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docs: bootconfig: Update file format on initrd image 

To align the total file size, add padding null character when appending
the bootconfig to initrd image.

Link: https://lkml.kernel.org/r/160576522916.320071.4145530996151028855.stgit@devnote2

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
This commit is contained in:
Masami Hiramatsu 2020-11-19 14:53:49 +09:00 committed by Steven Rostedt (VMware)
parent e1cef2d4c3
commit fbc6e1c6e0
1 changed files with 13 additions and 5 deletions
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18
Documentation/admin-guide/bootconfig.rst
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@ -137,15 +137,22 @@ Boot Kernel With a Boot Config
==============================
Since the boot configuration file is loaded with initrd, it will be added
to the end of the initrd (initramfs) image file with size, checksum and
12-byte magic word as below.
to the end of the initrd (initramfs) image file with padding, size,
checksum and 12-byte magic word as below.
[initrd][bootconfig][size(u32)][checksum(u32)][#BOOTCONFIG\n]
[initrd][bootconfig][padding][size(u32)][checksum(u32)][#BOOTCONFIG\n]
When the boot configuration is added to the initrd image, the total
file size is aligned to 4 bytes. To fill the gap, null characters
(``\0``) will be added. Thus the ``size`` is the length of the bootconfig
file + padding bytes.
The Linux kernel decodes the last part of the initrd image in memory to
get the boot configuration data.
Because of this "piggyback" method, there is no need to change or
update the boot loader and the kernel image itself.
update the boot loader and the kernel image itself as long as the boot
loader passes the correct initrd file size. If by any chance, the boot
loader passes a longer size, the kernel feils to find the bootconfig data.
To do this operation, Linux kernel provides "bootconfig" command under
tools/bootconfig, which allows admin to apply or delete the config file
@ -176,7 +183,8 @@ up to 512 key-value pairs. If keys contains 3 words in average, it can
contain 256 key-value pairs. In most cases, the number of config items
will be under 100 entries and smaller than 8KB, so it would be enough.
If the node number exceeds 1024, parser returns an error even if the file
size is smaller than 32KB.
size is smaller than 32KB. (Note that this maximum size is not including
the padding null characters.)
Anyway, since bootconfig command verifies it when appending a boot config
to initrd image, user can notice it before boot.