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Personal fork of crosspoint-reader firmware for Xteink4
xteink crosspoint-reader
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2.3 kB · 68 lines
Python
at main
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869"""PlatformIO pre-build script: patch JPEGDEC for MCU_SKIP wild pointer crash.
Problem: JPEGDecodeMCU_P computes pMCU = &sMCUs[iMCU & 0xffffff]. When iMCU is MCU_SKIP (-8), the bitmask produces index 0xFFFFF8 (16 777 208), creating a pointer ~33 MB past the 392-entry sMCUs array. If the progressive JPEG's first scan includes AC coefficients (iScanEnd > 0), the AC decode loop writes through this wild pointer and crashes with a store-access fault.
Upstream commit 8628297 guarded the DC coefficient write (pMCU[0]) but not the AC coefficient writes at indices 1-63.
Fix: Redirect pMCU to sMCUs[0] when MCU_SKIP is active. Writes to sMCUs[1..63] are harmless: for JPEG_SCALE_EIGHTH only sMCUs[0] is read for output, and the DC write at sMCUs[0] is already guarded by the existing `if (iMCU >= 0)` check.
Applied idempotently — safe to run on every build."""
Import("env")import os
def patch_jpegdec(env): libdeps_dir = os.path.join(env["PROJECT_DIR"], ".pio", "libdeps") if not os.path.isdir(libdeps_dir): return for env_dir in os.listdir(libdeps_dir): jpeg_inl = os.path.join(libdeps_dir, env_dir, "JPEGDEC", "src", "jpeg.inl") if os.path.isfile(jpeg_inl): _apply_mcu_skip_pointer_fix(jpeg_inl)
def _apply_mcu_skip_pointer_fix(filepath): MARKER = "// CrossPoint patch: safe pMCU for MCU_SKIP" with open(filepath, "r") as f: content = f.read()
if MARKER in content: return # already patched
# The wild-pointer line in JPEGDecodeMCU_P: OLD = " signed short *pMCU = &pJPEG->sMCUs[iMCU & 0xffffff];"
NEW = ( " " + MARKER + "\n" " signed short *pMCU = (iMCU < 0) ? pJPEG->sMCUs\n" " : &pJPEG->sMCUs[iMCU & 0xffffff];" )
if OLD not in content: print( "WARNING: JPEGDEC MCU_SKIP pointer patch target not found in %s " "— library may have been updated" % filepath ) return
content = content.replace(OLD, NEW, 1) with open(filepath, "w") as f: f.write(content) print("Patched JPEGDEC: safe pMCU for MCU_SKIP in JPEGDecodeMCU_P: %s" % filepath)
# Run immediately at script import time (before compilation).patch_jpegdec(env)