前言

在看本文之前，请乖乖的答应我去看这个专利最开始的几篇，先知道一下AVB是什么东西，用在什么场景。再来看这个是怎么使用的。

没有规律的撸撸AVB代码：AVB中CPP到C

今天有点头晕，老早就溜了，回来闲着无事，便记录一下最近学习AVB的东西。

之前在这个专栏里写了很多的东西，也转载记录了一些前辈的优秀博客，这里自己最近也在看代码，于是就再来梳理一下。

之前也有涉及到源码，或者有涉及到应用，总是将两者拆分开的。于是这里记录一个小点，AVB怎么用。你要去跟踪的时候你的打印应该加在哪里？

那么废话不多说，跟着老习惯，自顶向下分为应用层+实现层

1、应用层

应用层的东西就是展示android是怎么用到AVB的，以及它是怎么使用的。

那么这里我们从以下几个问题？

1、AVB哪里用的？

1、Stage one

这个我之前在文章多次提到过，AVB的校验本质上分为两个阶段。

在bootloader中，需要校验分区的函数变量：

// Library/avb/VerifiedBoot.cstatic CHAR8 *avb_verify_partition_name[] = {"boot","dtbo","vbmeta","recovery","vendor_boot"
};

从上面的变量可以知道，在bootloader中，可能需要校验上面的5个分区。

为什么说是可能？因为最终的校验是需要根据vbmeta分区来决定的。

解析vbmeta镜像：

avbtool info_image --image vbmeta.img > vbmeta.img.info

Minimum libavb version:   1.0
Header Block:             256 bytes
Authentication Block:     576 bytes
Auxiliary Block:          4032 bytes
Public key (sha1):        2e22ae4a46cf9db9c24ab74cee91fa005ccb30e4
Algorithm:                SHA256_RSA4096
Rollback Index:           0
Flags:                    0
Release String:           'avbtool 1.1.0'
Descriptors:Chain Partition descriptor:Partition Name:          vbmeta_systemRollback Index Location: 2Public key (sha1):       fc92d9cba0628858d846fb9a18a7af72b05d7dc8...Hash descriptor:Image Size:            59932672 bytesHash Algorithm:        sha256Partition Name:        bootSalt:                  e691366c1c43ee5e23b342d65555ad8cfbadf77118dceb77e240c8e7d3e63ea6Digest:                239648eb41f5a491c7c4d6b51b52a533bd9da98ba8800f58a0957f7341dd1686Flags:                 0Hash descriptor:Image Size:            740733 bytesHash Algorithm:        sha256Partition Name:        dtboSalt:                  d445a36d8154a774589dd51c49029ee388ecaac28212c8c6899f45dc5a51dbcfDigest:                cac0bd59091464292bc83d6f1193afb1520c12a3849f2673ad3160bb951acf6dFlags:                 0Hash descriptor:Image Size:            1916928 bytesHash Algorithm:        sha256Partition Name:        vendor_bootSalt:                  5f7b7c3592142d4f3645d7e675fb7865915e52e8b361ba330fccf00aeb1c4028Digest:                cf153ab9df9a5c34024417ea2b0b4dfd716d01ba9cd2d5bcf964fe7e25cdd802Flags:                 0Hashtree descriptor:Version of dm-verity:  1Image Size:            864256 bytesTree Offset:           864256Tree Size:             12288 bytesData Block Size:       4096 bytesHash Block Size:       4096 bytesFEC num roots:         2FEC offset:            876544FEC size:              8192 bytesHash Algorithm:        sha1Partition Name:        odmSalt:                  b6e1f57ae6939659355e83ad7fa57feb6b5eb15a3d16b96752f43cdc14918708Root Digest:           da99875b16661e72eec81d05d58dffdf09fe228dFlags:                 0Hashtree descriptor:Version of dm-verity:  1Image Size:            1002233856 bytesTree Offset:           1002233856Tree Size:             7897088 bytesData Block Size:       4096 bytesHash Block Size:       4096 bytesFEC num roots:         2FEC offset:            1010130944FEC size:              7987200 bytesHash Algorithm:        sha1Partition Name:        vendorSalt:                  b6e1f57ae6939659355e83ad7fa57feb6b5eb15a3d16b96752f43cdc14918708Root Digest:           336bd4885da274ae0de38b32b899f7b6169e676fFlags:                 0

