diff mbox series

[v3] initramfs: Expose retained initrd as sysfs file

Message ID 20231207235654.16622-1-graf@amazon.com
State New
Headers
Series [v3] initramfs: Expose retained initrd as sysfs file |

Commit Message

Alexander Graf Dec. 7, 2023, 11:56 p.m. UTC 
  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>

---

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

Comments

Bagas Sanjaya Dec. 9, 2023, 3:51 a.m. UTC | #1 
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 <graf@amazon.com>
> Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>
> 
> ---
> 
> 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 <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 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/2a86a7f2-bbf3-43fb-82b9-40d3397d5578@amazon.com/
Alexander Graf Dec. 9, 2023, 9:23 a.m. UTC | #2 
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 <graf@amazon.com>
>> Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>
>>
>> ---
>>
>> 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 <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 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
diff mbox series

Patch

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 <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 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;