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>

diff --git a/Documentation/ABI/testing/sysfs-firmware-initrd b/Documentation/ABI/testing/sysfs-firmware-initrd
new file mode 100644 (file)
index 0000000..20bf7cf
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-firmware-initrd
@@ -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.

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 65731b060e3fef98cb97f03695ca8757ee197700..51575cd31741ef9df09d6cb41dc17d16afa08658 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -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)

diff --git a/init/initramfs.c b/init/initramfs.c
index 8d0fd946cdd2b3fc8eae59ae6715928fa70e6f18..76deb48c38cb16dd779de7ee91b35785b6890579 100644 (file)
--- a/init/initramfs.c
+++ b/init/initramfs.c
@@ -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;