通过vbmeta.img.info可以知道，bootloader中需要校验 vbmeta、boot 、dtbo、vendor_boot四个分区。

当然这些都是可以自己设置的，比如说你的产品的boot\dtbo等有些分区你觉得不想用AVB的方式，你把这些分区放到SecureBoot启动链中去校验也可以的。

2、Stage two

init阶段的avb校验，可以通过dts和fstab确定需要校验的分区。

 android {compatible = "android,firmware";vbmeta {compatible = "android,vbmeta";parts = "vbmeta,boot,system,vendor,dtbo";};fstab {compatible = "android,fstab";vendor {compatible = "android,vendor";dev = "/dev/block/platform/soc/1d84000.ufshc/by-name/vendor";type = "ext4";mnt_flags = "ro,barrier=1,discard";fsmgr_flags = "wait,slotselect,avb";status = "ok";};};};

具体的看一下这篇—>Android AVB的校验宏观的两个阶段

这个部分你需要知道的就是这两步是哪两步，以及这两步是可以自己定义校验哪些分区的。

那么下面就需要看看AVB这个玩意在代码中是怎么使用的呢？

2、AVB怎么用的？

怎么要看干了什么？

1、设备检测

如果说AVB只是校验了镜像就真的有点肤浅了哈！！！

如果你有看过之前的AVB的流程图。

就这个图，就知道这个里面涉及到了四个操作：
（详细的内容：【Uboot阶段AVB2.0校验流程】）

1、Device Lock设备上锁
2、Valid Os found-检察vbmeta.img是否合法
3、Was the root of trust set by the user
4、Update rollback

那这个里面我就知道了有自定义的东西，要实现这些功能，就移植libavb库到UBOOT或者UEFI中，然后实现avb_ops.c中的几个接口就可以了：

文件：libavb/avb_ops.h

  AvbIOResult (*read_from_partition)(AvbOps* ops,const char *partition,int64_t offset,size_t num_bytes,void* buffer,size_t* out_num_read);AvbIOResult (*read_rollback_index)(AvbOps* ops,size_t rollback_index_location,uint64_t* out_rollback_index);AvbIOResult (*write_rollback_index)(AvbOps* ops,size_t rollback_index_location,uint64_t rollback_index);  AvbIOResult (*validate_vbmeta_public_key)(AvbOps* ops,const uint8_t* public_key_data,size_t public_key_length,const uint8_t* public_key_metadata,size_t public_key_metadata_length,bool* out_is_trusted);

