From: Steven Rostedt <rostedt@goodmis.org>
To: linux-kernel@vger.kernel.org
Cc: Ingo Molnar <mingo@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Masami Hiramatsu <mhiramat@kernel.org>
Subject: [for-linus][PATCH 04/12] docs: bootconfig: Update file format on initrd image
Date: Tue, 01 Dec 2020 10:58:39 -0500 [thread overview]
Message-ID: <20201201160005.618794714@goodmis.org> (raw)
In-Reply-To: <20201201155835.647858317@goodmis.org>
From: Masami Hiramatsu <mhiramat@kernel.org>
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>
---
Documentation/admin-guide/bootconfig.rst | 18 +++++++++++++-----
1 file changed, 13 insertions(+), 5 deletions(-)
diff --git a/Documentation/admin-guide/bootconfig.rst b/Documentation/admin-guide/bootconfig.rst
index a22024f9175e..363599683784 100644
--- a/Documentation/admin-guide/bootconfig.rst
+++ b/Documentation/admin-guide/bootconfig.rst
@@ -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.
--
2.28.0
next prev parent reply other threads:[~2020-12-01 16:01 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-01 15:58 [for-linus][PATCH 00/12] tracing: Fixes for 5.10-rc6 Steven Rostedt
2020-12-01 15:58 ` [for-linus][PATCH 01/12] tools/bootconfig: Fix errno reference after printf() Steven Rostedt
2020-12-01 15:58 ` [for-linus][PATCH 02/12] tools/bootconfig: Fix to check the write failure correctly Steven Rostedt
2020-12-01 15:58 ` [for-linus][PATCH 03/12] tools/bootconfig: Align the bootconfig applied initrd image size to 4 Steven Rostedt
2020-12-01 15:58 ` Steven Rostedt [this message]
2020-12-01 15:58 ` [for-linus][PATCH 05/12] ring-buffer: Update write stamp with the correct ts Steven Rostedt
2020-12-01 15:58 ` [for-linus][PATCH 06/12] ring-buffer: Set the right timestamp in the slow path of __rb_reserve_next() Steven Rostedt
2020-12-01 15:58 ` [for-linus][PATCH 07/12] samples/ftrace: Mark my_tramp[12]? global Steven Rostedt
2020-12-01 15:58 ` [for-linus][PATCH 08/12] tracing: Remove WARN_ON in start_thread() Steven Rostedt
2020-12-01 15:58 ` [for-linus][PATCH 09/12] tracing: Fix alignment of static buffer Steven Rostedt
2020-12-01 15:58 ` [for-linus][PATCH 10/12] ftrace: Fix updating FTRACE_FL_TRAMP Steven Rostedt
2020-12-01 15:58 ` [for-linus][PATCH 11/12] ftrace: Fix DYNAMIC_FTRACE_WITH_DIRECT_CALLS dependency Steven Rostedt
2020-12-01 15:58 ` [for-linus][PATCH 12/12] ring-buffer: Always check to put back before stamp when crossing pages Steven Rostedt
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