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 <[email protected]>
Tested-by: Bagas Sanjaya <[email protected]>
---
v1 -> v2:
- Reword commit message to explain the new file path
- Add a Documentation/ABI/testing/sysfs-firmware-initrd file
v2 -> v3:
- Only expose file when initrd is present (James Gowans)
---
.../ABI/testing/sysfs-firmware-initrd | 8 ++++++++
.../admin-guide/kernel-parameters.txt | 5 +++--
init/initramfs.c | 18 +++++++++++++++++-
3 files changed, 28 insertions(+), 3 deletions(-)
create mode 100644 Documentation/ABI/testing/sysfs-firmware-initrd
diff --git a/Documentation/ABI/testing/sysfs-firmware-initrd b/Documentation/ABI/testing/sysfs-firmware-initrd
new file mode 100644
index 000000000000..20bf7cf77a19
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-firmware-initrd
@@ -0,0 +1,8 @@
+What: /sys/firmware/initrd
+Date: December 2023
+Contact: Alexander Graf <[email protected]>
+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 65731b060e3f..51575cd31741 100644
--- 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 8d0fd946cdd2..76deb48c38cb 100644
--- 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 @@ static void __init do_populate_rootfs(void *unused, async_cookie_t cookie)
* 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;
--
2.40.1
Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
Sitz: Berlin
Ust-ID: DE 289 237 879
On Thu, Dec 07, 2023 at 11:56:54PM +0000, Alexander Graf wrote:
> 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 <[email protected]>
> Tested-by: Bagas Sanjaya <[email protected]>
>
> ---
>
> v1 -> v2:
>
> - Reword commit message to explain the new file path
> - Add a Documentation/ABI/testing/sysfs-firmware-initrd file
>
> v2 -> v3:
>
> - Only expose file when initrd is present (James Gowans)
> ---
> .../ABI/testing/sysfs-firmware-initrd | 8 ++++++++
> .../admin-guide/kernel-parameters.txt | 5 +++--
> init/initramfs.c | 18 +++++++++++++++++-
> 3 files changed, 28 insertions(+), 3 deletions(-)
> create mode 100644 Documentation/ABI/testing/sysfs-firmware-initrd
>
> diff --git a/Documentation/ABI/testing/sysfs-firmware-initrd b/Documentation/ABI/testing/sysfs-firmware-initrd
> new file mode 100644
> index 000000000000..20bf7cf77a19
> --- /dev/null
> +++ b/Documentation/ABI/testing/sysfs-firmware-initrd
> @@ -0,0 +1,8 @@
> +What: /sys/firmware/initrd
> +Date: December 2023
> +Contact: Alexander Graf <[email protected]>
> +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 65731b060e3f..51575cd31741 100644
> --- 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)
You may want to adjust documentation to address my testing [1]. In summary,
only the first initrd blob that was passed to the kernel will be exposed
in /sys/firmware/initrd.
Thanks.
[1]: https://lore.kernel.org/linux-doc/[email protected]/
--
An old man doll... just what I always wanted! - Clara
On 09.12.23 04:51, Bagas Sanjaya wrote:
> On Thu, Dec 07, 2023 at 11:56:54PM +0000, Alexander Graf wrote:
>> 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 <[email protected]>
>> Tested-by: Bagas Sanjaya <[email protected]>
>>
>> ---
>>
>> v1 -> v2:
>>
>> - Reword commit message to explain the new file path
>> - Add a Documentation/ABI/testing/sysfs-firmware-initrd file
>>
>> v2 -> v3:
>>
>> - Only expose file when initrd is present (James Gowans)
>> ---
>> .../ABI/testing/sysfs-firmware-initrd | 8 ++++++++
>> .../admin-guide/kernel-parameters.txt | 5 +++--
>> init/initramfs.c | 18 +++++++++++++++++-
>> 3 files changed, 28 insertions(+), 3 deletions(-)
>> create mode 100644 Documentation/ABI/testing/sysfs-firmware-initrd
>>
>> diff --git a/Documentation/ABI/testing/sysfs-firmware-initrd b/Documentation/ABI/testing/sysfs-firmware-initrd
>> new file mode 100644
>> index 000000000000..20bf7cf77a19
>> --- /dev/null
>> +++ b/Documentation/ABI/testing/sysfs-firmware-initrd
>> @@ -0,0 +1,8 @@
>> +What: /sys/firmware/initrd
>> +Date: December 2023
>> +Contact: Alexander Graf <[email protected]>
>> +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 65731b060e3f..51575cd31741 100644
>> --- 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)
> You may want to adjust documentation to address my testing [1]. In summary,
> only the first initrd blob that was passed to the kernel will be exposed
> in /sys/firmware/initrd.
From the kernel's point of view, there is only a single initrd binary
:). Whether your boot loader concatenates multiple archives to create
that single one out of multiple individual files is a boot loader
implementation detail.
Alex
Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
Sitz: Berlin
Ust-ID: DE 289 237 879