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

Pavel Machek pavel at ucw.cz
Tue Jul 8 14:53:15 PDT 2008


On Tue 2008-07-08 16:47:21, Serge E. Hallyn wrote:
> Quoting Pavel Machek (pavel at ucw.cz):
> > Hi!
> > 
> > >>> 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.
> > 
> > > I had a look at the lwn.net article that describes the sys_indirect() 
> > > interface.
> > > It does exactly what we need here, so I do like it, but it has the same 
> > > drawbacks as the one you're complaining about:
> > > . a new field is needed in the task structure
> > > . looks like many people found it ugly...
> > 
> > > Now, coming back to what I'm proposing: what we need is actually to change 
> > > the behavior of *existing* syscalls, since we are in a very particular 
> > > context (restarting an application).
> > 
> > Changing existing syscalls is _bad_: for backwards compatibility
> > reasons. strace will be very confusing to read, etc...
> 
> I dunno...  if you normally open(), you get back a random fd.  If you do
> it having set the next_id inadvertently, then as far as you know you get
> back a random fd, no?

Sorry?!

No, open does not return random fds. It allocates them bottom-up. So
you do not need any changes in open case.

(If you want to open "/foo/bar" as fd #50, open /dev/zero 49 times,
then open "/foo/bar"; bash already uses that trick.) 
								Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html


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