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.c static 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_system Rollback Index Location: 2 Public key (sha1): fc92d9cba0628858d846fb9a18a7af72b05d7dc8 ... Hash descriptor: Image Size: 59932672 bytes Hash Algorithm: sha256 Partition Name: boot Salt: e691366c1c43ee5e23b342d65555ad8cfbadf77118dceb77e240c8e7d3e63ea6 Digest: 239648eb41f5a491c7c4d6b51b52a533bd9da98ba8800f58a0957f7341dd1686 Flags: 0 Hash descriptor: Image Size: 740733 bytes Hash Algorithm: sha256 Partition Name: dtbo Salt: d445a36d8154a774589dd51c49029ee388ecaac28212c8c6899f45dc5a51dbcf Digest: cac0bd59091464292bc83d6f1193afb1520c12a3849f2673ad3160bb951acf6d Flags: 0 Hash descriptor: Image Size: 1916928 bytes Hash Algorithm: sha256 Partition Name: vendor_boot Salt: 5f7b7c3592142d4f3645d7e675fb7865915e52e8b361ba330fccf00aeb1c4028 Digest: cf153ab9df9a5c34024417ea2b0b4dfd716d01ba9cd2d5bcf964fe7e25cdd802 Flags: 0 Hashtree descriptor: Version of dm-verity: 1 Image Size: 864256 bytes Tree Offset: 864256 Tree Size: 12288 bytes Data Block Size: 4096 bytes Hash Block Size: 4096 bytes FEC num roots: 2 FEC offset: 876544 FEC size: 8192 bytes Hash Algorithm: sha1 Partition Name: odm Salt: b6e1f57ae6939659355e83ad7fa57feb6b5eb15a3d16b96752f43cdc14918708 Root Digest: da99875b16661e72eec81d05d58dffdf09fe228d Flags: 0 Hashtree descriptor: Version of dm-verity: 1 Image Size: 1002233856 bytes Tree Offset: 1002233856 Tree Size: 7897088 bytes Data Block Size: 4096 bytes Hash Block Size: 4096 bytes FEC num roots: 2 FEC offset: 1010130944 FEC size: 7987200 bytes Hash Algorithm: sha1 Partition Name: vendor Salt: b6e1f57ae6939659355e83ad7fa57feb6b5eb15a3d16b96752f43cdc14918708 Root Digest: 336bd4885da274ae0de38b32b899f7b6169e676f Flags: 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/
