Currently, the bootloader-provided command line can be prepended to with
the built-in command line. This is done by enabling CONFIG_CMDLINE_BOOL
and specifying a CONFIG_CMDLINE value with CONFIG_CMDLINE_OVERRIDE
disabled.

However, there is currently no way to append the built-in command line
to the bootloader-provided command line, like there is on some other
architectures. This is necessary to work around bootloaders that are
difficult to update, where we want to override a subset of the
bootloader-provided values and keep the others.

To solve this limitation, we add CONFIG_CMDLINE_EXTEND, which is already
available on several other architectures, to make the built-in command
line append to the bootloader-provided command line.

Signed-off-by: Brian Mak <makb@juniper.net>
---

Hi all,

I'm not sure if I'm reaching the right audience since I haven't received a
response in the past two times I sent this patch, so I'm adding Andrew
Morton to this patch as well.

Best,
Brian

 arch/x86/Kconfig        |  9 +++++++++
 arch/x86/kernel/setup.c | 13 +++++++++----
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 58d890fe2100..8da39ebaddf4 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2275,6 +2275,15 @@ config CMDLINE_BOOL
 	  Systems with fully functional boot loaders (i.e. non-embedded)
 	  should leave this option set to 'N'.
 
+config CMDLINE_EXTEND
+	bool "Extend bootloader kernel arguments"
+	depends on CMDLINE_BOOL && !CMDLINE_OVERRIDE
+	help
+	  The built-in command line will be appended to the command-
+	  line arguments provided during boot. This is useful in
+	  cases where the provided arguments are insufficient and
+	  you don't want to or cannot modify them.
+
 config CMDLINE
 	string "Built-in kernel command string"
 	depends on CMDLINE_BOOL
diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 1b2edd07a3e1..86e4d8ab8558 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -911,10 +911,15 @@ void __init setup_arch(char **cmdline_p)
 	strscpy(boot_command_line, builtin_cmdline, COMMAND_LINE_SIZE);
 #else
 	if (builtin_cmdline[0]) {
-		/* append boot loader cmdline to builtin */
-		strlcat(builtin_cmdline, " ", COMMAND_LINE_SIZE);
-		strlcat(builtin_cmdline, boot_command_line, COMMAND_LINE_SIZE);
-		strscpy(boot_command_line, builtin_cmdline, COMMAND_LINE_SIZE);
+		if (!IS_ENABLED(CONFIG_CMDLINE_EXTEND)) {
+			/* append boot loader cmdline to builtin */
+			strlcat(builtin_cmdline, " ", COMMAND_LINE_SIZE);
+			strlcat(builtin_cmdline, boot_command_line, COMMAND_LINE_SIZE);
+			strscpy(boot_command_line, builtin_cmdline, COMMAND_LINE_SIZE);
+		} else {
+			strlcat(boot_command_line, " ", COMMAND_LINE_SIZE);
+			strlcat(boot_command_line, builtin_cmdline, COMMAND_LINE_SIZE);
+		}
 	}
 #endif
 	builtin_cmdline_added = true;

base-commit: 1b237f190eb3d36f52dffe07a40b5eb210280e00
-- 
2.25.1

On 10/1/25 16:04, Brian Mak wrote:
> To solve this limitation, we add CONFIG_CMDLINE_EXTEND, which is already
> available on several other architectures, to make the built-in command
> line append to the bootloader-provided command line.

I'd really rather not have another copy-and-paste of another
architecture's Kconfig bits into x86.

At the _very_ least, we'd get a boolean ARCH_HAS_CMDLIND_EXTEND which
would then expose an arch-independent CMDLINE_EXTEND option. Literally
duplicating Kconfig options just isn't scalable.

I also cringe every time I see code like this get added to arch/x86 that
really doesn't have anything to do with x86 and really only gets dumped
in to arch/ because there's never been a proper refactoring of all the
copy-and-pasted code.

In the end, refactoring Kconfig is dirt simple. Refactoring
builtin_cmdline[] into arch-independent code would be a lot harder.

On Oct 1, 2025, at 5:13 PM, Dave Hansen <dave.hansen@intel.com> wrote:

> On 10/1/25 16:04, Brian Mak wrote:
>> To solve this limitation, we add CONFIG_CMDLINE_EXTEND, which is already
>> available on several other architectures, to make the built-in command
>> line append to the bootloader-provided command line.
> 
> I'd really rather not have another copy-and-paste of another
> architecture's Kconfig bits into x86.
> 
> At the _very_ least, we'd get a boolean ARCH_HAS_CMDLIND_EXTEND which
> would then expose an arch-independent CMDLINE_EXTEND option. Literally
> duplicating Kconfig options just isn't scalable.

Hi Dave,

Thanks for your comments!

Sure, I'll introduce an arch-independent (ARCH_HAS_)CMDLINE_EXTEND option
in v2.

> I also cringe every time I see code like this get added to arch/x86 that
> really doesn't have anything to do with x86 and really only gets dumped
> in to arch/ because there's never been a proper refactoring of all the
> copy-and-pasted code.
> 
> In the end, refactoring Kconfig is dirt simple. Refactoring
> builtin_cmdline[] into arch-independent code would be a lot harder.

In the past, there have been efforts to add arch-independent cmdline
processing [1] (CC: Daniel Walker). These efforts have been stalled for a
long time now though.

[1] https://lore.kernel.org/lkml/20231110013817.2378507-1-danielwa@cisco.com/

Thanks,
Brian

On Thu, Oct 02, 2025 at 05:30:10PM +0000, Brian Mak wrote:
> On Oct 1, 2025, at 5:13 PM, Dave Hansen <dave.hansen@intel.com> wrote:
> 
> > On 10/1/25 16:04, Brian Mak wrote:
> >> To solve this limitation, we add CONFIG_CMDLINE_EXTEND, which is already
> >> available on several other architectures, to make the built-in command
> >> line append to the bootloader-provided command line.
> > 
> > I'd really rather not have another copy-and-paste of another
> > architecture's Kconfig bits into x86.
> > 
> > At the _very_ least, we'd get a boolean ARCH_HAS_CMDLIND_EXTEND which
> > would then expose an arch-independent CMDLINE_EXTEND option. Literally
> > duplicating Kconfig options just isn't scalable.
> 
> Hi Dave,
> 
> Thanks for your comments!
> 
> Sure, I'll introduce an arch-independent (ARCH_HAS_)CMDLINE_EXTEND option
> in v2.
> 
> > I also cringe every time I see code like this get added to arch/x86 that
> > really doesn't have anything to do with x86 and really only gets dumped
> > in to arch/ because there's never been a proper refactoring of all the
> > copy-and-pasted code.
> > 
> > In the end, refactoring Kconfig is dirt simple. Refactoring
> > builtin_cmdline[] into arch-independent code would be a lot harder.
> 
> In the past, there have been efforts to add arch-independent cmdline
> processing [1] (CC: Daniel Walker). These efforts have been stalled for a
> long time now though.


We use my change at Cisco in production, but I don't submit it often enough.

Daniel