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SDFS filesystem

SDFS is the filesystem used by the resource partitions. It is deliberately small: a header, a fixed-size entry table, then aligned file bodies.

Layout

Section titled “Layout”
0x00  magic "sdfs.bin"
0x08  u32 zero
0x0C  u32 file count
0x10  u32 total image size
0x14  u32 zero
0x18  u32 checksum of the entry table
0x1C  u32 checksum of the body
0x20  entries, 32 bytes each:
        name[12] | u32 offset | u32 size | u32 0 | u32 0 | u32 sum32(data)

File offsets are absolute inside the image and aligned to 0x40. The checksum is a 32-bit word sum (sum32), not CRC32: the data is read as little-endian u32 values, summed modulo 2^32; trailing bytes are ignored.

Tools

Section titled “Tools”
Terminal window
uv run fwkit sdfs-list    work/res_e.bin
uv run fwkit sdfs-extract work/res_e.bin work/res_e
uv run fwkit sdfs-pack    work/res_e work/res_e_new.bin

Replacing one file inside an AOTA partition without unpacking everything:

Terminal window
uv run fwkit edit-sdfs firmware/JX402_V1.0.0.73.bin res_e.bin 9.ajs work/9.ajs out/custom.bin

Fixed partition length

Section titled “Fixed partition length”

res_e.bin is a fixed 58,777,088-byte partition. Replacing a file can change the natural SDFS size, so fwkit.sdfs.pad_to(img, size) rebuilds the filesystem and pads it back to the original partition length, refreshing total_size and the body checksum.

This applies whenever the partition is embedded in an AOTA image. Shrinking the partition is unsafe unless the way the bootloader maps it is known.

Capacity

Section titled “Capacity”

The stock res_e uses 58,776,014 of 58,777,088 bytes. There is almost no headroom. Replacements are planned around the size of the file removed, not the free bytes reported by a naive directory listing.