Re: Upcoming: Notifications, FS notifications and fsinfo()

[Miklos Szeredi <miklos@szeredi.hu>]

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On Fri, Apr 3, 2020 at 1:11 PM Lennart Poettering <mzxreary@0pointer.de> wrote:
>
> On Fr, 03.04.20 09:44, Ian Kent (raven@themaw.net) wrote:
>
> > > Currently the only way to find the mount id from a path is by parsing
> > > /proc/self/fdinfo/$fd.  It is trivial, however, to extend statx(2) to
> > > return it directly from a path.   Also the mount notification queue
> > > that David implemented contains the mount ID of the changed mount.
>
> I would love to have the mount ID exposed via statx().

Here's a patch.

Thanks,
Miklos

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From: Miklos Szeredi <mszeredi@redhat.com>
Subject: statx: add mount ID

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

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
 fs/stat.c                 |    6 ++++++
 include/linux/stat.h      |    1 +
 include/uapi/linux/stat.h |    6 +++++-
 3 files changed, 12 insertions(+), 1 deletion(-)

--- a/fs/stat.c
+++ b/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 +200,10 @@ int vfs_statx(int dfd, const char __user
 		goto out;
 
 	error = vfs_getattr(&path, stat, request_mask, flags);
+	if (!error) {
+		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 +568,7 @@ cp_statx(const struct kstat *stat, struc
 	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;
 }
--- a/include/linux/stat.h
+++ b/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
--- a/include/uapi/linux/stat.h
+++ b/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_ALL		0x00000fffU	/* All currently supported flags */
 #define STATX__RESERVED		0x80000000U	/* Reserved for future struct statx expansion */