[RFC PATCH v7 9/9] vfs: expose STATX_VERSION to userland

[Jeff Layton <jlayton@kernel.org>]

From: Jeff Layton <jlayton@redhat.com>

Claim one of the spare fields in struct statx to hold a 64-bit inode
version attribute. When userland requests STATX_VERSION, copy the
value from the kstat struct there, and stop masking off
STATX_ATTR_VERSION_MONOTONIC.

Update the test-statx sample program to output the change attr and
MountId.

Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/stat.c                 | 12 +++---------
 include/linux/stat.h      |  9 ---------
 include/uapi/linux/stat.h |  6 ++++--
 samples/vfs/test-statx.c  |  8 ++++++--
 4 files changed, 13 insertions(+), 22 deletions(-)

Posting this as an RFC as we're still trying to sort out what semantics
we want to present to userland. In particular, this patch leaves the
problem of crash resilience in to userland applications on filesystems
that don't report as MONOTONIC.

Trond is of the opinion that monotonicity is a hard requirement, and
that we should not allow filesystems that can't provide that quality to
report STATX_VERSION at all. His rationale is that one of the main uses
for this is for backup applications, and for those a counter that could
go backward is worse than useless.

I don't have strong feelings either way, but if we want that then we
will not be able to offload the crash counter handling to userland.

Thoughts?

diff --git a/fs/stat.c b/fs/stat.c
index e7f8cd4b24e1..8396c372022f 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -593,11 +593,9 @@ cp_statx(const struct kstat *stat, struct statx __user *buffer)
 
 	memset(&tmp, 0, sizeof(tmp));
 
-	/* STATX_VERSION is kernel-only for now */
-	tmp.stx_mask = stat->result_mask & ~STATX_VERSION;
+	tmp.stx_mask = stat->result_mask;
 	tmp.stx_blksize = stat->blksize;
-	/* STATX_ATTR_VERSION_MONOTONIC is kernel-only for now */
-	tmp.stx_attributes = stat->attributes & ~STATX_ATTR_VERSION_MONOTONIC;
+	tmp.stx_attributes = stat->attributes;
 	tmp.stx_nlink = stat->nlink;
 	tmp.stx_uid = from_kuid_munged(current_user_ns(), stat->uid);
 	tmp.stx_gid = from_kgid_munged(current_user_ns(), stat->gid);
@@ -621,6 +619,7 @@ cp_statx(const struct kstat *stat, struct statx __user *buffer)
 	tmp.stx_mnt_id = stat->mnt_id;
 	tmp.stx_dio_mem_align = stat->dio_mem_align;
 	tmp.stx_dio_offset_align = stat->dio_offset_align;
+	tmp.stx_version = stat->version;
 
 	return copy_to_user(buffer, &tmp, sizeof(tmp)) ? -EFAULT : 0;
 }
@@ -636,11 +635,6 @@ int do_statx(int dfd, struct filename *filename, unsigned int flags,
 	if ((flags & AT_STATX_SYNC_TYPE) == AT_STATX_SYNC_TYPE)
 		return -EINVAL;
 
-	/* STATX_VERSION is kernel-only for now. Ignore requests
-	 * from userland.
-	 */
-	mask &= ~STATX_VERSION;
-
 	error = vfs_statx(dfd, filename, flags, &stat, mask);
 	if (error)
 		return error;
diff --git a/include/linux/stat.h b/include/linux/stat.h
index 4e9428d86a3a..69c79e4fd1b1 100644
--- a/include/linux/stat.h
+++ b/include/linux/stat.h
@@ -54,13 +54,4 @@ struct kstat {
 	u32		dio_offset_align;
 	u64		version;
 };
-
-/* These definitions are internal to the kernel for now. Mainly used by nfsd. */
-
-/* mask values */
-#define STATX_VERSION		0x40000000U	/* Want/got stx_change_attr */
-
-/* file attribute values */
-#define STATX_ATTR_VERSION_MONOTONIC	0x8000000000000000ULL /* version monotonically increases */
-
 #endif
diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
index 7cab2c65d3d7..4a0a1f27c059 100644
--- a/include/uapi/linux/stat.h
+++ b/include/uapi/linux/stat.h
@@ -127,7 +127,8 @@ struct statx {
 	__u32	stx_dio_mem_align;	/* Memory buffer alignment for direct I/O */
 	__u32	stx_dio_offset_align;	/* File offset alignment for direct I/O */
 	/* 0xa0 */
-	__u64	__spare3[12];	/* Spare space for future expansion */
+	__u64	stx_version; /* Inode change attribute */
+	__u64	__spare3[11];	/* Spare space for future expansion */
 	/* 0x100 */
 };
 
@@ -154,6 +155,7 @@ struct statx {
 #define STATX_BTIME		0x00000800U	/* Want/got stx_btime */
 #define STATX_MNT_ID		0x00001000U	/* Got stx_mnt_id */
 #define STATX_DIOALIGN		0x00002000U	/* Want/got direct I/O alignment info */
+#define STATX_VERSION		0x00004000U	/* Want/got stx_version */
 
 #define STATX__RESERVED		0x80000000U	/* Reserved for future struct statx expansion */
 
@@ -189,6 +191,6 @@ struct statx {
 #define STATX_ATTR_MOUNT_ROOT		0x00002000 /* Root of a mount */
 #define STATX_ATTR_VERITY		0x00100000 /* [I] Verity protected file */
 #define STATX_ATTR_DAX			0x00200000 /* File is currently in DAX state */
-
+#define STATX_ATTR_VERSION_MONOTONIC	0x00400000 /* stx_version increases w/ every change */
 
 #endif /* _UAPI_LINUX_STAT_H */
diff --git a/samples/vfs/test-statx.c b/samples/vfs/test-statx.c
index 49c7a46cee07..868c9394e038 100644
--- a/samples/vfs/test-statx.c
+++ b/samples/vfs/test-statx.c
@@ -107,6 +107,8 @@ static void dump_statx(struct statx *stx)
 	printf("Device: %-15s", buffer);
 	if (stx->stx_mask & STATX_INO)
 		printf(" Inode: %-11llu", (unsigned long long) stx->stx_ino);
+	if (stx->stx_mask & STATX_MNT_ID)
+		printf(" MountId: %llx", stx->stx_mnt_id);
 	if (stx->stx_mask & STATX_NLINK)
 		printf(" Links: %-5u", stx->stx_nlink);
 	if (stx->stx_mask & STATX_TYPE) {
@@ -145,7 +147,9 @@ static void dump_statx(struct statx *stx)
 	if (stx->stx_mask & STATX_CTIME)
 		print_time("Change: ", &stx->stx_ctime);
 	if (stx->stx_mask & STATX_BTIME)
-		print_time(" Birth: ", &stx->stx_btime);
+		print_time("Birth: ", &stx->stx_btime);
+	if (stx->stx_mask & STATX_VERSION)
+		printf("Inode Version: %llu\n", stx->stx_version);
 
 	if (stx->stx_attributes_mask) {
 		unsigned char bits, mbits;
@@ -218,7 +222,7 @@ int main(int argc, char **argv)
 	struct statx stx;
 	int ret, raw = 0, atflag = AT_SYMLINK_NOFOLLOW;
 
-	unsigned int mask = STATX_BASIC_STATS | STATX_BTIME;
+	unsigned int mask = STATX_BASIC_STATS | STATX_BTIME | STATX_MNT_ID | STATX_VERSION;
 
 	for (argv++; *argv; argv++) {
 		if (strcmp(*argv, "-F") == 0) {
-- 
2.37.3