Re: [PATCH] common: setting the root= command line parameter

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hi Fabian,

On 11/24/25 11:16 AM, Fabian Pflug wrote:
> In verified boot settings, it might not be wanted to have a root=
> command line parameter for the kernel, because if the initramfs is not
> configured, or barebox boots the kernel directly due to
> misconfiguration, the device might boot correctly, but without verified
> boot, because the kernel will parse root=
> 
> Allowing to change the parameter name will help circumvent such errors,
> as there now needs to be multiple misconfigurations for the verified
> boot to go wrong and the initramfs can use a the different name from the
> commandline to get the rootfs.

Thanks for your patch!

This is a useful thing to have, but I think deciding replacement of
root= across all functionality at compile-time is too invasive:

Imagine you use the same barebox both for development and in release
mode (or you support runtime unlocking with a token).
When booting securely, you want to use verity_root= for example, but
when you do boot /mnt/nfs, you'll want barebox to fix up root= as before.

I thus think that this should be a $global.bootm.root_arg evaluated at
runtime instead.

By adding it to bootm_data_restore_defaults(), its value will then be
restored after the boot script has done executing.

What do you think?

Cheers,
Ahmad

> 
> Signed-off-by: Fabian Pflug <f.pflug@pengutronix.de>
> ---
>  common/Kconfig         | 13 +++++++++++++
>  common/block.c         |  3 +--
>  common/bootm.c         |  8 ++++----
>  drivers/mci/mci-core.c |  2 +-
>  fs/9p/vfs_super.c      |  2 +-
>  fs/nfs.c               |  2 +-
>  fs/squashfs/squashfs.c |  2 +-
>  fs/ubifs/ubifs.c       |  2 +-
>  8 files changed, 23 insertions(+), 11 deletions(-)
> 
> diff --git a/common/Kconfig b/common/Kconfig
> index d923d4c4b6..62797a9e69 100644
> --- a/common/Kconfig
> +++ b/common/Kconfig
> @@ -1252,6 +1252,19 @@ config SERIAL_NUMBER_FIXUP_SYSTEMD_HOSTNAME
>  	  This option without effect if global.bootm.provide_hostname
>  	  is unset.
>  
> +config ROOT_COMMANDLINE_PARAMETER
> +	string "parameter name for root partition"
> +	default "root"
> +	help
> +	  When setting the root partition on the commandline string to the os, use
> +	  this name as an indicator of the root partition.
> +
> +	  Changing this could be useful in verified boot contexts to not give the
> +	  kernel the chance to boot a non-secure image, but still use the power of
> +	  barebox to get the right disk for the rootfs.
> +
> +	  If unsure, leave as default (root)
> +
>  config SYSTEMD_OF_WATCHDOG
>  	bool "inform devicetree-enabled kernel of used watchdog"
>  	depends on WATCHDOG && OFTREE && FLEXIBLE_BOOTARGS
> diff --git a/common/block.c b/common/block.c
> index ca2ed37dbd..bb8b01c0a0 100644
> --- a/common/block.c
> +++ b/common/block.c
> @@ -601,11 +601,10 @@ char *cdev_get_linux_rootarg(const struct cdev *partcdev)
>  	if (blk->ops->get_rootarg)
>  		rootarg = blk->ops->get_rootarg(blk, partcdev);
>  	if (!rootarg && partcdev->partuuid[0] != 0)
> -		rootarg = basprintf("root=PARTUUID=%s", partcdev->partuuid);
> +		rootarg = basprintf(CONFIG_ROOT_COMMANDLINE_PARAMETER "=PARTUUID=%s", partcdev->partuuid);
>  
>  	if (IS_ENABLED(CONFIG_ROOTWAIT_BOOTARG) && blk->rootwait)
>  		rootarg = linux_bootargs_append_rootwait(rootarg);
>  
>  	return rootarg;
>  }
> -
> diff --git a/common/bootm.c b/common/bootm.c
> index 17792b2a1d..4549bfeb8b 100644
> --- a/common/bootm.c
> +++ b/common/bootm.c
> @@ -852,10 +852,10 @@ int bootm_boot(struct bootm_data *bootm_data)
>  				rootarg = ERR_PTR(-EINVAL);
>  
>  				if (!root_cdev)
> -					pr_err("no cdev found for %s, cannot set root= option\n",
> +					pr_err("no cdev found for %s, cannot set " CONFIG_ROOT_COMMANDLINE_PARAMETER "= option\n",
>  						root_dev_name);
>  				else if (!root_cdev->partuuid[0])
> -					pr_err("%s doesn't have a PARTUUID, cannot set root= option\n",
> +					pr_err("%s doesn't have a PARTUUID, cannot set " CONFIG_ROOT_COMMANDLINE_PARAMETER "= option\n",
>  						root_dev_name);
>  			}
>  
> @@ -866,7 +866,7 @@ int bootm_boot(struct bootm_data *bootm_data)
>  		}
>  
>  		if (IS_ERR(rootarg)) {
> -			pr_err("Failed to append kernel cmdline parameter 'root='\n");
