[PATCH 3/6] ntfs3: map ptime to NTFS creation time with rename-over

[Sean Smith <defendthedisabled@gmail.com>]

Map ptime to the NTFS Date Created field in $STANDARD_INFORMATION.
This is a mapped-ptime implementation: setting ptime overwrites the
creation time. Justified because Windows treats NTFS creation time
as mutable via SetFileTime() - it was never truly immutable.

Getattr: report NTFS creation time as ptime.
Setattr: write ptime to NTFS creation time via frecord cr_time path.
Rename-over: save target creation time before unlink, restore to
  source after rename. Replicates Windows behavior where creation
  time survives application atomic saves.

Round-trip: NTFS Date Created -> Btrfs ptime -> NTFS Date Created
preserves the original creation date through cross-FS copies.

Signed-off-by: Sean Smith <DefendTheDisabled@gmail.com>
---
 fs/ntfs3/file.c    | 13 +++++++++++++
 fs/ntfs3/frecord.c |  8 ++++++++
 fs/ntfs3/namei.c   | 14 ++++++++++++++
 3 files changed, 35 insertions(+)

diff --git a/fs/ntfs3/file.c b/fs/ntfs3/file.c
index 13d014b87..8688a48b1 100644
--- a/fs/ntfs3/file.c
+++ b/fs/ntfs3/file.c
@@ -161,6 +161,13 @@ int ntfs_getattr(struct mnt_idmap *idmap, const struct path *path,
 
 	stat->result_mask |= STATX_BTIME;
 	stat->btime = ni->i_crtime;
+
+	/* Map NTFS creation time to ptime (provenance time) */
+	if (request_mask & STATX_PTIME) {
+		stat->ptime = ni->i_crtime;
+		stat->result_mask |= STATX_PTIME;
+	}
+
 	stat->blksize = ni->mi.sbi->cluster_size; /* 512, 1K, ..., 2M */
 
 	if (inode->i_flags & S_IMMUTABLE)
@@ -857,6 +864,12 @@ int ntfs_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
 		i_size_write(inode, newsize);
 	}
 
+	/* Accept ptime and store as NTFS creation time */
+	if (ia_valid & ATTR_PTIME) {
+		ni->i_crtime = attr->ia_ptime;
+		ni->ni_flags |= NI_FLAG_UPDATE_PARENT;
+	}
+
 	setattr_copy(idmap, inode, attr);
 
 	if (mode != inode->i_mode) {
diff --git a/fs/ntfs3/frecord.c b/fs/ntfs3/frecord.c
index d5bbd47e1..b164b2f50 100644
--- a/fs/ntfs3/frecord.c
+++ b/fs/ntfs3/frecord.c
@@ -3197,6 +3197,14 @@ int ni_write_inode(struct inode *inode, int sync, const char *hint)
 			modified = true;
 		}
 
+		/* Write creation time (ptime maps to NTFS cr_time) */
+		ts = ni->i_crtime;
+		dup.cr_time = kernel2nt(&ts);
+		if (std->cr_time != dup.cr_time) {
+			std->cr_time = dup.cr_time;
+			modified = true;
+		}
+
 		dup.fa = ni->std_fa;
 		if (std->fa != dup.fa) {
 			std->fa = dup.fa;
diff --git a/fs/ntfs3/namei.c b/fs/ntfs3/namei.c
index b2af8f695..40d06884f 100644
--- a/fs/ntfs3/namei.c
+++ b/fs/ntfs3/namei.c
@@ -292,6 +292,16 @@ static int ntfs_rename(struct mnt_idmap *idmap, struct inode *dir,
 		return -EINVAL;
 	}
 
+	/* ptime rename-over: save target creation time before unlink */
+	struct timespec64 saved_crtime = {};
+	bool inherit_crtime = false;
+
+	if (new_inode && S_ISREG(inode->i_mode) &&
+	    S_ISREG(new_inode->i_mode) && inode->i_nlink == 1) {
+		saved_crtime = ntfs_i(new_inode)->i_crtime;
+		inherit_crtime = true;
+	}
+
 	if (new_inode) {
 		/* Target name exists. Unlink it. */
 		dget(new_dentry);
@@ -330,6 +340,10 @@ static int ntfs_rename(struct mnt_idmap *idmap, struct inode *dir,
 
 	err = ni_rename(dir_ni, new_dir_ni, ni, de, new_de);
 	if (!err) {
+		/* ptime rename-over: inherit target creation time */
+		if (inherit_crtime)
+			ni->i_crtime = saved_crtime;
+
 		simple_rename_timestamp(dir, dentry, new_dir, new_dentry);
 		mark_inode_dirty(inode);
 		mark_inode_dirty(dir);
-- 
2.53.0