Edward J. Schwartz: Computer Science Researcher

I'm trying to document a few things that I do infrequently enough that I tend to forget how to do them, and need to rediscover the process each time. Next in this series is debugging the Ghidra decompiler. This is one of the only resources I know of that discusses this!
The process is roughly:
cd /path/to/ghidra/Ghidra/Features/Decompiler/src/decompile/cpp
make decomp_dbg
Inside of Ghidra, right click on the "Debug Function Decompilation" menu item and save the XML file somewhere.
Run SLEIGHHOME=/path/to/ghidra ./decomp_dbg. You should now be in the decomp_dbg interpreter.
Use restore /path/to/debug.xml
Use load function target_func
Optionally use trace address insn_address
Finally run decompile
Here's a full example:
~/g/g/G/F/D/s/d/cpp $ env SLEIGHHOME=/home/ed/ghidra/ghidra_10.4_PUBLIC/ ./decomp_dbg
[decomp]> restore /tmp/mydebug.xml
/tmp/mydebug.xml successfully loaded: Intel/AMD 32-bit x86
[decomp]> load function main
Function main: 0x00411530
[decomp]> trace address 0x4115ec
OK (1 ranges)
[decomp]> decompile
Decompiling main
DEBUG 0: extrapopsetup
0x004115ec:200: **
   0x004115ec:200: ESP(0x004115ec:200) = ESP(free) + #0x4

DEBUG 1: funclink
0x004115ec:a5: call fCls3:8(free)
   0x004115ec:a5: EAX(0x004115ec:a5) = call fCls3:8(free)(ECX(free),u0x10000019:1(0x004115ec:212))
0x004115ec:211: **
   0x004115ec:211: u0x10000015(0x004115ec:211) = ESP(free) + #0x0
0x004115ec:212: **
   0x004115ec:212: u0x10000019:1(0x004115ec:212) = *(ram,u0x10000015(0x004115ec:211))

DEBUG 2: heritage
0x004115ec:248: **
   0x004115ec:248: ECX(0x004115ec:248) = [create] i0x004115ec:a5:8(free)
0x004115ec:25a: **
   0x004115ec:25a: EDX(0x004115ec:25a) = [create] i0x004115ec:a5:8(free)
0x004115ec:26f: **
   0x004115ec:26f: CF(0x004115ec:26f) = CF(0x004115dd:96) [] i0x004115ec:a5:8(free)
0x004115ec:280: **
   0x004115ec:280: PF(0x004115ec:280) = PF(0x004115dd:9e) [] i0x004115ec:a5:8(free)
0x004115ec:291: **
   0x004115ec:291: ZF(0x004115ec:291) = ZF(0x004115dd:9a) [] i0x004115ec:a5:8(free)
0x004115ec:2a2: **
   0x004115ec:2a2: SF(0x004115ec:2a2) = SF(0x004115dd:99) [] i0x004115ec:a5:8(free)
0x004115ec:2b3: **
   0x004115ec:2b3: OF(0x004115ec:2b3) = OF(0x004115dd:97) [] i0x004115ec:a5:8(free)
0x004115ec:2c4: **
   0x004115ec:2c4: EIP(0x004115ec:2c4) = EIP(0x004115c8:2c3) [] i0x004115ec:a5:8(free)
0x004115ec:a3: ESP(0x004115ec:a3) = ESP(free) - #0x4
   0x004115ec:a3: ESP(0x004115ec:a3) = ESP(0x004115cd:86) - #0x4
0x004115ec:a4: *(ram,ESP(free)) = #0x4115f1
   0x004115ec:a4: *(ram,ESP(0x004115ec:a3)) = #0x4115f1
0x004115ec:211: u0x10000015(0x004115ec:211) = ESP(free) + #0x0
   0x004115ec:211: u0x10000015(0x004115ec:211) = ESP(0x004115ec:a3) + #0x0
0x004115ec:a5: EAX(0x004115ec:a5) = call fCls3:8(free)(ECX(free),u0x10000019:1(0x004115ec:212))
   0x004115ec:a5: EAX(0x004115ec:a5) = call fCls3:8(free)(ECX(0x004115e6:a2),u0x10000019:1(0x004115ec:212))
0x004115ec:200: ESP(0x004115ec:200) = ESP(free) + #0x4
   0x004115ec:200: ESP(0x004115ec:200) = ESP(0x004115ec:a3) + #0x4

