When using MbedCRC, init value must be non-reversed, regardless of
`reflect_data` or `reflect_out` settings. This means we need to reflect
the intermediate output before passing using it as the next init value.
(In GCC this ends up putting in two `RBIT` instructions back-to-back,
because it's implemented as assembler, so it doesn't know how to
optimise. In ARMC6, `__RBIT` is implemented as an intrinsic, so adding
this reflection cancels the existing reflection and makes the code
smaller).
The most common issue with using littlefs in mbed-os is when users
change from littlefs->FAT->littlefs (or with MBR or similar). When this
corrupts the superblock, littlefs tries to fall back to the backup
superblock. However, at this point in the time the old superblock may be
very out-of-date and pointing to an incorrect filesystem.
There's no complete solution to a malicious modification of the
filesystem (short of checking all metadata+data, a very expensive
operation), but we can at least expand our validation to all of the
metadata for the filesystem. This at least catches the common issues
with changing between different filesystems.
CRC used in LittleFS is Reversed ANSI, hence new polynomial added.
Reversed polynomials perform shift in reverse direction of standard
polynomial, and we do not have option to notify reverse shift to hardware.
Hence this option is available in software only.
Kevin Bracey
Christopher Haster
Michal Paszta
Cruz Monrreal II
Veijo Pesonen
David Saada
Deepika
Yossi Levy