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initramfs: Expose retained initrd as sysfs file 

When the kernel command line option "retain_initrd" is set, we do not
free the initrd memory. However, we also don't expose it to anyone for
consumption. That leaves us in a weird situation where the only user of
this feature is ppc64 and arm64 specific kexec tooling.

To make it more generally useful, this patch adds a kobject to the
firmware object that contains the initrd context when "retain_initrd"
is set. That way, we can access the initrd any time after boot from
user space and for example hand it into kexec as --initrd parameter
if we want to reboot the same initrd. Or inspect it directly locally.

With this patch applied, there is a new /sys/firmware/initrd file when
the kernel was booted with an initrd and "retain_initrd" command line
option is set.

Signed-off-by: Alexander Graf <graf@amazon.com>
Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20231207235654.16622-1-graf@amazon.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Alexander Graf 2023-12-07 23:56:54 +00:00 committed by Greg Kroah-Hartman
parent 5133bee62f
commit 2678fd2fe9
3 changed files with 28 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
Documentation/ABI/testing/sysfs-firmware-initrd Normal file
View File

@ -0,0 +1,8 @@
What: /sys/firmware/initrd
Date: December 2023
Contact: Alexander Graf <graf@amazon.com>
Description:
When the kernel was booted with an initrd and the
"retain_initrd" option is set on the kernel command
line, /sys/firmware/initrd contains the contents of the
initrd that the kernel was booted with.

5
Documentation/admin-guide/kernel-parameters.txt
View File

@ -2438,7 +2438,7 @@
between unregistering the boot console and initializing
the real console.
keepinitrd [HW,ARM]
keepinitrd [HW,ARM] See retain_initrd.
kernelcore= [KNL,X86,IA-64,PPC]
Format: nn[KMGTPE] | nn% | "mirror"
@ -5580,7 +5580,8 @@
Useful for devices that are detected asynchronously
(e.g. USB and MMC devices).
retain_initrd [RAM] Keep initrd memory after extraction
retain_initrd [RAM] Keep initrd memory after extraction. After boot, it will
be accessible via /sys/firmware/initrd.
retbleed= [X86] Control mitigation of RETBleed (Arbitrary
Speculative Code Execution with Return Instructions)

18
init/initramfs.c
View File

@ -574,6 +574,16 @@ extern unsigned long __initramfs_size;
#include <linux/initrd.h>
#include <linux/kexec.h>
static ssize_t raw_read(struct file *file, struct kobject *kobj,
struct bin_attribute *attr, char *buf,
loff_t pos, size_t count)
{
memcpy(buf, attr->private + pos, count);
return count;
}
static BIN_ATTR(initrd, 0440, raw_read, NULL, 0);
void __init reserve_initrd_mem(void)
{
phys_addr_t start;
@ -715,8 +725,14 @@ done:
* If the initrd region is overlapped with crashkernel reserved region,
* free only memory that is not part of crashkernel region.
*/
if (!do_retain_initrd && initrd_start && !kexec_free_initrd())
if (!do_retain_initrd && initrd_start && !kexec_free_initrd()) {
free_initrd_mem(initrd_start, initrd_end);
} else if (do_retain_initrd && initrd_start) {
bin_attr_initrd.size = initrd_end - initrd_start;
bin_attr_initrd.private = (void *)initrd_start;
if (sysfs_create_bin_file(firmware_kobj, &bin_attr_initrd))
pr_err("Failed to create initrd sysfs file");
}
initrd_start = 0;
initrd_end = 0;