DEBUG 3: deadcode
0x004115ec:248: ECX(0x004115ec:248) = [create] i0x004115ec:a5:8(free)
   0x004115ec:248: **
0x004115ec:25a: EDX(0x004115ec:25a) = [create] i0x004115ec:a5:8(free)
   0x004115ec:25a: **
0x004115ec:26f: CF(0x004115ec:26f) = CF(0x004115dd:96) [] i0x004115ec:a5:8(free)
   0x004115ec:26f: **
0x004115ec:280: PF(0x004115ec:280) = PF(0x004115dd:9e) [] i0x004115ec:a5:8(free)
   0x004115ec:280: **
0x004115ec:291: ZF(0x004115ec:291) = ZF(0x004115dd:9a) [] i0x004115ec:a5:8(free)
   0x004115ec:291: **
0x004115ec:2a2: SF(0x004115ec:2a2) = SF(0x004115dd:99) [] i0x004115ec:a5:8(free)
   0x004115ec:2a2: **
0x004115ec:2b3: OF(0x004115ec:2b3) = OF(0x004115dd:97) [] i0x004115ec:a5:8(free)
   0x004115ec:2b3: **
0x004115ec:2c4: EIP(0x004115ec:2c4) = EIP(0x004115c8:2c3) [] i0x004115ec:a5:8(free)
   0x004115ec:2c4: **

DEBUG 4: sub2add
0x004115ec:2e4: **
   0x004115ec:2e4: u0x100000a0(0x004115ec:2e4) = #0x4 * #0xffffffff
0x004115ec:a3: ESP(0x004115ec:a3) = ESP(0x004115cd:86) - #0x4
   0x004115ec:a3: ESP(0x004115ec:a3) = ESP(0x004115cd:86) + u0x100000a0(0x004115ec:2e4)

DEBUG 5: propagatecopy
0x004115ec:a5: EAX(0x004115ec:a5) = call fCls3:8(free)(ECX(0x004115e6:a2),u0x10000019:1(0x004115ec:212))
   0x004115ec:a5: EAX(0x004115ec:a5) = call fCls3:8(free)(u0x00007a80(0x004115e6:a1),u0x10000019:1(0x004115ec:212))

DEBUG 6: identityel
0x004115ec:211: u0x10000015(0x004115ec:211) = ESP(0x004115ec:a3) + #0x0
   0x004115ec:211: u0x10000015(0x004115ec:211) = ESP(0x004115ec:a3)

DEBUG 7: propagatecopy
0x004115ec:212: u0x10000019:1(0x004115ec:212) = *(ram,u0x10000015(0x004115ec:211))
   0x004115ec:212: u0x10000019:1(0x004115ec:212) = *(ram,ESP(0x004115ec:a3))

DEBUG 8: collapseconstants
0x004115ec:2e4: u0x100000a0(0x004115ec:2e4) = #0x4 * #0xffffffff
   0x004115ec:2e4: u0x100000a0(0x004115ec:2e4) = #0xfffffffc

DEBUG 9: propagatecopy
0x004115ec:a3: ESP(0x004115ec:a3) = ESP(0x004115cd:86) + u0x100000a0(0x004115ec:2e4)
   0x004115ec:a3: ESP(0x004115ec:a3) = ESP(0x004115cd:86) + #0xfffffffc

DEBUG 10: addmultcollapse
0x004115ec:200: ESP(0x004115ec:200) = ESP(0x004115ec:a3) + #0x4
   0x004115ec:200: ESP(0x004115ec:200) = ESP(0x004115cd:86) + #0x0

DEBUG 11: earlyremoval
0x004115ec:211: u0x10000015(0x004115ec:211) = ESP(0x004115ec:a3)
   0x004115ec:211: **

DEBUG 12: earlyremoval
0x004115ec:2e4: u0x100000a0(0x004115ec:2e4) = #0xfffffffc
   0x004115ec:2e4: **

DEBUG 13: addmultcollapse
0x004115ec:a3: ESP(0x004115ec:a3) = ESP(0x004115cd:86) + #0xfffffffc
   0x004115ec:a3: ESP(0x004115ec:a3) = ESP(0x004115aa:26a) + #0xfffffffc

DEBUG 14: identityel
0x004115ec:200: ESP(0x004115ec:200) = ESP(0x004115cd:86) + #0x0
   0x004115ec:200: ESP(0x004115ec:200) = ESP(0x004115cd:86)

DEBUG 15: multicollapse
0x004115ec:a3: ESP(0x004115ec:a3) = ESP(0x004115aa:26a) + #0xfffffffc
   0x004115ec:a3: ESP(0x004115ec:a3) = ESP(0x00411574:4b) + #0xfffffffc

