Re: [PATCH v12] dm: add support to directly boot to a mapped device

[Helen Koike]

Hi Stephen,

On 6/3/19 8:02 PM, Stephen Boyd wrote:
> Quoting Helen Koike (2019-02-21 12:33:34)
>> Add a "create" module parameter, which allows device-mapper targets to be
>> configured at boot time. This enables early use of dm targets in the boot
>> process (as the root device or otherwise) without the need of an initramfs.
>>
>> The syntax used in the boot param is based on the concise format from the
>> dmsetup tool to follow the rule of least surprise:
>>
>>         sudo dmsetup table --concise /dev/mapper/lroot
>>
>> Which is:
>>         dm-mod.create=<name>,<uuid>,<minor>,<flags>,<table>[,<table>+][;<name>,<uuid>,<minor>,<flags>,<table>[,<table>+]+]
>>
>> Where,
>>         <name>          ::= The device name.
>>         <uuid>          ::= xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx | ""
>>         <minor>         ::= The device minor number | ""
>>         <flags>         ::= "ro" | "rw"
>>         <table>         ::= <start_sector> <num_sectors> <target_type> <target_args>
>>         <target_type>   ::= "verity" | "linear" | ...
>>
>> For example, the following could be added in the boot parameters:
>> dm-mod.create="lroot,,,rw, 0 4096 linear 98:16 0, 4096 4096 linear 98:32 0" root=/dev/dm-0
>>
>> Only the targets that were tested are allowed and the ones that doesn't
>> change any block device when the dm is create as read-only. For example,
>> mirror and cache targets are not allowed. The rationale behind this is
>> that if the user makes a mistake, choosing the wrong device to be the
>> mirror or the cache can corrupt data.
>>
>> The only targets allowed are:
>> * crypt
>> * delay
>> * linear
>> * snapshot-origin
>> * striped
>> * verity
>>
>> Co-developed-by: Will Drewry <wad@xxxxxxxxxxxx>
>> Co-developed-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> Co-developed-by: Enric Balletbo i Serra <enric.balletbo@xxxxxxxxxxxxx>
>> Signed-off-by: Helen Koike <helen.koike@xxxxxxxxxxxxx>
>>
>> ---
>>
> 
> I'm trying to boot a mainline linux kernel on a chromeos device with dm
> verity and a USB stick but it's not working for me even with this patch.
> I've had to hack around two problems:
> 
>  1) rootwait isn't considered
> 
>  2) verity doesn't seem to accept UUID for <hash_dev> or <dev>
> 
> For the first problem, it happens every boot for me because I'm trying
> to boot off of a USB stick and it's behind a hub that takes a few
> seconds to enumerate. If I hack up the code to call dm_init_init() after
> the 'rootdelay' cmdline parameter is used then I can make this work. It
> would be much nicer if the whole mechanism didn't use a late initcall
> though. If it used a hook from prepare_namespace() and then looped
> waiting for devices to create when rootwait was specified it would work.

The patch was implemented with late initcall partially to be contained
in drivers/md/*, but to support rootwait, adding a hook from
prepare_namespace seems the way to go indeed.

> 
> The second problem is that in chromeos we have the bootloader fill out
> the UUID of the kernel partition (%U) and then we have another parameter
> that indicates the offset from that kernel partition to add to the
> kernel partition (typically 1, i.e. PARTNROFF=1) to find the root
> filesystem partition. The way verity seems to work here is that we need
> to specify a path like /dev/sda3 or the major:minor number of the device
> on the commandline to make this work. It would be better if we could add
> in support for the PARTNROFF style that name_to_dev_t() handles so we
> can specify the root partition like we're currently doing. I suspect we
> should be able to add support for this into the device mapper layer so
> that we can specify devices this way.

hmm, I didn't test this yet but at least from what I can see in the
code, verity_ctr() calls dm_get_device() that ends up calling
name_to_dev_t() which should take care of PARTNROFF, this requires a bit
more investigation.

> 
> If it helps, an example commandline I've been using to test out a usb
> stick is as follows:
> 
> dm-mod.create="vroot,,0,ro, 0 4710400 verity 0 8:19 8:19 4096 4096 588800 588800 sha1 9b0a223aedbf74b06442b0f05fbff33c55edd010 414b21fba60a1901e23aec373e994942e991d6762631e54a39bc42411f244bd2"

Thanks

> 
> Also, the documentation (Documentation/device-mapper/dm-init.txt) says
> we can use a way that doesn't specify so many arguments, but dm verity
> complains about not enough arguments (10) when following the example:
> 
>   vroot,,,ro,
>   0 1740800 verity 254:0 254:0 1740800 sha1
>   76e9be054b15884a9fa85973e9cb274c93afadb6
>   5b3549d54d6c7a3837b9b81ed72e49463a64c03680c47835bef94d768e5646fe;    
> 
> So the documentation needs an update?
> 

Ack, I'll update this.

Thanks
Helen