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

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Message-ID: <ZXPkL1jyXwGH11gJ@archie.me>
Date:   Sat, 9 Dec 2023 10:51:11 +0700
From:   Bagas Sanjaya <bagasdotme@...il.com>
To:     Alexander Graf <graf@...zon.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:     Linux Documentation <linux-doc@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jan H . Schönherr <jschoenh@...zon.de>,
        James Gowans <jgowans@...zon.com>
Subject: Re: [PATCH v3] initramfs: Expose retained initrd as sysfs file

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@...zon.com>
> Tested-by: Bagas Sanjaya <bagasdotme@...il.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@...zon.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/

-- 
An old man doll... just what I always wanted! - Clara

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