statx: add mount ID

Systemd is hacking around to get it and it's trivial to add to statx, so...

Cc: linux-api@vger.kernel.org
Cc: linux-man@vger.kernel.org
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/stat.c

@@ -22,6 +22,7 @@
 #include <asm/unistd.h>
 
 #include "internal.h"
+#include "mount.h"
 
 /**
  * generic_fillattr - Fill in the basic attributes from the inode struct
@@ -199,6 +200,8 @@ int vfs_statx(int dfd, const char __user *filename, int flags,
 		goto out;
 
 	error = vfs_getattr(&path, stat, request_mask, flags);
+	stat->mnt_id = real_mount(path.mnt)->mnt_id;
+	stat->result_mask |= STATX_MNT_ID;
 	path_put(&path);
 	if (retry_estale(error, lookup_flags)) {
 		lookup_flags |= LOOKUP_REVAL;
@@ -563,6 +566,7 @@ cp_statx(const struct kstat *stat, struct statx __user *buffer)
 	tmp.stx_rdev_minor = MINOR(stat->rdev);
 	tmp.stx_dev_major = MAJOR(stat->dev);
 	tmp.stx_dev_minor = MINOR(stat->dev);
+	tmp.stx_mnt_id = stat->mnt_id;
 
 	return copy_to_user(buffer, &tmp, sizeof(tmp)) ? -EFAULT : 0;
 }

include/linux/stat.h

@@ -47,6 +47,7 @@ struct kstat {
 	struct timespec64 ctime;
 	struct timespec64 btime;			/* File creation time */
 	u64		blocks;
+	u64		mnt_id;
 };
 
 #endif

include/uapi/linux/stat.h

@@ -123,7 +123,10 @@ struct statx {
 	__u32	stx_dev_major;	/* ID of device containing file [uncond] */
 	__u32	stx_dev_minor;
 	/* 0x90 */
-	__u64	__spare2[14];	/* Spare space for future expansion */
+	__u64	stx_mnt_id;
+	__u64	__spare2;
+	/* 0xa0 */
+	__u64	__spare3[12];	/* Spare space for future expansion */
 	/* 0x100 */
 };
 
@@ -148,6 +151,7 @@ struct statx {
 #define STATX_BLOCKS		0x00000400U	/* Want/got stx_blocks */
 #define STATX_BASIC_STATS	0x000007ffU	/* The stuff in the normal stat struct */
 #define STATX_BTIME		0x00000800U	/* Want/got stx_btime */
+#define STATX_MNT_ID		0x00001000U	/* Got stx_mnt_id */
 
 #define STATX__RESERVED		0x80000000U	/* Reserved for future struct statx expansion */