From: David Disseldorp <ddiss@suse•de>
To: linux-kbuild@vger•kernel.org, linux-fsdevel@vger•kernel.org
Cc: linux-next@vger•kernel.org, David Disseldorp <ddiss@suse•de>
Subject: [PATCH v2 6/7] docs: initramfs: file data alignment via name padding
Date: Thu, 14 Aug 2025 15:18:04 +1000 [thread overview]
Message-ID: <20250814054818.7266-7-ddiss@suse.de> (raw)
In-Reply-To: <20250814054818.7266-1-ddiss@suse.de>
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>
---
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
next prev parent reply other threads:[~2025-08-14 5:48 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-08-14 5:17 [PATCH v2 0/7] gen_init_cpio: add copy_file_range / reflink support David Disseldorp
2025-08-14 5:17 ` [PATCH v2 1/7] gen_init_cpio: write to fd instead of stdout stream David Disseldorp
2025-08-18 18:40 ` Nicolas Schier
2025-08-14 5:18 ` [PATCH v2 2/7] gen_init_cpio: support -o <output_path> parameter David Disseldorp
2025-08-18 18:40 ` Nicolas Schier
2025-08-14 5:18 ` [PATCH v2 3/7] gen_init_cpio: attempt copy_file_range for file data David Disseldorp
2025-08-18 18:40 ` Nicolas Schier
2025-08-19 0:20 ` David Disseldorp
2025-08-14 5:18 ` [PATCH v2 4/7] gen_init_cpio: avoid duplicate strlen calls David Disseldorp
2025-08-14 5:18 ` [PATCH v2 5/7] gen_initramfs.sh: use gen_init_cpio -o parameter David Disseldorp
2025-08-18 18:40 ` Nicolas Schier
2025-08-14 5:18 ` David Disseldorp [this message]
2025-08-14 5:18 ` [PATCH v2 7/7] gen_init_cpio: add -a <data_align> as reflink optimization David Disseldorp
2025-08-18 19:23 ` Nicolas Schier
2025-08-21 8:10 ` David Disseldorp
2025-08-18 18:40 ` [PATCH v2 0/7] gen_init_cpio: add copy_file_range / reflink support Nicolas Schier
2025-08-18 23:46 ` David Disseldorp
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