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

[Serge E. Hallyn]

Quoting Pavel Machek (pavel at ucw.cz):
> Hi!
> 
> > This patchset is a part of an effort to change some syscalls behavior for
> > checkpoint restart.
> > 
> > When restarting an object that has previously been checkpointed, its state
> > should be unchanged compared to the checkpointed image.
> > For example, a restarted process should have the same upid nr as the one it
> > used to have when being checkpointed; an ipc object should have the same id
> > as the one it had when the checkpoint occured.
> > Also, talking about system V ipcs, they should be restored with the same
> > state (e.g. in terms of pid of last operation).
> > 
> > This means that several syscalls should not behave in a default mode when
> > they are called during a restart phase.
> > 
> > One solution consists in defining a new syscall for each syscall that is
> > called during restart:
> >  . sys_fork_with_id() would fork a process with a predefined id.
> >  . sys_msgget_with_id() would create a msg queue with a predefined id
> >  . sys_semget_with_id() would create a semaphore set with a predefined id
> >  . etc,
> > 
> > This solution requires defining a new syscall each time we need an existing
> > syscall to behave in a non-default way.
> 
> Yes, and I believe that's better than...
> 
> > 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.
> 									Pavel

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.

-serge
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