> +			pr_err("Failed to append kernel cmdline parameter '" CONFIG_ROOT_COMMANDLINE_PARAMETER "='\n");
>  		} else {
>  			pr_info("Adding \"%s\" to Kernel commandline\n", rootarg);
>  			globalvar_add_simple("linux.bootargs.bootm.appendroot",
> @@ -1165,7 +1165,7 @@ BAREBOX_MAGICVAR(global.bootm.dryrun, "bootm default dryrun level");
>  BAREBOX_MAGICVAR(global.bootm.verify, "bootm default verify level");
>  BAREBOX_MAGICVAR(global.bootm.verbose, "bootm default verbosity level (0=quiet)");
>  BAREBOX_MAGICVAR(global.bootm.earlycon, "Add earlycon option to Kernel for early log output");
> -BAREBOX_MAGICVAR(global.bootm.appendroot, "Add root= option to Kernel to mount rootfs from the device the Kernel comes from (default, device can be overridden via global.bootm.root_dev)");
> +BAREBOX_MAGICVAR(global.bootm.appendroot, "Add " CONFIG_ROOT_COMMANDLINE_PARAMETER "= option to Kernel to mount rootfs from the device the Kernel comes from (default, device can be overridden via global.bootm.root_dev)");
>  BAREBOX_MAGICVAR(global.bootm.root_dev, "bootm default root device (overrides default device in global.bootm.appendroot)");
>  BAREBOX_MAGICVAR(global.bootm.provide_machine_id, "If true, append systemd.machine_id=$global.machine_id to Kernel command line");
>  BAREBOX_MAGICVAR(global.bootm.provide_hostname, "If true, append systemd.hostname=$global.hostname to Kernel command line");
> diff --git a/drivers/mci/mci-core.c b/drivers/mci/mci-core.c
> index 57825bc849..79cebe19f3 100644
> --- a/drivers/mci/mci-core.c
> +++ b/drivers/mci/mci-core.c
> @@ -2691,7 +2691,7 @@ static char *mci_get_linux_mmcblkdev(struct block_device *blk,
>  		 * skipping it.
>  		 */
>  		if (cdev_partname_equal(partcdev, cdev))
> -			return basprintf("root=/dev/mmcblk%dp%d", id, partnum);
> +			return basprintf(CONFIG_ROOT_COMMANDLINE_PARAMETER "=/dev/mmcblk%dp%d", id, partnum);
>  		if (cdev->flags & DEVFS_PARTITION_FROM_TABLE)
>  			partnum++;
>  	}
> diff --git a/fs/9p/vfs_super.c b/fs/9p/vfs_super.c
> index 6a451d9ef5..39a8529a4e 100644
> --- a/fs/9p/vfs_super.c
> +++ b/fs/9p/vfs_super.c
> @@ -70,7 +70,7 @@ static void v9fs_set_rootarg(struct v9fs_session_info *v9ses,
>  	trans = v9ses->clnt->trans_mod->name;
>  	path = v9ses->aname;
>  
> -	str = basprintf("root=%s rootfstype=9p rootflags=trans=%s,msize=%d,"
> +	str = basprintf(CONFIG_ROOT_COMMANDLINE_PARAMETER "=%s rootfstype=9p rootflags=trans=%s,msize=%d,"
>  			"cache=loose,uname=%s,dfltuid=0,dfltgid=0,aname=%s",
>  			tag, trans, v9ses->clnt->msize, tag, path);
>  
> diff --git a/fs/nfs.c b/fs/nfs.c
> index 5c2476cd88..9db23c5d18 100644
> --- a/fs/nfs.c
> +++ b/fs/nfs.c
> @@ -1558,7 +1558,7 @@ static void nfs_set_rootarg(struct nfs_priv *npriv, struct fs_device *fsdev)
>  	char *str, *tmp;
>  	const char *bootargs;
>  
> -	str = basprintf("root=/dev/nfs nfsroot=%pI4:%s%s%s", &npriv->server, npriv->path,
> +	str = basprintf(CONFIG_ROOT_COMMANDLINE_PARAMETER "=/dev/nfs nfsroot=%pI4:%s%s%s", &npriv->server, npriv->path,
>  			  rootnfsopts[0] ? "," : "", rootnfsopts);
>  
>  	/* forward specific mount options on demand */
> diff --git a/fs/squashfs/squashfs.c b/fs/squashfs/squashfs.c
> index e30372627a..8267e3cba0 100644
> --- a/fs/squashfs/squashfs.c
> +++ b/fs/squashfs/squashfs.c
> @@ -60,7 +60,7 @@ static void squashfs_set_rootarg(struct fs_device *fsdev)
>  	ubi_get_device_info(vi.ubi_num, &di);
>  	mtd = di.mtd;
>  
> -	str = basprintf("root=/dev/ubiblock%d_%d ubi.mtd=%s ubi.block=%d,%d rootfstype=squashfs",
> +	str = basprintf(CONFIG_ROOT_COMMANDLINE_PARAMETER "=/dev/ubiblock%d_%d ubi.mtd=%s ubi.block=%d,%d rootfstype=squashfs",
>  			vi.ubi_num, vi.vol_id, mtd->cdev.partname, vi.ubi_num, vi.vol_id);
>  
>  	fsdev_set_linux_rootarg(fsdev, str);
> diff --git a/fs/ubifs/ubifs.c b/fs/ubifs/ubifs.c
> index 37986acbb2..34151a0c9f 100644
> --- a/fs/ubifs/ubifs.c
> +++ b/fs/ubifs/ubifs.c
> @@ -442,7 +442,7 @@ static void ubifs_set_rootarg(struct ubifs_priv *priv,
>  
>  	mtd = di.mtd;
>  
> -	str = basprintf("root=ubi0:%s ubi.mtd=%s rootfstype=ubifs",
> +	str = basprintf(CONFIG_ROOT_COMMANDLINE_PARAMETER "=ubi0:%s ubi.mtd=%s rootfstype=ubifs",
>  			  vi.name, mtd->cdev.partname);
>  
>  	fsdev_set_linux_rootarg(fsdev, str);

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