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

[Alexander Graf]

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@xxxxxxxxxx>
> > +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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