Multiversion data corruption

[vrjtikonen]

Hi, I don't know if what we're doing is supported but reporting in case this is a bug.
We have a following situation.

Filesystem has been created with LittleFS 2.5.0.
Library build flags LFS_NO_MALLOC.

// Major (top-nibble), incremented on backwards incompatible changes
// Minor (bottom-nibble), incremented on feature additions
#define LFS_VERSION 0x00020005
#define LFS_VERSION_MAJOR (0xffff & (LFS_VERSION >> 16))
#define LFS_VERSION_MINOR (0xffff & (LFS_VERSION >>  0))

// Version of On-disk data structures
// Major (top-nibble), incremented on backwards incompatible changes
// Minor (bottom-nibble), incremented on feature additions
#define LFS_DISK_VERSION 0x00020000
#define LFS_DISK_VERSION_MAJOR (0xffff & (LFS_DISK_VERSION >> 16))
#define LFS_DISK_VERSION_MINOR (0xffff & (LFS_DISK_VERSION >>  0))

The we upgraded code to use LittleFS 2.11.2
Library build flags LFS_NO_MALLOC and LFS_MULTIVERSION

// Major (top-nibble), incremented on backwards incompatible changes
// Minor (bottom-nibble), incremented on feature additions
#define LFS_VERSION 0x0002000b
#define LFS_VERSION_MAJOR (0xffff & (LFS_VERSION >> 16))
#define LFS_VERSION_MINOR (0xffff & (LFS_VERSION >>  0))

// Version of On-disk data structures
// Major (top-nibble), incremented on backwards incompatible changes
// Minor (bottom-nibble), incremented on feature additions
#define LFS_DISK_VERSION 0x00020001
#define LFS_DISK_VERSION_MAJOR (0xffff & (LFS_DISK_VERSION >> 16))
#define LFS_DISK_VERSION_MINOR (0xffff & (LFS_DISK_VERSION >>  0))

Reproduction (not sure if this is the minimal but it triggers the problem always):

1. Format file system in v2.5.0. Create directories e.g. "/a" and /b"
2. Mount file system in v2.11.2.
3. Write a file in directory a. e.g. "/a/x.bin"
4. Rename it to the other directory "/a/x.bin" -> "/b/x.bin"
5. And finally delete the file "/b/x.bin".
6. Attempt to mount file system with v2.5.0.

What happens:
lfs_mount returns error -84, filesystem corrupted.

[geky]

Hi @vrjtikonen, thanks for creating an issue.

Quick question before further investigation, did you set cfg.disk_version to 0x00020000?

LFS_MULTIVERSION enables support for both versions, but by default, littlefs will still upgrade the on-disk version v2.0 -> v2.1.

---

Typing this out, I'm realizing how clunky this API is when the version is unknown. I'd have to go through the issues to see if anyone suggested a better API, but unfortunately support for v2.0 was a bit of an afterthought.

[vrjtikonen]

Hi, thank you for taking a look into this. I didn't realize I would need to set disk_version to maintain backwards compatibility. After setting disk_version explicitly to 0x00020000 I cannot reproduce the issue anymore. Seems like this was an user error. I will resume with testing and come back if any compatibility issues arise.