Re: [PATCH v3] initramfs: Expose retained initrd as sysfs file

[Alexander Graf]

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@xxxxxxxxxx>
> > Tested-by: Bagas Sanjaya <bagasdotme@xxxxxxxxx>
> >
> > ---
> >
> > 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@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)
> 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