Actually, upon review my current needs technically only require read functionality (even though as I understand it the communication is meant to be bidrectional), so if it would be easier to accept I can resubmit a readonly version.
----- Original Message -----
From: "Shea Levy" <address@hidden>
To: <address@hidden>
Cc:
Sent: Mon, 01 Feb 2016 09:27:38 -0500
Subject: Re: [PATCH] Add module for interacting with the android bootloader control block
Hi Andrei,
There's no formal specification for any of this, all I have to go off
of is the bits of the android codebase I've read, but I'll try to answer
as best I can.
On 2016-01-31 23:18, Andrei Borzenkov wrote:
> 01.02.2016 02:55, Shea Levy пишет:
>>
>>
>> Android systems use a so-called "bootloader control block" to
>> coordinate between the bootloader and the recovery system (which
>> handles
>> factory resets, system updates, firmware updates, etc.). The BCB is
>> traditionally either an MTD device or an eMMC partition, and it
>> contains
>> a struct bootloader_message directly on the device/partition. The
>> recovery system will write a message into the control block, which
>> the
>> bootloader is then meant to check to see if it should boot into
>> recovery
>> mode or normal mode (the idea being that the control block is only
>> cleared once recovery is finished, ensuring fault-tolerance; if the
>> device is powered off mid-update, it will boot back into the same
>> recovery mode).
>>
>> This module exports the bcb fields as env variables.
>> First the user sets android_bcb_disk to the partition containing the
>> BCB
>> (I don't know how to support MTDs yet but I only deal with x86).
>> Then
>> variables such as android_bcb_command and android_bcb_recovery can
>> be
>> checked and used to determine which mode to boot into. They can
>> also, if
>> necessary, be modified by setting the variables in question. The
>> typical
>> use case will probably be to check if there is a recovery command
>> present and if so boot into recovery mode.
>>
>> Does that explain enough?
>>
>
> How these variables are intended to be used? Could you provide
> example
> of actual values in this block and example of grub code using them?
>
The recovery service that's started when booted into recovery mode
reads values out of the BCB to determine its operation. For example, it
checks that the first (newline-separated) string in the 'recovery' is
field is "recovery", and then uses the rest to set its argv (e.g. a
recovery field consisting of "recovery\n--wipe_data\0" would specify a
factory reset). The app itself writes in "boot-recovery" into the
"command" field and copies over its arguments, so that upon reboot the
command will be started over again (until the command actually finishes,
when it zeroes out the BCB).
>
> How BCB area is identified? Your code allows writing to arbitrary
> device
> without any sanity check.
>
On the android userspace side there's only a recovery.fstab file, which
just names a block device. The code simply reads a struct out of the
block with no magic number or other kind of verification. Sometimes
there's not even a real partition table, just a config file with an
offset into a block.
>
> Is bootloader even *allowed* to make any choice here to remain
> compliant? I.e. can bootloader ignore request stored in BCB?
>
I have been unable to find any formal spec on the matter, but at least
respecting the "boot-recovery" command would probably be a good idea for
stock android. But this seems more like policy than mechanism, and IMO
makes more sense to be determined at the grub.cfg level rather than the
module level.
~Shea
>
>>
>> Thanks,
>> Shea
>>
>> On 2016-01-31 18:41, Vladimir 'phcoder' Serbinenko
>> wrote:
>>
>>> Sorry but I don't quite understand what this module is for.
>> Can you please give an example of how it's used both in original
>> context
>> and in GRUB. I have couple of style comments as well but clarifying
>> the
>> usecase first is important.
>>>
>>> Le lun. 1 févr. 2016 00:01, Shea Levy
>> <address@hidden> a écrit :
>>>
>>>> ---
>>>> grub-core/Makefile.core.def
>> | 5 ++
>>>> grub-core/android/android_bcb.c | 163
>> ++++++++++++++++++++++++++++++++++++++++
>>>> 2 files changed, 168
>> insertions(+)
>>>> create mode 100644 grub-core/android/android_bcb.c
>>>>
>>
>>>> diff --git a/grub-core/Makefile.core.def
>> b/grub-core/Makefile.core.def
>>>> index 0cc40bb..b045ea4 100644
>>>> ---
>> a/grub-core/Makefile.core.def
>>>> +++ b/grub-core/Makefile.core.def
>>>> @@
>> -2327,3 +2327,8 @@ module = {
>>>> common = loader/i386/xen_file64.c;
>>>>
>> extra_dist = loader/i386/xen_fileXX.c;
>>>> };
>>>> +
>>>> +module = {
>>>> +
>> name = android_bcb;
>>>> + common = android/android_bcb.c;
>>>> +};
>>>> diff
>> --git a/grub-core/android/android_bcb.c
>> b/grub-core/android/android_bcb.c
>>>> new file mode 100644
>>>> index
>> 0000000..db49446
>>>> --- /dev/null
>>>> +++
>> b/grub-core/android/android_bcb.c
>>>> @@ -0,0 +1,163 @@
>>>> +/*
>> android_bcb.c - module for interacting with the android bootloader
>> control block */
>>>> +/*
>>>> + * GRUB -- GRand Unified Bootloader
>>>> + *
>> Copyright (C) 2016 Free Software Foundation, Inc.
