The existing cpio extraction logic reads (maximum PATH_MAX) name_len bytes from the archive into the collected name buffer and ensures that the trailing byte is a null-terminator. This allows the actual file name to be shorter than name_len, with the name string simply zero-terminated prior to the last byte. Initramfs generators, such as dracut-cpio[1], can take advantage of name zero-padding to align file data segments within the archive to filesystem block boundaries. Block boundary alignment may allow the copy_file_range syscall to reflink archive source and destination extents. Link: https://github.com/dracutdevs/dracut/commit/300e4b116c624bca1b9e7251708b1ae656fe9157 [1] Signed-off-by: David Disseldorp <ddiss@xxxxxxx> --- Documentation/driver-api/early-userspace/buffer-format.rst | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/Documentation/driver-api/early-userspace/buffer-format.rst b/Documentation/driver-api/early-userspace/buffer-format.rst index 726bfa2fe70da..4597a91100b7b 100644 --- a/Documentation/driver-api/early-userspace/buffer-format.rst +++ b/Documentation/driver-api/early-userspace/buffer-format.rst @@ -86,6 +86,11 @@ c_mtime is ignored unless CONFIG_INITRAMFS_PRESERVE_MTIME=y is set. The c_filesize should be zero for any file which is not a regular file or symlink. +c_namesize may account for more than one trailing '\0', as long as the +value doesn't exceed PATH_MAX. This can be useful for ensuring that a +subsequent file data segment is aligned, e.g. to a filesystem block +boundary. + The c_chksum field contains a simple 32-bit unsigned sum of all the bytes in the data field. cpio(1) refers to this as "crc", which is clearly incorrect (a cyclic redundancy check is a different and -- 2.43.0
