使用littlefs-fuse在PC端调试littlefs文件系统

背景

littlefs是arm面向嵌入式设备推出的一款掉电安全的小型文件系统，具有抗掉电，动态磨损均衡，RAM/ROM需求少等特点，具体介绍可见 https://github.com/ARMmbed/littlefs

作为一款在嵌入式设备上使用的文件系统，出问题时，一般是需要将数据dump出来进行分析的。此时就需要PC端的调试工具了。

littlefs-fuse简介

这个项目提供了一个littlefs的FUSE封装，也就是你可以借助此项目，在PC上直接将littlefs镜像挂载起来，并正常进行一些文件系统的操作。

源码位于：https://github.com/ARMmbed/littlefs-fuse

结构

我们直接下载下来

$ git clone https://github.com/ARMmbed/littlefs-fuse.git
$ cd littlefs-fuse

先看下结构

$ tree -L 2

.
├── lfs_fuse_bd.c
├── lfs_fuse_bd.h
├── lfs_fuse.c
├── LICENSE.md
├── littlefs
│   ├── DESIGN.md
│   ├── emubd
│   ├── lfs.c
│   ├── lfs.h
│   ├── lfs_util.c
│   ├── lfs_util.h
│   ├── LICENSE.md
│   ├── Makefile
│   ├── README.md
│   ├── scripts
│   ├── SPEC.md
│   └── tests
├── Makefile
└── README.md

外层是封装，里面则直接包含了littlefs文件夹。当我们需要特定版本的littlefs，例如使用跟设备上同一版本的littlefs时，只需要替换掉里层的这个littlefs即可，非常方便。

编译

根据README，项目依赖 FUSE version 2.6及以上的版本，可以使用如下命令查看版本

fusermount -V

另外还需要安装下 libfuse-dev:

sudo apt-get install libfuse-dev

依赖项满足之后，直接make即可生成lfs应用程序

make

生成设备并挂载使用

首先需要造一个块设备出来

sudo chmod a+rw /dev/loop0                  # make loop device user accessible
dd if=/dev/zero of=image bs=512 count=2048  # create a 1MB image
losetup /dev/loop0 image                    # attach the loop device

我们上一步是dd生成了一个空的image，所以需要先格式化，

./lfs --format /dev/loop0

再挂载

mkdir -p mount
./lfs /dev/loop0 mount

挂载成功后，执行mount可以看到挂载情况

$ mount | grep lfs
/home/zhuangqiubin/debug_littlefs/littlefs-fuse/lfs on /home/zhuangqiubin/debug_littlefs/littlefs-fuse/mount type fuse.lfs (rw,nosuid,nodev,relatime,user_id=1000,group_id=1000)

现在就可以在mount目录下，随意进行操作了，创建删除读出写入，都会被转换成littlefs的操作，最终作用到image上

过程中，可以随时dump出数据，就可以看到你的操作，最终存储到littlefs上时什么样的数据格式了，例如

$ echo "www.cnblogs.com/zqb-all/" >> mount/test_littlefs.txt

$ hexdump -C image
00000000  03 00 00 00 f0 0f ff f7  6c 69 74 74 6c 65 66 73  |........littlefs|
00000010  2f e0 00 10 00 00 02 00  00 02 00 00 00 08 00 00  |/...............|
00000020  ff 00 00 00 ff ff ff 7f  fe 03 00 00 20 00 04 09  |............ ...|
00000030  74 65 73 74 5f 6c 69 74  74 6c 65 66 73 2e 74 78  |test_littlefs.tx|
00000040  74 20 00 00 11 70 0f f9  b7 ee c0 48 bb ff ff ff  |t ...p.....H....|
00000050  ff ff ff ff ff ff ff ff  ff ff ff ff ff ff ff ff  |................|
*
00000200  04 00 00 00 f0 0f ff f7  6c 69 74 74 6c 65 66 73  |........littlefs|
00000210  2f e0 00 10 00 00 02 00  00 02 00 00 00 08 00 00  |/...............|
00000220  ff 00 00 00 ff ff ff 7f  fe 03 00 00 20 00 04 09  |............ ...|
00000230  74 65 73 74 5f 6c 69 74  74 6c 65 66 73 2e 74 78  |test_littlefs.tx|
00000240  74 20 00 00 08 77 77 77  2e 63 6e 62 6c 6f 67 73  |t ...www.cnblogs|
00000250  2e 63 6f 6d 2f 7a 71 62  2d 61 6c 6c 2f 0a 70 0f  |.com/zqb-all/.p.|
00000260  f9 87 46 fe c1 ad ff ff  ff ff ff ff ff ff ff ff  |..F.............|
00000270  ff ff ff ff ff ff ff ff  ff ff ff ff ff ff ff ff  |................|
*
00000400  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00100000

使用完毕之后，用以下命令卸载

umount mount
sudo losetup -d /dev/loop0

挂载一个指定镜像

以上是生成了空的镜像，格式化再挂载。如果要挂载一个现成的镜像，步骤也是差不多的，只是镜像换一下。假设现成的镜像叫littlefs.img，则

sudo chmod a+rw /dev/loop0                  # make loop device user accessible
losetup /dev/loop0 littlefs.img                    # attach the loop device
mkdir -p mount
./lfs /dev/loop0 mount

