[RFC] proc: don't show kernel-internal mnt_id in fdinfo (if possible)

[Dmitry Safonov]

Some kernel special fs could be mounted by userspace, let's show
userspace mnt_id in those cases.

Otherwise, I got:
  [~]# cat /proc/11299/fdinfo/3
  pos:	0
  flags:	02000002
  mnt_id:	14
  [~]# cat /proc/11299/mountinfo | grep '^14'
  [~]# ls -l /proc/11299/fd/3
  lrwx------. 1 root root 64 Nov  7 18:30 /proc/11299/fd/3 -> /test-queue
  [~]# ls /dev/mqueue/
  test-queue
  [~]# cat /proc/11299/mountinfo | grep mqueue
  32 18 0:14 / /dev/mqueue rw,relatime shared:17 - mqueue mqueue rw,seclabel

This happens because mqueue fs is mounted twice:
- the first is kernel-internal mnt on init:
  kernel_init=>do_one_initcall=>init_mqueue_fs=>mq_init_ns=>vfs_kern_mount
- the second time it's systemd's mount-unit:
  entry_SYSCALL_64_fastpath=>SyS_mount=>do_mount=>vfs_kern_mount

For the purpose of userspace parsing, having in-kernel mnt_id is less
useful then mnt_id of mount point: afterwards I'm able to see fs type,
path, etc:
  [~]# cat /proc/11152/mountinfo | grep mqueue
  32 18 0:14 / /dev/mqueue rw,relatime shared:18 - mqueue mqueue rw,seclabel
  [~]# cat /proc/11152/fdinfo/3
  pos:	0
  flags:	02000002
  mnt_id:	32

On a bad side - if we've no userspace mount, we still can't tell a thing
about opened fd..

Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Andrey Vagin <avagin@xxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Rob Landley <rob@xxxxxxxxxxx>
Signed-off-by: Dmitry Safonov <dsafonov@xxxxxxxxxxxxx>
---
 fs/proc/fd.c | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index d21dafef3102..bfa8699bcd8e 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -9,6 +9,7 @@
 #include <linux/file.h>
 #include <linux/seq_file.h>
 #include <linux/fs.h>
+#include <linux/nsproxy.h>
 
 #include <linux/proc_fs.h>
 
@@ -19,9 +20,10 @@
 static int seq_show(struct seq_file *m, void *v)
 {
 	struct files_struct *files = NULL;
-	int f_flags = 0, ret = -ENOENT;
+	int f_flags = 0, ret = -ENOENT, mnt_id = 0;
 	struct file *file = NULL;
 	struct task_struct *task;
+	struct mount *mount;
 
 	task = get_proc_task(m->private);
 	if (!task)
@@ -52,9 +54,25 @@ static int seq_show(struct seq_file *m, void *v)
 	if (ret)
 		return ret;
 
+	mount = real_mount(file->f_path.mnt);
+	if (mount->mnt_ns == MNT_NS_INTERNAL) {
+		struct mount *mnt;
+
+		lock_mount_hash();
+		list_for_each_entry(mnt, &mount->mnt_instance, mnt_instance) {
+			if (current->nsproxy->mnt_ns == mnt->mnt_ns) {
+				mnt_id = mnt->mnt_id;
+				break;
+			}
+		}
+		unlock_mount_hash();
+	}
+
+	if (mnt_id == 0)
+		mnt_id = mount->mnt_id;
+
 	seq_printf(m, "pos:\t%lli\nflags:\t0%o\nmnt_id:\t%i\n",
-		   (long long)file->f_pos, f_flags,
-		   real_mount(file->f_path.mnt)->mnt_id);
+		   (long long)file->f_pos, f_flags, mnt_id);
 
 	show_fd_locks(m, file, files);
 	if (seq_has_overflowed(m))
-- 
2.10.2