Android 7.0支持exfat文件系统

网上看了一圈，关于Android移植exfat支持的文章大多停留在Android 4.x，Android 6.0或7.0以上的基本空缺，所以决定自己写一篇，本文实践环境是MTK的Android 7.0系统。

部分代码参考了wince_lover的移植指南，感谢：

https://blog.csdn.net/wince_lover/article/details/80426796

一、关于fuse和no-fuse

简单概括，fuse是用户空间层的文件系统，no-fuse是位于内核驱动层的。

实际上，一开始是希望直接在内核层支持exfat。根据github上star最多的项目 https://github.com/dorimanx/exfat-nofuse，做完了移植后，插入exfat格式的SD卡也确实能挂载，但是只要开始读写SD卡，就会报'sector_read: out of range error'的错误，然后就被remount成ro模式。这在项目的issues中也有类似的讨论https://github.com/dorimanx/exfat-nofuse/issues/131

由于没能解决这个问题，所以最后还是放弃了no-fuse的方式，转为使用fuse方式。

二、fuse方式移植exfat过程

1.下载exfat相关代码，代码地址，https://github.com/Lurker00/Android-fs

只需下载其中的jni/external/exfat 和 jni/external/fuse
拷贝exfat目录到android源码中external目录下，
拷贝fusefuse目录到android源码中external目录下，并改名为libfuse_forexfat

2.修改libfuse_forexfat下的Android.mk

LOCAL_MODULE := libfuse

修改为 LOCAL_MODULE := libfuse_forexfat

3.修改exfat下的Android.mk
-I$(EXFAT_ROOT)/../fuse/include 修改为 -I$(EXFAT_ROOT)/../libfuse_forexfat/include
LOCAL_STATIC_LIBRARIES += libexfat libfuse 修改为 LOCAL_STATIC_LIBRARIES += libexfat libfuse_forexfat
然后在include $(BUILD_EXECUTABLE)之后加上以下代码

LINKS := fsck.exfat mkfs.exfat
SYMLINKS := $(addprefix $(TARGET_OUT)/bin/,$(LINKS))
$(SYMLINKS): EXFAT_BINARY := $(LOCAL_MODULE)
$(SYMLINKS): $(LOCAL_INSTALLED_MODULE) $(LOCAL_PATH)/Android.mk

@echo "Symlink: $@ -> $(EXFAT_BINARY)"
@mkdir -p $(dir $@)
@rm -rf $@
$(hide) ln -sf $(EXFAT_BINARY) $@

ALL_DEFAULT_INSTALLED_MODULES += $(SYMLINKS)
这样编译以后，会在out目录system/bin下生成可执行文件mount.exfat，另外有两个指向它的软连接fsck.exfat和mkfs.exfat

4.接下来做sd卡的自动挂载修改

system/vold/fs/Exfat.cpp
将
static const char* kMkfsPath = "/system/bin/mkexfat";
static const char* kFsckPath = "/system/bin/exfatfsck";
修改为
static const char* kMkfsPath = "/system/bin/mkfs.exfat";
static const char* kFsckPath = "/system/bin/fsck.exfat";
static const char* exfatMountPath = "/system/bin/mount.exfat";

然后修改exfat.cpp的Mount函数，注释其中的 rc = mount(c_source, c_target, "exfat", flags, mountData);
并替换为如下代码，不使用mount函数，直接用上面编译出来的mount.exfat进行挂载

//leung modify
//rc = mount(c_source, c_target, "exfat", flags, mountData);
std::string exec;
exec = exfatMountPath;
exec = exec + " " + c_source + " " + c_target;
rc = system(exec.c_str());
if (rc == 0) {

SLOGI("exfat Filesystem mounted OK");

} else {

SLOGE("exfat Filesystem mounted failed");
errno = EROFS;

}

//add end

5.修改系统中的blkid工具

该工具会在挂载之前调用，获取sd卡的uuid等信息，需要先让它支持exfat

blkid这个工具位于Android工程的external/e2fsprogs/lib/blkid/

查看github上项目https://github.com/tytso/e2fsprogs

其中commit a850bc56e0aa7370b7e0e81d8dbab93841829f98 已有支持exfat的提交记录，我们把该记录的修改导出来作为patch
加入到android工程的代码中即可