DEBUG 16: addmultcollapse
0x004115ec:a3: ESP(0x004115ec:a3) = ESP(0x00411574:4b) + #0xfffffffc
   0x004115ec:a3: ESP(0x004115ec:a3) = ESP(i) + #0xfffffe94

DEBUG 17: earlyremoval
0x004115ec:200: ESP(0x004115ec:200) = ESP(0x004115cd:86)
   0x004115ec:200: **

DEBUG 18: stackptrflow
0x004115ec:a3: ESP(0x004115ec:a3) = ESP(i) + #0xfffffe94
   0x004115ec:a3: ESP(0x004115ec:a3) = ESP(i) + #0xfffffe94

DEBUG 19: storevarnode
0x004115ec:a4: *(ram,ESP(0x004115ec:a3)) = #0x4115f1
   0x004115ec:a4: s0xfffffe94(0x004115ec:a4) = #0x4115f1

DEBUG 20: loadvarnode
0x004115ec:212: u0x10000019:1(0x004115ec:212) = *(ram,ESP(0x004115ec:a3))
   0x004115ec:212: **
0x004115ec:a5: EAX(0x004115ec:a5) = call fCls3:8(free)(u0x00007a80(0x004115e6:a1),u0x10000019:1(0x004115ec:212))
   0x004115ec:a5: EAX(0x004115ec:a5) = call fCls3:8(free)(u0x00007a80(0x004115e6:a1))

DEBUG 21: heritage
0x004115ec:2fc: **
   0x004115ec:2fc: r0x0041a048(0x004115ec:2fc) = r0x0041a048(0x004115c8:2fb) [] i0x004115ec:a5:8(free)
0x004115ec:312: **
   0x004115ec:312: s0xfffffe80(0x004115ec:312) = s0xfffffe80(0x004115c8:311) [] i0x004115ec:a5:8(free)
0x004115ec:325: **
   0x004115ec:325: s0xfffffe84(0x004115ec:325) = s0xfffffe84(0x004115c8:324) [] i0x004115ec:a5:8(free)
0x004115ec:338: **
   0x004115ec:338: s0xfffffe88(0x004115ec:338) = s0xfffffe88(0x004115c8:337) [] i0x004115ec:a5:8(free)
0x004115ec:34b: **
   0x004115ec:34b: s0xfffffe8c(0x004115ec:34b) = s0xfffffe8c(0x004115c8:34a) [] i0x004115ec:a5:8(free)
0x004115ec:35e: **
   0x004115ec:35e: s0xfffffe90(0x004115ec:35e) = s0xfffffe90(0x004115c8:35d) [] i0x004115ec:a5:8(free)
0x004115ec:371: **
   0x004115ec:371: s0xfffffe94(0x004115ec:371) = s0xfffffe94(0x004115ec:a4) [] i0x004115ec:a5:8(free)
0x004115ec:384: **
   0x004115ec:384: s0xfffffe98(0x004115ec:384) = s0xfffffe98(0x004115c8:383) [] i0x004115ec:a5:8(free)
0x004115ec:397: **
   0x004115ec:397: s0xfffffe9c(0x004115ec:397) = s0xfffffe9c(0x004115c8:396) [] i0x004115ec:a5:8(free)
0x004115ec:3aa: **
   0x004115ec:3aa: s0xfffffea0(0x004115ec:3aa) = s0xfffffea0(0x004115c8:3a9) [] i0x004115ec:a5:8(free)
0x004115ec:3bd: **
   0x004115ec:3bd: s0xfffffea4(0x004115ec:3bd) = s0xfffffea4(0x004115c8:3bc) [] i0x004115ec:a5:8(free)
0x004115ec:3d0: **
   0x004115ec:3d0: s0xfffffea8(0x004115ec:3d0) = s0xfffffea8(0x004115c8:3cf) [] i0x004115ec:a5:8(free)
0x004115ec:3e3: **
   0x004115ec:3e3: s0xfffffeb0(0x004115ec:3e3) = s0xfffffeb0(0x004115c8:3e2) [] i0x004115ec:a5:8(free)
0x004115ec:3f6: **
   0x004115ec:3f6: s0xfffffebc(0x004115ec:3f6) = s0xfffffebc(0x004115c8:3f5) [] i0x004115ec:a5:8(free)
0x004115ec:409: **
   0x004115ec:409: s0xfffffec8(0x004115ec:409) = s0xfffffec8(0x004115c8:408) [] i0x004115ec:a5:8(free)
0x004115ec:41c: **
   0x004115ec:41c: s0xfffffed4(0x004115ec:41c) = s0xfffffed4(0x004115d0:8f) [] i0x004115ec:a5:8(free)
0x004115ec:42f: **
   0x004115ec:42f: s0xfffffee0(0x004115ec:42f) = s0xfffffee0(0x004115c8:42e) [] i0x004115ec:a5:8(free)
0x004115ec:442: **
   0x004115ec:442: s0xfffffeec(0x004115ec:442) = s0xfffffeec(0x004115c8:441) [] i0x004115ec:a5:8(free)
0x004115ec:455: **
   0x004115ec:455: s0xfffffef8(0x004115ec:455) = s0xfffffef8(0x004115c8:454) [] i0x004115ec:a5:8(free)
0x004115ec:468: **
   0x004115ec:468: s0xffffff04(0x004115ec:468) = s0xffffff04(0x004115c8:467) [] i0x004115ec:a5:8(free)
0x004115ec:47b: **
   0x004115ec:47b: s0xffffffd0(0x004115ec:47b) = s0xffffffd0(0x004115c8:47a) [] i0x004115ec:a5:8(free)
0x004115ec:48e: **
   0x004115ec:48e: s0xffffffdc(0x004115ec:48e) = s0xffffffdc(0x004115c8:48d) [] i0x004115ec:a5:8(free)
0x004115ec:4a1: **
   0x004115ec:4a1: s0xffffffe8(0x004115ec:4a1) = s0xffffffe8(0x004115c8:4a0) [] i0x004115ec:a5:8(free)
0x004115ec:4b4: **
   0x004115ec:4b4: s0xfffffff0(0x004115ec:4b4) = s0xfffffff0(0x004115c8:4b3) [] i0x004115ec:a5:8(free)
0x004115ec:4c7: **
   0x004115ec:4c7: s0xfffffff4(0x004115ec:4c7) = s0xfffffff4(0x004115c8:4c6) [] i0x004115ec:a5:8(free)
0x004115ec:4da: **
   0x004115ec:4da: s0xfffffff8(0x004115ec:4da) = s0xfffffff8(0x004115d6:92) [] i0x004115ec:a5:8(free)
0x004115ec:4ed: **
   0x004115ec:4ed: s0xfffffffc(0x004115ec:4ed) = s0xfffffffc(0x004115c8:4ec) [] i0x004115ec:a5:8(free)