完整版（推荐阅读这个，接口上面有注释，英语较为薄弱的可以借助翻译，弄懂接口和结构体的含义）

/** Copyright (C) 2016 The Android Open Source Project** Permission is hereby granted, free of charge, to any person* obtaining a copy of this software and associated documentation* files (the "Software"), to deal in the Software without* restriction, including without limitation the rights to use, copy,* modify, merge, publish, distribute, sublicense, and/or sell copies* of the Software, and to permit persons to whom the Software is* furnished to do so, subject to the following conditions:** The above copyright notice and this permission notice shall be* included in all copies or substantial portions of the Software.** THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS* BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN* ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE* SOFTWARE.*/
#if !defined(AVB_INSIDE_LIBAVB_H) && !defined(AVB_COMPILATION)
#error "Never include this file directly, include libavb.h instead."
#endif
#ifndef AVB_OPS_H_
#define AVB_OPS_H_
#include "avb_sysdeps.h"
#ifdef __cplusplus
extern "C" {
#endif
/* Well-known names of named persistent values. */
#define AVB_NPV_PERSISTENT_DIGEST_PREFIX "avb.persistent_digest."
#define AVB_NPV_MANAGED_VERITY_MODE "avb.managed_verity_mode"
/* Return codes used for I/O operations.** AVB_IO_RESULT_OK is returned if the requested operation was* successful.** AVB_IO_RESULT_ERROR_IO is returned if the underlying hardware (disk* or other subsystem) encountered an I/O error.** AVB_IO_RESULT_ERROR_OOM is returned if unable to allocate memory.** AVB_IO_RESULT_ERROR_NO_SUCH_PARTITION is returned if the requested* partition does not exist.** AVB_IO_RESULT_ERROR_RANGE_OUTSIDE_PARTITION is returned if the* range of bytes requested to be read or written is outside the range* of the partition.** AVB_IO_RESULT_ERROR_NO_SUCH_VALUE is returned if a named persistent value* does not exist.** AVB_IO_RESULT_ERROR_INVALID_VALUE_SIZE is returned if a named persistent* value size is not supported or does not match the expected size.** AVB_IO_RESULT_ERROR_INSUFFICIENT_SPACE is returned if a buffer is too small* for the requested operation.*/
typedef enum {AVB_IO_RESULT_OK,AVB_IO_RESULT_ERROR_OOM,AVB_IO_RESULT_ERROR_IO,AVB_IO_RESULT_ERROR_NO_SUCH_PARTITION,AVB_IO_RESULT_ERROR_RANGE_OUTSIDE_PARTITION,AVB_IO_RESULT_ERROR_NO_SUCH_VALUE,AVB_IO_RESULT_ERROR_INVALID_VALUE_SIZE,AVB_IO_RESULT_ERROR_INSUFFICIENT_SPACE,
} AvbIOResult;
struct AvbOps;
typedef struct AvbOps AvbOps;
/* Forward-declaration of operations in libavb_ab. */
struct AvbABOps;
/* Forward-declaration of operations in libavb_atx. */
struct AvbAtxOps;
/* High-level operations/functions/methods that are platform* dependent.** Operations may be added in the future so when implementing it* always make sure to zero out sizeof(AvbOps) bytes of the struct to* ensure that unimplemented operations are set to NULL.*/
struct AvbOps {/* This pointer can be used by the application/bootloader using* libavb and is typically used in each operation to get a pointer* to platform-specific resources. It cannot be used by libraries.*/void* user_data;/* If libavb_ab is used, this should point to the* AvbABOps. Otherwise it must be set to NULL.*/struct AvbABOps* ab_ops;/* If libavb_atx is used, this should point to the* AvbAtxOps. Otherwise it must be set to NULL.*/struct AvbAtxOps* atx_ops;/* Reads |num_bytes| from offset |offset| from partition with name* |partition| (NUL-terminated UTF-8 string). If |offset| is* negative, its absolute value should be interpreted as the number* of bytes from the end of the partition.** This function returns AVB_IO_RESULT_ERROR_NO_SUCH_PARTITION if* there is no partition with the given name,* AVB_IO_RESULT_ERROR_RANGE_OUTSIDE_PARTITION if the requested* |offset| is outside the partition, and AVB_IO_RESULT_ERROR_IO if* there was an I/O error from the underlying I/O subsystem.  If the* operation succeeds as requested AVB_IO_RESULT_OK is returned and* the data is available in |buffer|.** The only time partial I/O may occur is if reading beyond the end* of the partition. In this case the value returned in* |out_num_read| may be smaller than |num_bytes|.*/AvbIOResult (*read_from_partition)(AvbOps* ops,const char* partition,int64_t offset,size_t num_bytes,void* buffer,size_t* out_num_read);/* Gets the starting pointer of a partition that is pre-loaded in memory, and* save it to |out_pointer|. The preloaded partition is expected to be* |num_bytes|, where the actual preloaded byte count is returned in* |out_num_bytes_preloaded|. |out_num_bytes_preloaded| must be no larger than* |num_bytes|.** This provides an alternative way to access a partition that is preloaded* into memory without a full memory copy. When this function pointer is not* set (has value NULL), or when the |out_pointer| is set to NULL as a result,* |read_from_partition| will be used as the fallback. This function is mainly* used for accessing the entire partition content to calculate its hash.** Preloaded partition data must outlive the lifespan of the* |AvbSlotVerifyData| structure that |avb_slot_verify| outputs.*/AvbIOResult (*get_preloaded_partition)(AvbOps* ops,const char* partition,size_t num_bytes,uint8_t** out_pointer,size_t* out_num_bytes_preloaded);/* Writes |num_bytes| from |bffer| at offset |offset| to partition* with name |partition| (NUL-terminated UTF-8 string). If |offset|* is negative, its absolute value should be interpreted as the* number of bytes from the end of the partition.** This function returns AVB_IO_RESULT_ERROR_NO_SUCH_PARTITION if* there is no partition with the given name,* AVB_IO_RESULT_ERROR_RANGE_OUTSIDE_PARTITION if the requested* byterange goes outside the partition, and AVB_IO_RESULT_ERROR_IO* if there was an I/O error from the underlying I/O subsystem.  If* the operation succeeds as requested AVB_IO_RESULT_OK is* returned.** This function never does any partial I/O, it either transfers all* of the requested bytes or returns an error.