[PATCH 5/6] fat: map ptime to FAT creation time with rename-over

[Sean Smith <defendthedisabled@gmail.com>]

Map ptime to the FAT/VFAT creation time field. Only active on VFAT
(long filename) mounts since plain FAT12/FAT16 lack creation time.
FAT32 creation time has 2-second precision.

Getattr: report creation time as ptime (VFAT only, via isvfat check).
Setattr: write ptime to i_crtime.
Rename-over: save target creation time before detach, restore to
  source after attach. Preserves creation time across atomic saves.

Signed-off-by: Sean Smith <DefendTheDisabled@gmail.com>
---
 fs/fat/file.c       |  6 ++++++
 fs/fat/namei_vfat.c | 20 ++++++++++++++++++--
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/fs/fat/file.c b/fs/fat/file.c
index 4fc49a614..9d1fcc554 100644
--- a/fs/fat/file.c
+++ b/fs/fat/file.c
@@ -413,6 +413,10 @@ int fat_getattr(struct mnt_idmap *idmap, const struct path *path,
 		stat->result_mask |= STATX_BTIME;
 		stat->btime = MSDOS_I(inode)->i_crtime;
 	}
+	if (sbi->options.isvfat && (request_mask & STATX_PTIME)) {
+		stat->result_mask |= STATX_PTIME;
+		stat->ptime = MSDOS_I(inode)->i_crtime;
+	}
 
 	return 0;
 }
@@ -564,6 +568,8 @@ int fat_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
 		fat_truncate_time(inode, &attr->ia_mtime, S_MTIME);
 	attr->ia_valid &= ~(ATTR_ATIME|ATTR_CTIME|ATTR_MTIME);
 
+	if (attr->ia_valid & ATTR_PTIME)
+		MSDOS_I(inode)->i_crtime = attr->ia_ptime;
 	setattr_copy(idmap, inode, attr);
 	mark_inode_dirty(inode);
 out:
diff --git a/fs/fat/namei_vfat.c b/fs/fat/namei_vfat.c
index 47ff083cf..f1e2eadf8 100644
--- a/fs/fat/namei_vfat.c
+++ b/fs/fat/namei_vfat.c
@@ -975,8 +975,24 @@ static int vfat_rename(struct inode *old_dir, struct dentry *old_dentry,
 	}
 	inode_inc_iversion(new_dir);
 
-	fat_detach(old_inode);
-	fat_attach(old_inode, new_i_pos);
+	/* ptime rename-over: save target creation time */
+	{
+		struct timespec64 saved_crtime = {};
+		bool inherit_crtime = false;
+
+		if (new_inode && S_ISREG(old_inode->i_mode) &&
+		    S_ISREG(new_inode->i_mode) && old_inode->i_nlink == 1) {
+			saved_crtime = MSDOS_I(new_inode)->i_crtime;
+			inherit_crtime = true;
+		}
+
+		fat_detach(old_inode);
+		fat_attach(old_inode, new_i_pos);
+
+		if (inherit_crtime)
+			MSDOS_I(old_inode)->i_crtime = saved_crtime;
+	}
+
 	err = vfat_sync_ipos(new_dir, old_inode);
 	if (err)
 		goto error_inode;
-- 
2.53.0