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

[Serge E. Hallyn]

Quoting Pavel Machek (pavel at ucw.cz):
> On Mon 2008-07-07 14:01:19, Serge E. Hallyn wrote:
> > 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.
> > 
> > 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.

-serge
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