[RFC PATCH 0/5] Resend - Use procfs to change a syscall behavior

[Nadia Derbey]

Serge E. Hallyn wrote:
> Quoting Pavel Machek (pavel at ucw.cz):
> 
>>>>>>>An alternative to this solution consists in defining a new field in the
>>>>>>>task structure (let's call it next_syscall_data) that, if set, would change
>>>>>>>the behavior of next syscall to be called. The sys_fork_with_id() previously
>>>>>>>cited can be replaced by
>>>>>>> 1) set next_syscall_data to a target upid nr
>>>>>>> 2) call fork().
>>>>>>
>>>>>>...bloat task struct and
>>>>>>
>>>>>>
>>>>>>>A new file is created in procfs: /proc/self/task/<my_tid>/next_syscall_data.
>>>>>>>This makes it possible to avoid races between several threads belonging to
>>>>>>>the same process.
>>>>>>
>>>>>>...introducing this kind of uglyness.
>>>>>>
>>>>>>Actually, there were proposals for sys_indirect(), which is slightly
>>>>>>less ugly, but IIRC we ended up with adding syscalls, too.
>>>>>
>>>>>Silly question...
>>>>>
>>>>>Oren, would you object to defining sys_fork_with_id(),
>>>>>sys_msgget_with_id(), and sys_semget_with_id()?
>>>>>
>>>>>Eric, Pavel (Emelyanov), Dave, do you have preferences?
>>>>>
>>>>>For the cases Nadia has implemented here I'd be tempted to side with
>>>>>Pavel Machek, but once we get to things like open() and socket(), (a)
>>>>>the # new syscalls starts to jump, and (b) the per-syscall api starts to
>>>>>seem a lot more cumbersome.
>>>>
>>>>You should not need to modify open/socket. You can already select fd
>>>>by creatively using open/dup/close...
>>>
>>>That's what we do right now in cryo.  And if we end up patching up every
>>>API with separate syscalls, then we wouldn't create open_with_id().  But
>>>so long as the next_id were to exist, exploiting it in open is nigh on
>>>trivial and much nicer.
>>
>>Ok, so ignore previous email. You know how unix works.
>>
>>I believe you should just introduce syscalls you need. Yes,
>>introducing new syscalls is hard/expensive, but changing existing
>>syscalls is simply bad idea.
> 
> 
> Ok, thanks, Pavel.  I'm really far more inclined to agree with you than
> it probably sounds like.  I'll go ahead and implement a clone_with_id()
> syscall for starters later this week just as a comparison.
> 
> Unless, Nadia, you have already started that?

Actually, what I've started working on these days is replace the proc 
interface by a syscall to set the next_syscall_data field: I think this 
might help us avoid defining a precise list of the new syscalls we need?

Regards,
Nadia

> 
> 
>>So what new syscalls do you _really_ need? Not open_this_fd, nor
>>socket_this_fd.
> 
> 
> Oren, do you have a list of the syscalls which were modified to use the
> next_id in zap?
> 
> -serge
> 
>