Re: [PATCH v2] initramfs: Expose retained initrd as sysfs file

[Alexander Graf <graf@amazon.com>]

Hi Bagas,

On 07.12.23 13:37, Bagas Sanjaya wrote:
> On Wed, Dec 06, 2023 at 09:33:23PM +0000, Alexander Graf wrote:
>> 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..25244e2a5739 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) {
>> +		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;
>>   
> On my Arch Linux system, /sys/firmware/initrd is not same as initramfs image
> from /boot partition that is uncompressed. `ls -l` listing shows
> (with /tmp/initramfs-boot is unzstd'ed initramfs of the same kernel booted):
>
> ```
> -r--r----- 1 root root 22967535 Dec  7 19:32 /sys/firmware/initrd
> -rw------- 1 root root 40960000 Dec  7 19:26 /tmp/initramfs-boot
> ```
>
> And thus, `cpio -i -v` listing differs. While in uncompressed initramfs,
> I got expected initramfs contents (early userpace for booting), doing the same
> to /sys/firmware/initrd only shows Intel microcode.
>
> Regardless, exposing initramfs as advertised in the patch description works for
> me.


Thanks a bunch for testing the patch!

The reason you're seeing microcode is that something in your boot chain 
(grub maybe? sd-boot?) sends multiple initrd blobs to Linux: One that 
contains microcode and another that contains the real initrd. Linux 
continues extracting past the first cpio archive.


Alex




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