*/AvbIOResult (*write_to_partition)(AvbOps* ops,const char* partition,int64_t offset,size_t num_bytes,const void* buffer);/* Checks if the given public key used to sign the 'vbmeta'* partition is trusted. Boot loaders typically compare this with* embedded key material generated with 'avbtool* extract_public_key'.** The public key is in the array pointed to by |public_key_data|* and is of |public_key_length| bytes.** If there is no public key metadata (set with the avbtool option* --public_key_metadata) then |public_key_metadata| will be set to* NULL. Otherwise this field points to the data which is* |public_key_metadata_length| bytes long.** If AVB_IO_RESULT_OK is returned then |out_is_trusted| is set -* true if trusted or false if untrusted.** NOTE: If AVB_SLOT_VERIFY_FLAGS_NO_VBMETA_PARTITION is passed to* avb_slot_verify() then this operation is never used. Instead, the* validate_public_key_for_partition() operation is used*/AvbIOResult (*validate_vbmeta_public_key)(AvbOps* ops,const uint8_t* public_key_data,size_t public_key_length,const uint8_t* public_key_metadata,size_t public_key_metadata_length,bool* out_is_trusted);/* Gets the rollback index corresponding to the location given by* |rollback_index_location|. The value is returned in* |out_rollback_index|. Returns AVB_IO_RESULT_OK if the rollback* index was retrieved, otherwise an error code.** A device may have a limited amount of rollback index locations (say,* one or four) so may error out if |rollback_index_location| exceeds* this number.*/AvbIOResult (*read_rollback_index)(AvbOps* ops,size_t rollback_index_location,uint64_t* out_rollback_index);/* Sets the rollback index corresponding to the location given by* |rollback_index_location| to |rollback_index|. Returns* AVB_IO_RESULT_OK if the rollback index was set, otherwise an* error code.** A device may have a limited amount of rollback index locations (say,* one or four) so may error out if |rollback_index_location| exceeds* this number.*/AvbIOResult (*write_rollback_index)(AvbOps* ops,size_t rollback_index_location,uint64_t rollback_index);/* Gets whether the device is unlocked. The value is returned in* |out_is_unlocked| (true if unlocked, false otherwise). Returns* AVB_IO_RESULT_OK if the state was retrieved, otherwise an error* code.*/AvbIOResult (*read_is_device_unlocked)(AvbOps* ops, bool* out_is_unlocked);/* Gets the unique partition GUID for a partition with name in* |partition| (NUL-terminated UTF-8 string). The GUID is copied as* a string into |guid_buf| of size |guid_buf_size| and will be NUL* terminated. The string must be lower-case and properly* hyphenated. For example:**  527c1c6d-6361-4593-8842-3c78fcd39219** Returns AVB_IO_RESULT_OK on success, otherwise an error code.*/AvbIOResult (*get_unique_guid_for_partition)(AvbOps* ops,const char* partition,char* guid_buf,size_t guid_buf_size);/* Gets the size of a partition with the name in |partition|* (NUL-terminated UTF-8 string). Returns the value in* |out_size_num_bytes|.** If the partition doesn't exist the AVB_IO_RESULT_ERROR_NO_SUCH_PARTITION* error code should be returned.** Returns AVB_IO_RESULT_OK on success, otherwise an error code.*/AvbIOResult (*get_size_of_partition)(AvbOps* ops,const char* partition,uint64_t* out_size_num_bytes);/* Reads a persistent value corresponding to the given |name|. The value is* returned in |out_buffer| which must point to |buffer_size| bytes. On* success |out_num_bytes_read| contains the number of bytes read into* |out_buffer|. If AVB_IO_RESULT_ERROR_INSUFFICIENT_SPACE is returned,* |out_num_bytes_read| contains the number of bytes that would have been read* which can be used to allocate a buffer.** The |buffer_size| may be zero and the |out_buffer| may be NULL, but if* |out_buffer| is NULL then |buffer_size| *must* be zero.** Returns AVB_IO_RESULT_OK on success, otherwise an error code.** If the value does not exist, is not supported, or is not populated, returns* AVB_IO_RESULT_ERROR_NO_SUCH_VALUE. If |buffer_size| is smaller than the* size of the stored value, returns AVB_IO_RESULT_ERROR_INSUFFICIENT_SPACE.** This operation is currently only used to support persistent digests or the* AVB_HASHTREE_ERROR_MODE_MANAGED_RESTART_AND_EIO hashtree error mode. If a* device does not use one of these features this function pointer can be set* to NULL.*/AvbIOResult (*read_persistent_value)(AvbOps* ops,const char* name,size_t buffer_size,uint8_t* out_buffer,size_t* out_num_bytes_read);/* Writes a persistent value corresponding to the given |name|. The value is* supplied in |value| which must point to |value_size| bytes. Any existing* value with the same name is overwritten. If |value_size| is zero, future* calls to |read_persistent_value| will return* AVB_IO_RESULT_ERROR_NO_SUCH_VALUE.** Returns AVB_IO_RESULT_OK on success, otherwise an error code.** If the value |name| is not supported, returns* AVB_IO_RESULT_ERROR_NO_SUCH_VALUE. If the |value_size| is not supported,* returns AVB_IO_RESULT_ERROR_INVALID_VALUE_SIZE.** This operation is currently only used to support persistent digests or the* AVB_HASHTREE_ERROR_MODE_MANAGED_RESTART_AND_EIO hashtree error mode. If a* device does not use one of these features this function pointer can be set* to NULL.*/AvbIOResult (*write_persistent_value)(AvbOps* ops,const char* name,size_t value_size,const uint8_t* value);/* Like validate_vbmeta_public_key() but for when the flag* AVB_SLOT_VERIFY_FLAGS_NO_VBMETA_PARTITION is being used. The name of the* partition to get the public key for is passed in |partition_name|.** Also returns the rollback index location to use for the partition, in* |out_rollback_index_location|.** Returns AVB_IO_RESULT_OK on success, otherwise an error code.*/AvbIOResult (*validate_public_key_for_partition)(AvbOps* ops,const char* partition,const uint8_t* public_key_data,size_t public_key_length,const uint8_t* public_key_metadata,size_t public_key_metadata_length,bool* out_is_trusted,uint32_t* out_rollback_index_location);
};
#ifdef __cplusplus
}
#endif
#endif /* AVB_OPS_H_ */

