--incremental uses a 32-bit additive byte-sum, not a CRC — sum-preserving content changes are silently skipped

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--incremental uses a 32-bit additive byte-sum, not a CRC — sum-preserving content changes are silently skipped

Product: STM32CubeProgrammer 2.22.0 (Linux x86-64) Interface: SWD / STLINK-V3 Target: STM32F427 (DEV_ID 0x419)

Description

The --incremental download mode decides whether to skip a flash sector by comparing a 32-bit additive byte-sum of the sector's current chip content against the same sum of the new image.

The on-target .stldr CheckSum export (@ 0x200001e9, verified in two loader binaries) uses additive accumulation.

Because addition is commutative and linear, any two images whose bytes differ only in ways that cancel (adjacent-byte swap, +1/−1 pair, any permutation within a sector) produce identical sums. Those sectors are silently skipped — old bytes remain on chip and the tool reports success.

Reproduction

Test images A and B (64 KB, four 16 KB sectors) are constructed so that:

• Sector 1: bytes at offsets 0x4100/0x4101 are 0x11,0x22 in A and 0x22,0x11 in B — an adjacent swap. Additive sum of the full 16 KB sector is identical in both.
• Sector 2: byte at offset 0x8100 is 0x00 in A and 0xFF in B — single-byte flip. Sum differs; this is the control.

Steps:

1. Write image A to the device using st-flash (open-source, independent of ST software).
2. Run STM32_Programmer_CLI -c port=SWD sn=<SN> freq=8000 mode=UR -d B.bin 0x08000000 incremental
3. Read back with st-flash.

Result: sector 1 reads back as A (swap silently dropped); sector 2 reads back as B (control correctly written). CubeProgrammer reported success throughout.

Control: repeat with the same B without incremental — sector 1 is written correctly. The skip is incremental-mode-specific.

All setup and read-back uses st-flash, not CubeProgrammer, so the result is not self-certified by the tool under test.

Also noted during day to day programming. In all of the other options CRC-32 is used.