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| author | David Disseldorp <ddiss@suse.de> | 2025-08-19 13:05:49 +1000 |
|---|---|---|
| committer | Nathan Chancellor <nathan@kernel.org> | 2025-08-20 16:02:56 -0700 |
| commit | 7c1f14f6e8e7f288350faec02b3fbc25971da289 (patch) | |
| tree | c59f3a3d3467a01d58d6a3b8a6445f3a82dde13f | |
| parent | 9135564db4904e82a3207bd2d134ca64ce618047 (diff) | |
docs: initramfs: file data alignment via name padding
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@suse.de>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://lore.kernel.org/r/20250819032607.28727-7-ddiss@suse.de
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
| -rw-r--r-- | Documentation/driver-api/early-userspace/buffer-format.rst | 5 |
1 files changed, 5 insertions, 0 deletions
diff --git a/Documentation/driver-api/early-userspace/buffer-format.rst b/Documentation/driver-api/early-userspace/buffer-format.rst index 726bfa2fe70d..4597a91100b7 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 |