[PATCH] initramfs: Provide a way to ignore image provided by bootloader

[Daniel Thompson <daniel.thompson@linaro.org>]

Many "embedded" architectures provide CMDLINE_FORCE to allow the kernel
to override the command line provided by an inflexible bootloader.
However there is currrently no way for the kernel to override the
initramfs image provided by the bootloader meaning there are still ways
for bootloaders to make things difficult for us.

Fix this by introducing INITRAMFS_FORCE which can prevent the kernel
from loading the bootloader supplied image.

We use CMDLINE_FORCE (and its friend CMDLINE_EXTEND) to imply that the
system has an inflexible bootloader. This allow us to avoid presenting
this config option to users of systems where inflexible bootloaders
aren't usually a problem.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
---
 init/initramfs.c |  2 +-
 usr/Kconfig      | 10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/init/initramfs.c b/init/initramfs.c
index b32ad7d97ac9..cc9d9ec4abba 100644
--- a/init/initramfs.c
+++ b/init/initramfs.c
@@ -610,7 +610,7 @@ static int __init populate_rootfs(void)
 	char *err = unpack_to_rootfs(__initramfs_start, __initramfs_size);
 	if (err)
 		panic("%s", err); /* Failed to decompress INTERNAL initramfs */
-	if (initrd_start) {
+	if (initrd_start && !IS_ENABLED(CONFIG_INITRAMFS_FORCE)) {
 #ifdef CONFIG_BLK_DEV_RAM
 		int fd;
 		printk(KERN_INFO "Trying to unpack rootfs image as initramfs...\n");
diff --git a/usr/Kconfig b/usr/Kconfig
index 6278f135256d..c0c48507e44e 100644
--- a/usr/Kconfig
+++ b/usr/Kconfig
@@ -21,6 +21,16 @@ config INITRAMFS_SOURCE

 	  If you are not sure, leave it blank.

+config INITRAMFS_FORCE
+	bool "Ignore the initramfs passed by the bootloader"
+	depends on CMDLINE_EXTEND || CMDLINE_FORCE
+	help
+	  This option causes the kernel to ignore the initramfs image
+	  (or initrd image) passed to it by the bootloader. This is
+	  analogous to CMDLINE_FORCE, which is found on some architectures,
+	  and is useful if you cannot or don't want to change the image
+	  your bootloader passes to the kernel.
+
 config INITRAMFS_ROOT_UID
 	int "User ID to map to 0 (user root)"
 	depends on INITRAMFS_SOURCE!=""
--
2.9.3