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

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hello Adrian,

On 11/28/19 1:46 PM, 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

s/is used/may be used/

Because it's not required to use this array, right? I mean, the
default is that the kernel chooses the PIDs. Perhaps this needs to be
more clearly stated at the start of this subsection.

How about:

[[
By default, the kernel chooses the next sequential PID for the new
process in each of the PID namespaces where it is present.
When creating a process with
.BR clone3 (),
the
.I set_tid
array can be used to select specific PIDs for the process in some
or all of the PID namespaces where it is present.
]]

?

> +.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

most *deeply* nested

> +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.
> +.PP
> +To create a process with the following PIDs:
> +.RS
> +.TS
> +lb lb
> +l l .
> +PID NS level	Requested PID
> +0 (host)	31496
> +1	42
> +2	7
> +.TE
> +.RE
> +.PP
> +The
> +.I set_tid
> +array would need to be filled with:
> +.PP
> +.EX
> +	set_tid[0] = 7;
> +	set_tid[1] = 42;
> +	set_tid[2] = 31496;
> +	set_tid_size = 3;
> +.EE
> +.PP
> +If only the PID of the two innermost PID namespaces
> +should be defined it needs to be set like this:
> +.PP
> +.EX
> +	set_tid[0] = 7;
> +	set_tid[1] = 42;
> +	set_tid_size = 2;
> +.EE
> +.PP
> +The PID in the PID namespaces outside the two innermost PID namespaces
> +is then selected the same way as any other PID is selected.
> +.PP
> +Only a privileged process
> +.RB ( CAP_SYS_ADMIN )
> +can set
> +.I set_tid
> +to select a PID for the process to be created.
> +.\"
>   .SS The flags mask
>   .PP
>   Both
> @@ -1379,6 +1451,16 @@ in the
>   .I flags
>   mask.
>   .TP
> +.BR EINVAL " (" clone3 "() only)"
> +.I set_tid_size
> +larger than current number of nested PID namespaces or maximum number of
> +nested PID namespaces was specified.
> +.TP
> +.BR EINVAL " (" clone3 "() only)"
> +If one of the PIDs specified in
> +.I set_tid
> +was an invalid PID.
> +.TP
>   .B ENOMEM
>   Cannot allocate sufficient memory to allocate a task structure for the
>   child, or to copy those parts of the caller's context that need to be
> @@ -1450,6 +1532,14 @@ mask and the caller is in a chroot environment
>   (i.e., the caller's root directory does not match the root directory
>   of the mount namespace in which it resides).
>   .TP
> +.BR EPERM " (" clone3 "() only)"
> +If
> +.I set_tid
> +with
> +.I set_tid_size
> +larger than 0 was specified by an unprivileged process (process without
> +\fBCAP_SYS_ADMIN\fP).
> +.TP
>   .BR ERESTARTNOINTR " (since Linux 2.6.17)"
>   .\" commit 4a2c7a7837da1b91468e50426066d988050e4d56
>   System call was interrupted by a signal and will be restarted.

Thanks,

Michael