2、镜像检查

这个你肯定不陌生，这也是其的关键部分。

其实在前面引用层那里，有个关键词**“可能”**，这个是很关键的。就我自己接触的，其实两个阶段不是分的那么界限的明显。为什么那么分是因为有的分区很大，如果等待校验好了再启动肯定很慢，所以有部分就放在整过系统启动之后去后面校验。（verdo\userdata等）

如果是在UBoot里面去实现校验，我觉得代码是比较好移植和阅读的，因为都是C语言。

但是在我们加载kernel，然后进入到了init进程后，那么事情就会稍微复杂点。

init会在第一阶段进行校验，为后面的挂载分区做准备，需要校验的我们前面知道在这个阶段是在fstab中配置的。

整个的逻辑详细参考：【AVB源码学习（三）：AVB2.0 Init阶段安全启动流程】

这里面详细的展开了内容，我这里就不再重复，我使用一个图展示一下调用关系：

这是init阶段，main函数是main.cpp里面的main函数。

（IsDtVbmetaCompatible是检查kernel dtsi中是否有配置类似如下格式的数据(前面说了借助于dts或者rootfs中的fstab配置））

下面来看看FirstStageMountVBootV2函数：

这三个函数完成AVB的校验，其中InitAvbHandle：AVB调用exterlan/avb/libavb库完成分区的校验。下面就到了实现层，这些调用的内容到底是什么？

2、实现层

1、AVB的实现

上面应用层以及调到实现了吗？其实还没，这里我留了一点小尾巴，方便这下面的串联起来。

继续整个图：

（想看源码的友友可以看这篇详细的blog：【AVB源码学习（四）：AVB2.0-libavb库介绍1】）

这里就到了我们的AVB的源码实现，在android中进行的avb校验都是在这里进行的。

如果你想要跟踪一些校验的流程，那么可以在avb_slot_verify.c文件里进行一些打印，就会在校验的时候打出你想要的信息

2、AVB的源码目录

根据场景需要使用到的一半就是上面的五个文件夹。

我理解libavb应该是下面几个的基础，然后_ab是适配AB系统；_atx可能是针对设备集群（百度的，有知道的友友麻烦补充一下）；_user应该是提供一些可以在用户态操作AVB的接口。

所以基础的还是得看libavb目录

1、libavb

3、小结

以上就是我再看了前辈们的优秀blog，以及源码，大概梳理了一下思路。
可能有很多我觉得逻辑清晰的地方你觉得不通，欢迎在评论区指正，一定要看看这个专栏其他的文章再来看这篇，才会对你有贯穿整过流程的效果吧。

大家好好注意防护、戴好口罩！！！

AVB源码的链接：https://android.googlesource.com/platform/external/avb/+/master/libavb/

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前言