commit a850bc56e0aa7370b7e0e81d8dbab93841829f98
Author: liminghao <liminghao@xiaomi.com>
Date:   Wed Mar 1 17:54:42 2017 +0800AOSP: blkid: add support to recognize exfat to blkid.we can now identify exfat filesystem.Change-Id: I870e59a14b6bcd8b45562cdd02c2502d60a9eeffSigned-off-by: liminghao <liminghao@xiaomi.com>From AOSP commit: 1206f6d8c5ed47ba19cfc30a19dba51fcd2cd5cbSigned-off-by: Theodore Ts'o <tytso@mit.edu>diff --git a/lib/blkid/probe.c b/lib/blkid/probe.c
index 66ecbc4..fae74a7 100644
--- a/lib/blkid/probe.c
+++ b/lib/blkid/probe.c
@@ -106,7 +106,6 @@ static int check_mdraid(int fd, unsigned char *ret_uuid)if (blkid_llseek(fd, offset, 0) < 0 ||read(fd, buf, 4096) != 4096)return -BLKID_ERR_IO;
-/* Check for magic number */if (memcmp("\251+N\374", buf, 4) && memcmp("\374N+\251", buf, 4))return -BLKID_ERR_PARAM;
@@ -330,7 +329,7 @@ static int probe_ext4dev(struct blkid_probe *probe,EXT3_FEATURE_INCOMPAT_JOURNAL_DEV)return -BLKID_ERR_PARAM;- /*
+  /** If the filesystem does not have a journal and ext2 and ext4* is not present, then force this to be detected as an* ext4dev filesystem.
@@ -374,7 +373,7 @@ static int probe_ext4(struct blkid_probe *probe, struct blkid_magic *id,EXT3_FEATURE_INCOMPAT_JOURNAL_DEV)return -BLKID_ERR_PARAM;-    /*
+  /** If the filesystem does not have a journal and ext2 is not* present, then force this to be detected as an ext2* filesystem.
@@ -452,7 +451,7 @@ static int probe_ext2(struct blkid_probe *probe, struct blkid_magic *id,EXT2_FEATURE_INCOMPAT_UNSUPPORTED))return -BLKID_ERR_PARAM;-   /*
+  /** If ext2 is not present, but ext4 or ext4dev are, then* disclaim we are ext2*/
@@ -1403,6 +1402,99 @@ static int probe_f2fs(struct blkid_probe *probe,return 0;}+static uint64_t exfat_block_to_offset(const struct exfat_super_block *sb,
+                                      uint64_t block)
+{
+    return block << sb->block_bits;
+}
+
+static uint64_t exfat_cluster_to_block(const struct exfat_super_block *sb,
+                                       uint32_t cluster)
+{
+    return sb->cluster_block_start +
+            ((uint64_t)(cluster - EXFAT_FIRST_DATA_CLUSTER) << sb->bpc_bits);
+}
+
+static uint64_t exfat_cluster_to_offset(const struct exfat_super_block *sb,
+                                        uint32_t cluster)
+{
+    return exfat_block_to_offset(sb, exfat_cluster_to_block(sb, cluster));
+}
+
+static uint32_t exfat_next_cluster(struct blkid_probe *probe,
+                                   const struct exfat_super_block *sb,
+                                   uint32_t cluster)
+{
+    uint32_t *next;
+    uint64_t offset;
+
+    offset = exfat_block_to_offset(sb, sb->fat_block_start)
+            + (uint64_t) cluster * sizeof (cluster);
+    next = (uint32_t *)get_buffer(probe, offset, sizeof (uint32_t));
+
+    return next ? *next : 0;
+}
+
+static struct exfat_entry_label *find_exfat_entry_label(
+    struct blkid_probe *probe, const struct exfat_super_block *sb)
+{
+    uint32_t cluster = sb->rootdir_cluster;
+    uint64_t offset = exfat_cluster_to_offset(sb, cluster);
+    uint8_t *entry;
+    const size_t max_iter = 10000;
+    size_t i = 0;
+
+    for (; i < max_iter; ++i) {
+        entry = (uint8_t *)get_buffer(probe, offset, EXFAT_ENTRY_SIZE);
+        if (!entry)
+            return NULL;
+        if (entry[0] == EXFAT_ENTRY_EOD)
+            return NULL;
+        if (entry[0] == EXFAT_ENTRY_LABEL)
+            return (struct exfat_entry_label*) entry;
+
+        offset += EXFAT_ENTRY_SIZE;
+        if (offset % CLUSTER_SIZE(sb) == 0) {
+            cluster = exfat_next_cluster(probe, sb, cluster);
