Re: [PATCH 0/2] ptrace_vm: ptrace for syscall emulation virtual machines

["Américo Wang" <xiyou.wangcong@gmail.com>]

On Wed, Apr 08, 2009 at 02:18:51PM +0200, Renzo Davoli wrote:
>> > #endif
>> > #ifdef PTRACE_SINGLEBLOCK
>> >   case PTRACE_SINGLEBLOCK:
>> > #endif
>> > #ifdef PTRACE_SYSEMU
>> >   case PTRACE_SYSEMU:
>> >   case PTRACE_SYSEMU_SINGLESTEP:
>> > #endif
>> >   case PTRACE_SYSCALL:
>> >   case PTRACE_CONT:
>> >     return ptrace_resume(child, request, 0, data);
>> >+/* statements added for PTRACE_VM management */
>> >+#ifdef PTRACE_VM
>> >+  case PTRACE_VM:
>> >+#ifdef PTRACE_VM_SINGLESTEP
>> >+  case PTRACE_VM_SINGLESTEP:
>> >+#endif
>> >+#ifdef PTRACE_VM_SINGLEBLOCK
>> >+  case PTRACE_VM_SINGLEBLOCK:
>> >+#endif
>> >+    return ptrace_resume(child, PTRACE_VM_TAGS_MAPPING(request), addr, data);
>> >+#endif
>> >....
>> >  
>> 
>> Hmmm, I see your points. Thanks for your analysis.
>> 
>The "resume tags" SYSCALL, SINGLESTEP/SINGLEBLOCK, CONT give to ptrace
>the command to resume and indicate when ptrace must stop next time.
>The VM_SKIPCALL, VM_SKIPEXIT tags refer to the current system call.
>The two sets are independent, orthogonal.

I see.

>
>I may want to skip this system call and stop either at the next system call
>or at the next block, instruction or never.
>As usual, everything is possible with or without some tags, the difference
>in in this case in terms of clearness of the interface.

Yes, sure.

>
>If we'd provide only a PTRACE_VM call (namely a PTRACE_VM_SYSCALL)
>to resume up to the next syscall it was not possible to use it 
>to implement a virtualized "ptrace".
>The virtual ptrace call may need to stop the process after an instruction
>or a block as it was requested to do so.
>In this case the VM monitor should use PTRACE_SINGLE* without the
>VM_SKIP* optimization (maybe faking the execution of a getpid
>to skip a system call, like in the old times of User mode Linux).
>For a similar reason PTRACE_SYSEMY_SINGLESTEP was added in the kernel.
>
>WHy we should make life harder to VM monitor designer?
>
>We could also have a unique PTRACE_VM tag and encode both
>SYSCALL/SINGLESTEP/SINGLEBLOCK/BLOCK
>and
>SKIPCALL/SKIPEXIT
>in different bits inside the addr field.
>
>Again, this is a trick to use just one tag.
>It is a matter of values.
>Efficiency is the meaning of this patch, so it is a conditio
>sine qua non.
>Apart from efficiency, what do we want most?
>Clearness of interface design?
>Back compatibility for very improbable cases?
>
>I bet on the former, usually it is an insurance for the future.

This is almost exactly what I said in my first mail, I
have no objection to your patch, I like it, I just wanted to try
to find a balance.

Anyway, I will test your patch tomorrow, and will send you more
feedbacks soon.

Thanks.

-- 
Live like a child, think like the god.