Re: [PATCH 2/2] clone.2: added clone3() set_tid information

[Adrian Reber <areber@redhat.com>]

On Thu, Nov 28, 2019 at 06:24:05PM +0100, Christian Brauner wrote:
> On Thu, Nov 28, 2019 at 01:46:50PM +0100, Adrian Reber wrote:
> > Signed-off-by: Adrian Reber <areber@redhat.com>
> > ---
> >  man2/clone.2 | 90 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 90 insertions(+)
> > 
> > diff --git a/man2/clone.2 b/man2/clone.2
> > index 076b9258e..59c13ec35 100644
> > --- a/man2/clone.2
> > +++ b/man2/clone.2
> > @@ -195,6 +195,8 @@ struct clone_args {
> >      u64 stack;        /* Pointer to lowest byte of stack */
> >      u64 stack_size;   /* Size of stack */
> >      u64 tls;          /* Location of new TLS */
> > +    u64 set_tid;      /* Pointer to a \fIpid_t\fP array */
> > +    u64 set_tid_size; /* Number of elements in \fIset_tid\fP */
> >  };
> >  .EE
> >  .in
> > @@ -262,6 +264,8 @@ flags & 0xff	exit_signal
> >  stack	stack
> >  \fP---\fP	stack_size
> >  tls	tls	See CLONE_SETTLS
> > +\fP---\fP	set_tid	See below for details
> > +\fP---\fP	set_tid_size
> >  .TE
> >  .RE
> >  .\"
> > @@ -285,6 +289,74 @@ options when waiting for the child with
> >  If no signal (i.e., zero) is specified, then the parent process is not signaled
> >  when the child terminates.
> >  .\"
> > +.SS The set_tid array
> > +.PP
> > +The
> > +.I set_tid
> > +array is used to select a certain PID for the process to be created by
> > +.BR clone3 ().
> > +If the PID of the newly created process should only be set for the current
> > +PID namespace or in the newly created PID namespace (if
> > +.I flags
> > +contains
> > +.BR CLONE_NEWPID )
> > +then the first element in the
> > +.I set_tid
> > +array has to be the desired PID and
> > +.I set_tid_size
> > +needs to be 1.
> > +.PP
> > +If the PID of the newly created process should have a certain value in
> > +multiple PID namespaces the
> > +.I set_tid
> > +array can have multiple entries. The first entry defines the PID in the most
> > +nested PID namespace and all following entries contain the PID of the
> > +corresponding parent PID namespace. The number of PID namespaces in which a PID
> > +should be set is defined by
> > +.I set_tid_size
> > +which cannot be larger than the number of currently nested PID namespaces.
> 
> "It's upper cap is the kernel-enforced general nesting limit."
> or sm like that

Is that an addition to my sentence or a replacement. I think at this
point it is more important to point out that it cannot be larger than
the number of currently nested PID namespaces. Later (at EPERM) I am
also mentioning that it cannot be larger than the maximum number of
nested PID namespaces. The code does indeed check if set_tid_size is
larger than the maximum number of possible nested PID namespaces for
the user, I think, when calling clone3(), it is more relevant that
set_tid_size is not larger than the number of currently nested PID
namespaces. The maximum number of possible nested PID namespaces is more
likely enforced during unshare() or CLONE_NEWPID (which could be
happening at the same point in time as set_tid_size larger than maximum
number of nested PID namespace).

This definitely feels like too much discussion for a single sentence ;)

I can add a sentence about the maximum number of nested PID namespaces
here in addition to the one at EPERM.
I do not think it is relevant for the user at this point in time.

		Adrian