+            if (cluster < EXFAT_FIRST_DATA_CLUSTER)
+                return NULL;
+            if (cluster > EXFAT_LAST_DATA_CLUSTER)
+                return NULL;
+            offset = exfat_cluster_to_offset(sb, cluster);
+        }
+    }
+
+    return NULL;
+}
+
+static int probe_exfat(struct blkid_probe *probe, struct blkid_magic *id,
+                       unsigned char *buf)
+{
+    struct exfat_super_block *sb;
+    struct exfat_entry_label *label;
+    uuid_t uuid;
+    sb = (struct exfat_super_block *)buf;
+    if (!sb || !CLUSTER_SIZE(sb)) {
+        DBG(DEBUG_PROBE, printf("bad exfat superblock.\n"));
+        return errno ? - errno : 1;
+    }
+
+    label = find_exfat_entry_label(probe, sb);
+    if (label) {
+        blkid_set_tag(probe->dev, "LABEL", label->name, label->length);
+    } else {
+        blkid_set_tag(probe->dev, "LABEL", "disk", 4);
+    }
+
+    snprintf(uuid, sizeof (uuid), "%02hhX%02hhX-%02hhX%02hhX",
+             sb->volume_serial[3], sb->volume_serial[2],
+             sb->volume_serial[1], sb->volume_serial[0]);
+
+    set_uuid(probe->dev, uuid, 0);
+
+    return 0;
+}
+/** Various filesystem magics that we can check for.  Note that kboff and* sboff are in kilobytes and bytes respectively.  All magics are in
@@ -1512,6 +1604,7 @@ static struct blkid_magic type_array[] = {{ "lvm2pv",  1,  0x218,  8, "LVM2 001",       probe_lvm2 },{ "btrfs",    64,  0x40,  8, "_BHRfS_M",       probe_btrfs },{ "f2fs",    1,      0,  4, "\x10\x20\xf5\xf2",   probe_f2fs },
+  { "exfat",     0,      3,  8, "EXFAT   ",             probe_exfat },{   NULL,    0,  0,  0, NULL,           NULL }};diff --git a/lib/blkid/probe.h b/lib/blkid/probe.h
index 9d02695..c46a332 100644
--- a/lib/blkid/probe.h
+++ b/lib/blkid/probe.h
@@ -14,6 +14,8 @@#ifndef _BLKID_PROBE_H#define _BLKID_PROBE_H+#include <stdint.h>
+#include <blkid/blkid_types.h>struct blkid_magic;
@@ -763,6 +765,46 @@ struct f2fs_super_block {__u8 extension_list[F2FS_MAX_EXTENSION][8]; /* extension array */} attribute((packed));+struct exfat_super_block {
+    uint8_t jump[3];
+    uint8_t oem_name[8];
+    uint8_t __unused1[53];
+    uint64_t block_start;
+    uint64_t block_count;
+    uint32_t fat_block_start;
+    uint32_t fat_block_count;
+    uint32_t cluster_block_start;
+    uint32_t cluster_count;
+    uint32_t rootdir_cluster;
+    uint8_t volume_serial[4];
+    struct {
+        uint8_t vermin;
+        uint8_t vermaj;
+    } version;
+    uint16_t volume_state;
+    uint8_t block_bits;
+    uint8_t bpc_bits;
+    uint8_t fat_count;
+    uint8_t drive_no;
+    uint8_t allocated_percent;
+} attribute((packed));
+
+struct exfat_entry_label {
+    uint8_t type;
+    uint8_t length;
+    uint8_t name[30];
+} attribute((packed));
+
+#define BLOCK_SIZE(sb)   (1 << (sb)->block_bits)
+#define CLUSTER_SIZE(sb) (BLOCK_SIZE(sb) << (sb)->bpc_bits)
+
+#define EXFAT_FIRST_DATA_CLUSTER 2
+#define EXFAT_LAST_DATA_CLUSTER  0xffffff6
+#define EXFAT_ENTRY_SIZE         32
+
+#define EXFAT_ENTRY_EOD   0x00
+#define EXFAT_ENTRY_LABEL 0x83
+/** Byte swap functions*/

6.下一步，在mk文件中，将一开始添加的external/exfat，/external/libfuse_forexfat加入编译，如
device/mediatek/common/device.mk
+PRODUCT_PACKAGES += libfuse_forexfat
+PRODUCT_PACKAGES += mount.exfat

7.然后make一次，测试在7.0上使用fuse方式支持exfat文件系统是否ok

8.要是发现无法自动挂载上，如果有adb root权限，可以先在adb中手动挂载试试

mount.exfat /dev/block/vold/publicxxx /mnt/media_rw/xxx

因为自动挂载的代码可能会遇到selinux的问题，需要按照不同平台，关闭selinux或者让报错的地方能被通过。

文章中的移植代码已经是完整可用的，有兴趣的朋友参考移植即可。

移植好的代码补丁也已传到csdn，有积分的童鞋可以自行下载。

https://download.csdn.net/download/huolinliang/10944533