Maksytka/linux
1
1
Fork 0
mirror of https://github.com/torvalds/linux.git synced 2025-12-01 07:26:02 +07:00
Code Issues Packages Projects Releases Wiki Activity

docs: initramfs: file data alignment via name padding

Browse Source 
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: 300e4b116c [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>
This commit is contained in:
David Disseldorp
2025-08-19 13:05:49 +10:00
committed by Nathan Chancellor
parent 9135564db4
commit 7c1f14f6e8
1 changed files with 5 additions and 0 deletions
Download Patch File Download Diff File Expand all files Collapse all files

5
Documentation/driver-api/early-userspace/buffer-format.rst
View File

@@ -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 The c_filesize should be zero for any file which is not a regular file
or symlink. 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 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 bytes in the data field. cpio(1) refers to this as "crc", which is
clearly incorrect (a cyclic redundancy check is a different and clearly incorrect (a cyclic redundancy check is a different and