>>>> + *
>>>> + * GRUB is
>> free software: you can redistribute it and/or modify
>>>> + * it under the
>> terms of the GNU General Public License as published by
>>>> + * the Free
>> Software Foundation, either version 3 of the License, or
>>>> + * (at your
>> option) any later version.
>>>> + *
>>>> + * GRUB is distributed in the hope
>> that it will be useful,
>>>> + * but WITHOUT ANY WARRANTY; without even
>> the implied warranty of
>>>> + * MERCHANTABILITY or FITNESS FOR A
>> PARTICULAR PURPOSE. See the
>>>> + * GNU General Public License for more
>> details.
>>>> + *
>>>> + * You should have received a copy of the GNU
>> General Public License
>>>> + * along with GRUB. If not, see
>> <http://www.gnu.org/licenses/ [1]>.
>>>> + */
>>>> +
>>>> +#include
>> <stddef.h>
>>>> +
>>>> +#include <grub/dl.h>
>>>> +#include <grub/env.h>
>>>>
>> +#include <grub/disk.h>
>>>> +
>>>> +GRUB_MOD_LICENSE ("GPLv3+");
>>>> +
>>>>
>> +/* Definition of struct bootloader message from
>>
>> https://android.googlesource.com/platform/bootable/recovery/+/9d72d4175b06a70c64c8867ff65b3c4c2181c9a9/bootloader.h#20
>> [2]
>>>> + * Available under the following copyright and terms:
>>>> + *
>>>>
>> + * Copyright (C) 2008 The Android Open Source Project
>>>> + *
>>>> + *
>> Licensed under the Apache License, Version 2.0 (the "License");
>>>> + *
>> you may not use this file except in compliance with the License.
>>>> + *
>> You may obtain a copy of the License at
>>>> + *
>>>> + *
>> http://www.apache.org/licenses/LICENSE-2.0 [3]
>>>> + *
>>>> + * Unless
>> required by applicable law or agreed to in writing, software
>>>> + *
>> distributed under the License is distributed on an "AS IS" BASIS,
>>>> + *
>> WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
>> implied.
>>>> + * See the License for the specific language governing
>> permissions and
>>>> + * limitations under the License.
>>>> + */
>>>> +struct
>> bootloader_message
>>>> +{
>>>> + char command[32];
>>>> + char status[32];
>>>>
>> + char recovery[768];
>>>> + // The 'recovery' field used to be 1024
>> bytes. It has only ever
>>>> + // been used to store the recovery command
>> line, so 768 bytes
>>>> + // should be plenty. We carve off the last 256
>> bytes to store the
>>>> + // stage string (for multistage packages) and
>> possible future
>>>> + // expansion.
>>>> + char stage[32];
>>>> + char
>> slot_suffix[32];
>>>> + char reserved[192];
>>>> +} GRUB_PACKED;
>>>> +
>>>>
>> +struct bcb_state
>>>> +{
>>>> + grub_disk_t disk;
>>>> + char
>> internal_write;
>>>> + struct bootloader_message msg;
>>>> +};
>>>> +
>>>>
>> +static struct bcb_state state = { 0 };
>>>> +
>>>> +#define
>> SET_FIELD(field)
>>>> + state.msg. field[sizeof state.msg. field - 1] =
>> '';
>>>> + grub_env_set ("android_bcb_" #field, state.msg. field)
>>>> +
>>>>
>> +static void open_disk (const char *name) {
>>>> + if (state.disk) {
>>>> +
>> grub_disk_close (state.disk);
>>>> + }
>>>> +
>>>> + state.disk =
>> grub_disk_open (name);
>>>> + if (state.disk) {
>>>> + grub_err_t err =
>> grub_disk_read (state.disk, 0, 0, sizeof state.msg,
>>>> + &state.msg);
>>>>
>> + if (err) {
>>>> + grub_disk_close (state.disk);
>>>> + state.disk = 0;
>>>>
>> + } else {
>>>> + state.internal_write = 1;
>>>> + SET_FIELD (command);
>>>> +
>> SET_FIELD (status);
>>>> + SET_FIELD (recovery);
>>>> + SET_FIELD
>> (stage);
>>>> + SET_FIELD (slot_suffix);
>>>> + state.internal_write = 0;
>>>>
>> + }
>>>> + }
>>>> +}
>>>> +
>>>> +#define MAYBE_UPDATE_FIELD(field)
>>>> + do {
>> if (!grub_strcmp (var->name, "android_bcb_" #field))
>>>> + {
>>>> +
>> grub_memcpy (state.msg. field, val, sizeof state.msg. field - 1);
>>>> +
>> state.msg. field[sizeof state.msg. field - 1] = '';
>>>> +
>> grub_disk_write (state.disk, 0,
>>>> + offsetof (struct
>> bootloader_message, field),
>>>> + sizeof state.msg. field, state.msg.
