Currently, we have no good way to get mount info for the mount a given
file descriptor is on. statx only provides it's mnt_id but we would have to
call statmount with every single mnt_ns_id in order to get mount info
since we have no way of knowing the mount namespace this mnt_id belongs
to.

This patch modifies statx to also export mnt_ns_id, allowing userspace
to easily get mount info for the mount a file descriptor is on by using
a combination of statx + statmount.

```
statx(fd, “”, AT_EMPTY_PATH,
	STATX_MNT_ID_UNIQUE | STATX_MNT_NS_ID, &stat);
struct mnt_id_req req = {
	.mnt_id = stat.stx_mnt_id,
	.mnt_ns_id = stat.stx_mnt_ns_id
};
statmount(&req, &statmount_buf, buf_size, 0);
```

Signed-off-by: Bhavik Sachdev <b.sachdev1904@gmail.com>
---
 fs/stat.c                       | 15 +++++++++++++--
 include/linux/stat.h            |  1 +
 include/uapi/linux/stat.h       |  4 +++-
 tools/include/uapi/linux/stat.h |  4 +++-
 4 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/fs/stat.c b/fs/stat.c
index f95c1dc3eaa4..1cc29946c0c3 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -296,18 +296,28 @@ static int statx_lookup_flags(int flags)
 static int vfs_statx_path(struct path *path, int flags, struct kstat *stat,
 			  u32 request_mask)
 {
+	struct mount *real_mnt;
 	int error = vfs_getattr(path, stat, request_mask, flags);
 	if (error)
 		return error;
 
+	real_mnt = real_mount(path->mnt);
+
 	if (request_mask & STATX_MNT_ID_UNIQUE) {
-		stat->mnt_id = real_mount(path->mnt)->mnt_id_unique;
+		stat->mnt_id = real_mnt->mnt_id_unique;
 		stat->result_mask |= STATX_MNT_ID_UNIQUE;
 	} else {
-		stat->mnt_id = real_mount(path->mnt)->mnt_id;
+		stat->mnt_id = real_mnt->mnt_id;
 		stat->result_mask |= STATX_MNT_ID;
 	}
 
+	if (request_mask & STATX_MNT_NS_ID) {
+		if (!real_mnt->mnt_ns)
+			/* returning EINVAL for now */
+			return -EINVAL;
+		stat->mnt_ns_id = real_mnt->mnt_ns->seq;
+	}
+
 	if (path_mounted(path))
 		stat->attributes |= STATX_ATTR_MOUNT_ROOT;
 	stat->attributes_mask |= STATX_ATTR_MOUNT_ROOT;
@@ -745,6 +755,7 @@ cp_statx(const struct kstat *stat, struct statx __user *buffer)
 	tmp.stx_atomic_write_unit_max = stat->atomic_write_unit_max;
 	tmp.stx_atomic_write_segments_max = stat->atomic_write_segments_max;
 	tmp.stx_atomic_write_unit_max_opt = stat->atomic_write_unit_max_opt;
+	tmp.stx_mnt_ns_id = stat->mnt_ns_id;
 
 	return copy_to_user(buffer, &tmp, sizeof(tmp)) ? -EFAULT : 0;
 }
diff --git a/include/linux/stat.h b/include/linux/stat.h
index e3d00e7bb26d..c62b70ce30d9 100644
--- a/include/linux/stat.h
+++ b/include/linux/stat.h
@@ -59,6 +59,7 @@ struct kstat {
 	u32		atomic_write_unit_max;
 	u32		atomic_write_unit_max_opt;
 	u32		atomic_write_segments_max;
+	u64		mnt_ns_id;
 };
 
 /* These definitions are internal to the kernel for now. Mainly used by nfsd. */
diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
index 1686861aae20..1071f5097d26 100644
--- a/include/uapi/linux/stat.h
+++ b/include/uapi/linux/stat.h
@@ -187,7 +187,8 @@ struct statx {
 	__u32	__spare2[1];
 
 	/* 0xc0 */
-	__u64	__spare3[8];	/* Spare space for future expansion */
+	__u64   stx_mnt_ns_id;
+	__u64	__spare3[7];	/* Spare space for future expansion */
 
 	/* 0x100 */
 };
@@ -219,6 +220,7 @@ struct statx {
 #define STATX_SUBVOL		0x00008000U	/* Want/got stx_subvol */
 #define STATX_WRITE_ATOMIC	0x00010000U	/* Want/got atomic_write_* fields */
 #define STATX_DIO_READ_ALIGN	0x00020000U	/* Want/got dio read alignment info */
+#define STATX_MNT_NS_ID		0x00040000U	/* Want/got stx_mnt_ns_id */
 
 #define STATX__RESERVED		0x80000000U	/* Reserved for future struct statx expansion */
 
diff --git a/tools/include/uapi/linux/stat.h b/tools/include/uapi/linux/stat.h
index 1686861aae20..1071f5097d26 100644
--- a/tools/include/uapi/linux/stat.h
+++ b/tools/include/uapi/linux/stat.h
@@ -187,7 +187,8 @@ struct statx {
 	__u32	__spare2[1];
 
 	/* 0xc0 */
-	__u64	__spare3[8];	/* Spare space for future expansion */
+	__u64   stx_mnt_ns_id;
+	__u64	__spare3[7];	/* Spare space for future expansion */
 
 	/* 0x100 */
 };
@@ -219,6 +220,7 @@ struct statx {
 #define STATX_SUBVOL		0x00008000U	/* Want/got stx_subvol */
 #define STATX_WRITE_ATOMIC	0x00010000U	/* Want/got atomic_write_* fields */
 #define STATX_DIO_READ_ALIGN	0x00020000U	/* Want/got dio read alignment info */
+#define STATX_MNT_NS_ID		0x00040000U	/* Want/got stx_mnt_ns_id */
 
 #define STATX__RESERVED		0x80000000U	/* Reserved for future struct statx expansion */
 
-- 
2.51.0