forked from Imagelibrary/littlefs
a60a986c9c
The documentation does not match the implementation here. The intended
behavior of superblock expansion was to duplicate the current superblock
entry into the new superblock:
.--------. .--------.
.|littlefs|->|littlefs|
||bs=4096 | ||bs=4096 |
||bc=256 | ||bc=256 |
||crc32 | ||root dir|
|| | ||crc32 |
|'--------' |'--------'
'--------' '--------'
The main benefit is that we can rely on the magic string "littlefs"
always residing in blocks 0x{0,1}, even if the superblock chain has
multiple superblocks.
The downside is that earlier superblocks in the superblock chain may
contain out-of-date configuration. This is a bit annoying, and risks
hard-to-reach bugs, but in theory shouldn't break anything as long as
the filesystem is aware of this.
Unfortunately this was lost at some point during refactoring in the
early v2-alpha work. A lot of code was moving around in this stage, so
it's a bit hard to track down the change and if it was intentional. The
result is superblock expansion creates a valid linked-list of
superblocks, but only the last superblock contains a valid superblock
entry:
.--------. .--------.
.|crc32 |->|littlefs|
|| | ||bs=4096 |
|| | ||bc=256 |
|| | ||root dir|
|| | ||crc32 |
|'--------' |'--------'
'--------' '--------'
What's interesting is this isn't invalid as far as lfs_mount is
concerned. lfs_mount is happy as long as a superblock entry exists
anywhere in the superblock chain. This is good for compat flexibility,
but is the main reason this has gone unnoticed for so long.
---
With the benefit of more time to think about the problem, it may have
been more preferable to copy only the "littlefs" magic string and NOT
the superblock entry:
.--------. .--------.
.|littlefs|->|littlefs|
||crc32c | ||bs=4096 |
|| | ||bc=256 |
|| | ||root dir|
|| | ||crc32 |
|'--------' |'--------'
'--------' '--------'
This would allow for simple "littlefs" magic string checks without the
risks associated with out-of-date superblock entries.
Unfortunately the current implementation errors if it finds a "littlefs"
magic string without an associated superblock entry, so such a change
would not be compatible with old drivers.
---
This commit tweaks superblock expansion to duplicate the superblock
entry instead of simply moving it to the new superblock. And adds tests
over the magic string "littlefs" both before and after superblock
expansion.
Found by rojer and Nikola Kosturski