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Subject[RFC] proc: don't show kernel-internal mnt_id in fdinfo (if possible)
Date
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@zeniv.linux.org.uk>
Cc: Andrey Vagin <avagin@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Rob Landley <rob@landley.net>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
---
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
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