[RFC PATCH v6 1/1] cmdline: Add capability to both append and prepend at the same time

One user has expressed the need to both append and prepend some
built-in parameters to the command line provided by the bootloader.

Allthough it is a corner case, it is easy to implement so let's do it.

When the user chooses to prepend the bootloader provided command line
with the built-in command line, he is offered the possibility to enter
an additionnal built-in command line to be appended after the
bootloader provided command line.

It is a complementary feature which has no impact on the already
existing ones and/or the existing defconfig.

Suggested-by: Daniel Walker <redacted>
Signed-off-by: Christophe Leroy <redacted>
---
Sending this out as an RFC, applies on top of the series
("Implement GENERIC_CMDLINE"). I will add it to the series next spin
unless someone is against it.
---
 include/linux/cmdline.h |  3 +++
 init/Kconfig            | 12 +++++++++++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/include/linux/cmdline.h b/include/linux/cmdline.h
index 020028e2bdf0..fb274a4d5519 100644
--- a/include/linux/cmdline.h
+++ b/include/linux/cmdline.h

@@ -36,6 +36,9 @@ static __always_inline bool __cmdline_build(char *dst, const char *src)
 
 	len = cmdline_strlcat(dst, src, COMMAND_LINE_SIZE);
 
+	if (IS_ENABLED(CONFIG_CMDLINE_PREPEND))
+		len = cmdline_strlcat(dst, " " CONFIG_CMDLINE_MORE, COMMAND_LINE_SIZE);
+
 	if (IS_ENABLED(CONFIG_CMDLINE_APPEND))
 		len = cmdline_strlcat(dst, " " CONFIG_CMDLINE, COMMAND_LINE_SIZE);
 

diff --git a/init/Kconfig b/init/Kconfig
index fa002e3765ab..cd3087ff4f28 100644
--- a/init/Kconfig
+++ b/init/Kconfig

@@ -128,6 +128,14 @@ config CMDLINE
 	  If this string is not empty, additional choices are proposed
 	  below to determine how it will be used by the kernel.
 
+config CMDLINE_MORE
+	string "Additional default kernel command string" if GENERIC_CMDLINE && CMDLINE_PREPEND
+	default ""
+	help
+	  Defines an additional default kernel command string.
+	  If this string is not empty, it is appended to the
+	  command-line arguments provided by the bootloader
+
 choice
 	prompt "Kernel command line type" if CMDLINE != ""
 	default CMDLINE_PREPEND if ARCH_WANT_CMDLINE_PREPEND_BY_DEFAULT

@@ -154,7 +162,9 @@ config CMDLINE_PREPEND
 	bool "Prepend to the bootloader kernel arguments"
 	help
 	  The default kernel command string will be prepended to the
-	  command-line arguments provided by the bootloader.
+	  command-line arguments provided by the bootloader. When this
+	  option is selected, another string can be added which will
+	  be appended.
 
 config CMDLINE_FORCE
 	bool "Always use the default kernel command string"

-- 
2.25.0

On Sun, Apr 4, 2021 at 12:20 PM Christophe Leroy
[off-list ref] wrote:

One user has expressed the need to both append and prepend some
built-in parameters to the command line provided by the bootloader.

Allthough it is a corner case, it is easy to implement so let's do it.

When the user chooses to prepend the bootloader provided command line
with the built-in command line, he is offered the possibility to enter
an additionnal built-in command line to be appended after the
bootloader provided command line.

It is a complementary feature which has no impact on the already
existing ones and/or the existing defconfig.

Suggested-by: Daniel Walker <redacted>
Signed-off-by: Christophe Leroy <redacted>
---
Sending this out as an RFC, applies on top of the series
("Implement GENERIC_CMDLINE"). I will add it to the series next spin
unless someone is against it.

Well, it works, but you are working around the existing kconfig and
the result is not great. You'd never design it this way.

Rob