>> field);
>>>> + } } while (0)
>>>> +
>>>> +static char *handle_write (struct
>> grub_env_var *var,
>>>> + const char *val)
>>>> +{
>>>> + if (!grub_strcmp
>> (var->name, "android_bcb_disk"))
>>>> + {
>>>> + open_disk (val);
>>>> + if
>> (!state.disk)
>>>> + grub_print_error ();
>>>> + }
>>>> + else if (state.disk
>> && !state.internal_write)
>>>> + {
>>>> + MAYBE_UPDATE_FIELD (command);
>>>> +
>> MAYBE_UPDATE_FIELD (status);
>>>> + MAYBE_UPDATE_FIELD (recovery);
>>>> +
>> MAYBE_UPDATE_FIELD (stage);
>>>> + MAYBE_UPDATE_FIELD (slot_suffix);
>>>> +
>> }
>>>> +
>>>> + return grub_strdup (val);
>>>> +}
>>>> +
>>>>
>> +GRUB_MOD_INIT(android_bcb)
>>>> +{
>>>> + const char *disk = grub_env_get
>> ("android_bcb_disk");
>>>> + if (disk)
>>>> + {
>>>> + open_disk (disk);
>>>> +
>> if (!state.disk)
>>>> + grub_print_error ();
>>>> + }
>>>> +
>>>> + if
>> (!grub_register_variable_hook ("android_bcb_disk", 0, handle_write))
>>>>
>> + grub_print_error ();
>>>> + if (grub_register_variable_hook
>> ("android_bcb_command", 0, handle_write))
>>>> + grub_print_error ();
>>>> +
>> if (grub_register_variable_hook ("android_bcb_status", 0,
>> handle_write))
>>>> + grub_print_error ();
>>>> + if
>> (grub_register_variable_hook ("android_bcb_recovery", 0,
>> handle_write))
>>>> + grub_print_error ();
>>>> + if
>> (grub_register_variable_hook ("android_bcb_stage", 0, handle_write))
>>>>
>> + grub_print_error ();
>>>> + if (grub_register_variable_hook
>> ("android_bcb_slot_suffix", 0, handle_write))
>>>> + grub_print_error
>> ();
>>>> +}
>>>> +
>>>> +GRUB_MOD_FINI(android_bcb)
>>>> +{
>>>> +
>> grub_register_variable_hook ("android_bcb_disk", 0, 0);
>>>> +
>> grub_register_variable_hook ("android_bcb_command", 0, 0);
>>>> +
>> grub_register_variable_hook ("android_bcb_status", 0, 0);
>>>> +
>> grub_register_variable_hook ("android_bcb_recovery", 0, 0);
>>>> +
>> grub_register_variable_hook ("android_bcb_stage", 0, 0);
>>>> +
>> grub_register_variable_hook ("android_bcb_slot_suffix", 0, 0);
>>>> +
>>>> +
>> if (state.disk)
>>>> + grub_disk_close (state.disk);
>>>> +}
>>>> --
>>>>
>> 2.7.0
>>>>
>>>> _______________________________________________
>>>>
>> Grub-devel mailing list
>>>> address@hidden
>>>>
>> https://lists.gnu.org/mailman/listinfo/grub-devel [4]
>>
>>
>>
>>
>> Links:
>> ------
>> [1] http://www.gnu.org/licenses/
>> [2]
>>
>> https://android.googlesource.com/platform/bootable/recovery/+/9d72d4175b06a70c64c8867ff65b3c4c2181c9a9/bootloader.h#20
>> [3]
>> http://www.apache.org/licenses/LICENSE-2.0
>> [4]
>> https://lists.gnu.org/mailman/listinfo/grub-devel
>>
>>
>>
>> _______________________________________________
>> Grub-devel mailing list
>> address@hidden
>> https://lists.gnu.org/mailman/listinfo/grub-devel
>>
>
>
> _______________________________________________
> Grub-devel mailing list
> address@hidden
> https://lists.gnu.org/mailman/listinfo/grub-devel
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