fs, proc: introduce CONFIG_PROC_CHILDREN

Commit 818411616b ("fs, proc: introduce /proc/<pid>/task/<tid>/children
entry") introduced the children entry for checkpoint restore and the
file is only available on kernels configured with CONFIG_EXPERT and
CONFIG_CHECKPOINT_RESTORE.

This is available in most distributions (Fedora, Debian, Ubuntu, CoreOS)
because they usually enable CONFIG_EXPERT and CONFIG_CHECKPOINT_RESTORE.
But Arch does not enable CONFIG_EXPERT or CONFIG_CHECKPOINT_RESTORE.

However, the children proc file is useful outside of checkpoint restore.
I would like to use it in rkt.  The rkt process exec() another program
it does not control, and that other program will fork()+exec() a child
process.  I would like to find the pid of the child process from an
external tool without iterating in /proc over all processes to find
which one has a parent pid equal to rkt.

This commit introduces CONFIG_PROC_CHILDREN and makes
CONFIG_CHECKPOINT_RESTORE select it.  This allows enabling
/proc/<pid>/task/<tid>/children without needing to enable
CONFIG_CHECKPOINT_RESTORE and CONFIG_EXPERT.

Alban tested that /proc/<pid>/task/<tid>/children is present when the
kernel is configured with CONFIG_PROC_CHILDREN=y but without
CONFIG_CHECKPOINT_RESTORE

Signed-off-by: Iago López Galeiras <iago@endocode.com>
Tested-by: Alban Crequy <alban@endocode.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Serge Hallyn <serge.hallyn@canonical.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Djalal Harouni <djalal@endocode.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/Kconfig

@@ -71,3 +71,7 @@ config PROC_PAGE_MONITOR
 	  /proc/pid/smaps, /proc/pid/clear_refs, /proc/pid/pagemap,
 	  /proc/kpagecount, and /proc/kpageflags. Disabling these
           interfaces will reduce the size of the kernel by approximately 4kb.
+
+config PROC_CHILDREN
+	bool "Include /proc/<pid>/task/<tid>/children file"
+	default n

fs/proc/array.c

@@ -577,7 +577,7 @@ int proc_pid_statm(struct seq_file *m, struct pid_namespace *ns,
 	return 0;
 }
 
-#ifdef CONFIG_CHECKPOINT_RESTORE
+#ifdef CONFIG_PROC_CHILDREN
 static struct pid *
 get_children_pid(struct inode *inode, struct pid *pid_prev, loff_t pos)
 {
@@ -700,4 +700,4 @@ const struct file_operations proc_tid_children_operations = {
 	.llseek  = seq_lseek,
 	.release = children_seq_release,
 };
-#endif /* CONFIG_CHECKPOINT_RESTORE */
+#endif /* CONFIG_PROC_CHILDREN */

fs/proc/base.c

@@ -3109,7 +3109,7 @@ static const struct pid_entry tid_base_stuff[] = {
 	ONE("stat",      S_IRUGO, proc_tid_stat),
 	ONE("statm",     S_IRUGO, proc_pid_statm),
 	REG("maps",      S_IRUGO, proc_tid_maps_operations),
-#ifdef CONFIG_CHECKPOINT_RESTORE
+#ifdef CONFIG_PROC_CHILDREN
 	REG("children",  S_IRUGO, proc_tid_children_operations),
 #endif
 #ifdef CONFIG_NUMA

init/Kconfig

@@ -1131,6 +1131,7 @@ endif # CGROUPS
 
 config CHECKPOINT_RESTORE
 	bool "Checkpoint/restore support" if EXPERT
+	select PROC_CHILDREN
 	default n
 	help
 	  Enables additional kernel features in a sake of checkpoint/restore.