卸载命令没有差别

umount mount
sudo losetup -d /dev/loop0

但从设备上dump出的littlefs镜像，其配置不一定会跟littlefs-fuse的默认配置匹配。

此时为了能正确挂载，以及准确地模拟littlefs在设备上运行的情况，我们需要将配置改成一样的。

解决方式一，直接修改源码，例如

diff --git a/lfs_fuse.c b/lfs_fuse.c
index 3c87dad..d1a99a2 100644
--- a/lfs_fuse.c
+++ b/lfs_fuse.c
@@ -26,7 +26,16 @@

// config and other state
-static struct lfs_config config = {0};
+/* static struct lfs_config config = {0}; */
+static struct lfs_config config = {
+    .read_size = 256,
+    .prog_size = 256,
+    .block_size = 4096,
+    .block_count = 1224,
+    .block_cycles = 512,
+    .cache_size = 256,
+    .lookahead_size = 32
+};

修改后重新make生成即可

解决方式二，在调用的时候，通过命令行参数指定，例如

./lfs --block_size=512 --format /dev/loop0
./lfs --block_size=512 /dev/loop0 mount

支持的选项，可以在help中找到

$lfs -h
usage: ./lfs [options] device mountpoint

general options:
-o opt,[opt...]        FUSE options
-h   --help            print help
-V   --version         print version

littlefs options:
--format               format instead of mounting
--migrate              migrate previous version  instead of mounting
-b   --block_size      logical block size, overrides the block device
--block_count          block count, overrides the block device
--block_cycles         number of erase cycles before eviction (512)
--read_size            readable unit (block_size)
--prog_size            programmable unit (block_size)
--cache_size           size of caches (block_size)
--lookahead_size       size of lookahead buffer (8192)
--name_max             max size of file names (255)
--file_max             max size of file contents (2147483647)
--attr_max             max size of custom attributes (1022)

FUSE options:
-d   -o debug          enable debug output (implies -f)
-f                     foreground operation
-s                     disable multi-threaded operation

-o allow_other         allow access to other users
-o allow_root          allow access to root
-o auto_unmount        auto unmount on process termination
-o nonempty            allow mounts over non-empty file/dir
-o default_permissions enable permission checking by kernel
-o fsname=NAME         set filesystem name
-o subtype=NAME        set filesystem type
-o large_read          issue large read requests (2.4 only)
-o max_read=N          set maximum size of read requests

-o hard_remove         immediate removal (don't hide files)
-o use_ino             let filesystem set inode numbers
-o readdir_ino         try to fill in d_ino in readdir
-o direct_io           use direct I/O
-o kernel_cache        cache files in kernel
-o [no]auto_cache      enable caching based on modification times (off)
-o umask=M             set file permissions (octal)
-o uid=N               set file owner
-o gid=N               set file group
-o entry_timeout=T     cache timeout for names (1.0s)
-o negative_timeout=T  cache timeout for deleted names (0.0s)
-o attr_timeout=T      cache timeout for attributes (1.0s)
-o ac_attr_timeout=T   auto cache timeout for attributes (attr_timeout)
-o noforget            never forget cached inodes
-o remember=T          remember cached inodes for T seconds (0s)
-o nopath              don't supply path if not necessary
-o intr                allow requests to be interrupted
-o intr_signal=NUM     signal to send on interrupt (10)
-o modules=M1[:M2...]  names of modules to push onto filesystem stack

-o max_write=N         set maximum size of write requests
-o max_readahead=N     set maximum readahead
-o max_background=N    set number of maximum background requests
-o congestion_threshold=N  set kernel's congestion threshold
-o async_read          perform reads asynchronously (default)
-o sync_read           perform reads synchronously
-o atomic_o_trunc      enable atomic open+truncate support
-o big_writes          enable larger than 4kB writes
-o no_remote_lock      disable remote file locking
-o no_remote_flock     disable remote file locking (BSD)
-o no_remote_posix_lock disable remove file locking (POSIX)
-o [no_]splice_write   use splice to write to the fuse device
-o [no_]splice_move    move data while splicing to the fuse device
-o [no_]splice_read    use splice to read from the fuse device

Module options:

[iconv]
-o from_code=CHARSET   original encoding of file names (default: UTF-8)
-o to_code=CHARSET      new encoding of the file names (default: UTF-8)

[subdir]
-o subdir=DIR       prepend this directory to all paths (mandatory)
-o [no]rellinks     transform absolute symlinks to relative

调试

既然可以在PC上模拟挂载和读写，那首先最直观的，就是可以在littlefs的源码中按需添加调试代码，再重新挂载dump出来的镜像，通过打印来分析问题。

其次，必要的时候，我们还可以上gdb

make DEBUG=1 clean all                # build with debug info
gdb --args ./lfs -d /dev/loop0 mount  # run with gdb

本文地址：https://www.cnblogs.com/zqb-all/p/12078659.html