hitachi djprobe mechanism         

On Thu, Jul 28, 2005 at 02:32:36PM -0400, Mathieu Desnoyers wrote:
> The goal of this int3 is indeed that, if another thread on another CPU
falls
> into it, the int3 handler will change its own return address to fall into
the
> push instruction, right after the nops.
Ah..I see you point now.

>
> For the IPI :
>
> - the on_each_cpu should be called from a context where no spinlock is
held. The
>   context from which it is called could be protected by a flag which
would cause
>   any concurrent processor to fail if they try to follow the same "code
>   alteration" function.
> - the IPI handler does not need to spin at all. It just returns. It only
has to
>   inform us about when interrupt disabled zone has ended. As this zone
will not
>   be reentered due to the int3 instruction, everything is fine.
If we are not spinning in IPI then I am fine.

>
> Or did I miss something ?

However I have one more issues to mentioni(for discussion sake), when we
have several nop
instructions(may be 4 nop instruction) in a landing zone, we have no
guarantee that
all of these nop instructions are with in a page boundary. This is very
important
because when you replace these instruction with a 5 byte jump instruction,
this
5 byte jump instruction should be with in a page boundary else the
processor
will generate unaligned instruction access voilation or it might
generate page fault while trying to execute an instruction which is a very
bad thing.
How are we going to address this issues?

Thanks,
Anil