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FAT file systems do not use inodes, instead all file meta-information is stored in directory entries. FAT12 and FAT16 use a fixed size area for root directories, with typically 512 entries of 32 bytes each (for a total of 16 KB) on hard disk formats. The file system data is stored in clusters of typically 512 to 4096 bytes, depending on the size of the file system. The current code uses the offset of a DOS 8.3 style directory entry as a pseudo-inode, which leads to inode values of 0 to 16368 for typical root directories with 512 entries. Sub-directories use 2 cluster length plus the byte offset of the directory entry in the data area for the pseudo-inode, which may be as low as 1024 in case of 512 byte clusters. A sub-directory in cluster 2 and with 512 byte clusters will therefore lead to a re-use of inode 1024 when there are at least 32 DOS 8.3 style filenames in the root directory (or 11 14-character Windows long file names, each of which takes up 3 directory entries). FAT32 file systems are not affected by this issue and FAT12/FAT16 file systems with larger cluster sizes are unlikely to have as many directory entries in the root directory as are required to cause the collision. This commit leads to inode numbers that are guaranteed to not collide for all valid FAT12 and FAT16 file system parameters. It does also provide a small speed-up due to more efficient use of the vnode cache. Approved by: mckusick MFC after: 3 days Differential Revision: https://reviews.freebsd.org/D43978 |
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