[PATCH] pidfd: add NSpid entries to fdinfo

[Christian Brauner <christian.brauner@ubuntu.com>]

Currently, the fdinfo file of contains the field Pid:
It contains the pid a given pidfd refers to in the pid namespace of the
opener's procfs instance.
If the pid namespace of the process is not a descendant of the pid
namespace of the procfs instance 0 will be shown as its pid. This is
similar to calling getppid() on a process who's parent is out of it's
pid namespace (e.g. when moving a process into a sibling pid namespace
via setns()).

Add an NSpid field for easy retrieval of the pid in all descendant pid
namespaces:
If pid namespaces are supported this field will contain the pid a given
pidfd refers to for all descendant pid namespaces starting from the
current pid namespace of the opener's procfs instance, i.e. the first
pid entry for Pid and NSpid will be identical.
If the pid namespace of the process is not a descendant of the pid
namespace of the procfs instance 0 will be shown as its first NSpid and
no other NSpid entries will be shown.
Note that this differs from the Pid and NSpid fields in
/proc/<pid>/status where Pid and NSpid are always shown relative to the
pid namespace of the opener's procfs instace. The difference becomes
obvious when sending around a pidfd between pid namespaces from
different trees, i.e. where no ancestoral relation is present between
the pid namespaces:
1. sending around pidfd:
- create two new pid namespaces ns1 and ns2 in the initial pid namespace
  (Also take care to create new mount namespaces in the new pid
  namespace and mount procfs.)
- create a process with a pidfd in ns1
- send pidfd from ns1 to ns2
- read /proc/self/fdinfo/<pidfd> and observe that Pid and NSpid entry
  are 0
- create a process with a pidfd in
- open a pidfd for a process in the initial pid namespace
2. sending around /proc/<pid>/status fd:
- create two new pid namespaces ns1 and ns2 in the initial pid namespace
  (Also take care to create new mount namespaces in the new pid
  namespace and mount procfs.)
- create a process in ns1
- open /proc/<pid>/status in the initial pid namespace for the process
  you created in ns1
- send statusfd from initial pid namespace to ns2
- read statusfd and observe:
  - that Pid will contain the pid of the process as seen from the init
  - that NSpid will contain the pids of the process for all descendant
    pid namespaces starting from the initial pid namespace

Cc: Jann Horn <jannh@google.com>
Cc: linux-api@vger.kernel.org
Co-Developed-by: Christian Kellner <christian@kellner.me>
Signed-off-by: Christian Kellner <christian@kellner.me>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 kernel/fork.c | 73 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 72 insertions(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 1f6c45f6a734..b155bad92d9c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1695,12 +1695,83 @@ static int pidfd_release(struct inode *inode, struct file *file)
 }
 
 #ifdef CONFIG_PROC_FS
+/**
+ * pidfd_show_fdinfo - print information about a pidfd
+ * @m: proc fdinfo file
+ * @f: file referencing a pidfd
+ *
+ * Pid:
+ * This function will print the pid a given pidfd refers to in the pid
+ * namespace of the opener's procfs instance.
+ * If the pid namespace of the process is not a descendant of the pid
+ * namespace of the procfs instance 0 will be shown as its pid. This is
+ * similar to calling getppid() on a process who's parent is out of it's
+ * pid namespace (e.g. when moving a process into a sibling pid namespace
+ * via setns()).
+ *
+ * NSpid:
+ * If pid namespaces are supported then this function will also print the
+ * pid a given pidfd refers to for all descendant pid namespaces starting
+ * from the current pid namespace of the opener's procfs instance, i.e. the
+ * first pid entry for Pid and NSpid will be identical.
+ * If the pid namespace of the process is not a descendant of the pid
+ * namespace of the procfs instance 0 will be shown as its first NSpid and
+ * no other NSpid entries will be shown.
+ * Note that this differs from the Pid and NSpid fields in
+ * /proc/<pid>/status where Pid and NSpid are always shown relative to the
+ * pid namespace of the opener's procfs instace. The difference becomes
+ * obvious when sending around a pidfd between pid namespaces from
+ * different trees, i.e. where no ancestoral relation is present between
+ * the pid namespaces:
+ * 1. sending around pidfd:
+ * - create two new pid namespaces ns1 and ns2 in the initial pid namespace
+ *   (Also take care to create new mount namespaces in the new pid
+ *   namespace and mount procfs.)
+ * - create a process with a pidfd in ns1
+ * - send pidfd from ns1 to ns2
+ * - read /proc/self/fdinfo/<pidfd> and observe that Pid and NSpid entry
+ *   are 0
+ * - create a process with a pidfd in
+ * - open a pidfd for a process in the initial pid namespace
+ * 2. sending around /proc/<pid>/status fd:
+ * - create two new pid namespaces ns1 and ns2 in the initial pid namespace
+ *   (Also take care to create new mount namespaces in the new pid
+ *   namespace and mount procfs.)
+ * - create a process in ns1
+ * - open /proc/<pid>/status in the initial pid namespace for the process
+ *   you created in ns1
+ * - send statusfd from initial pid namespace to ns2
+ * - read statusfd and observe:
+ *   - that Pid will contain the pid of the process as seen from the init
+ *   - that NSpid will contain the pids of the process for all descendant
+ *     pid namespaces starting from the initial pid namespace
+ */
 static void pidfd_show_fdinfo(struct seq_file *m, struct file *f)
 {
 	struct pid_namespace *ns = proc_pid_ns(file_inode(m->file));
 	struct pid *pid = f->private_data;
+	pid_t nr = pid_nr_ns(pid, ns);
+
+	seq_put_decimal_ull(m, "Pid:\t", nr);
+
+#ifdef CONFIG_PID_NS
+	seq_puts(m, "\nNSpid:");
+	if (nr == 0) {
+		/*
+		 * If nr is zero the pid namespace of the procfs and the
+		 * pid namespace of the pidfd are neither the same pid
+		 * namespace nor are they ancestors. Since NSpid and Pid
+		 * are always identical in their first entry shortcut it
+		 * and simply print 0.
+		 */
+		seq_put_decimal_ull(m, "\t", nr);
+	} else {
+		int i;
+		for (i = ns->level; i <= pid->level; i++)
+			seq_put_decimal_ull(m, "\t", pid_nr_ns(pid, pid->numbers[i].ns));
+	}
+#endif
 
-	seq_put_decimal_ull(m, "Pid:\t", pid_nr_ns(pid, ns));
 	seq_putc(m, '\n');
 }
 #endif
-- 
2.23.0