DEBUG 22: deadcode
0x004115ec:a3: ESP(0x004115ec:a3) = ESP(i) + #0xfffffe94
   0x004115ec:a3: **
0x004115ec:312: s0xfffffe80(0x004115ec:312) = s0xfffffe80(0x004115c8:311) [] i0x004115ec:a5:8(free)
   0x004115ec:312: **
0x004115ec:325: s0xfffffe84(0x004115ec:325) = s0xfffffe84(0x004115c8:324) [] i0x004115ec:a5:8(free)
   0x004115ec:325: **
0x004115ec:338: s0xfffffe88(0x004115ec:338) = s0xfffffe88(0x004115c8:337) [] i0x004115ec:a5:8(free)
   0x004115ec:338: **
0x004115ec:34b: s0xfffffe8c(0x004115ec:34b) = s0xfffffe8c(0x004115c8:34a) [] i0x004115ec:a5:8(free)
   0x004115ec:34b: **
0x004115ec:397: s0xfffffe9c(0x004115ec:397) = s0xfffffe9c(0x004115c8:396) [] i0x004115ec:a5:8(free)
   0x004115ec:397: **
0x004115ec:3aa: s0xfffffea0(0x004115ec:3aa) = s0xfffffea0(0x004115c8:3a9) [] i0x004115ec:a5:8(free)
   0x004115ec:3aa: **
0x004115ec:3bd: s0xfffffea4(0x004115ec:3bd) = s0xfffffea4(0x004115c8:3bc) [] i0x004115ec:a5:8(free)
   0x004115ec:3bd: **
0x004115ec:409: s0xfffffec8(0x004115ec:409) = s0xfffffec8(0x004115c8:408) [] i0x004115ec:a5:8(free)
   0x004115ec:409: **
0x004115ec:42f: s0xfffffee0(0x004115ec:42f) = s0xfffffee0(0x004115c8:42e) [] i0x004115ec:a5:8(free)
   0x004115ec:42f: **
0x004115ec:442: s0xfffffeec(0x004115ec:442) = s0xfffffeec(0x004115c8:441) [] i0x004115ec:a5:8(free)
   0x004115ec:442: **
0x004115ec:455: s0xfffffef8(0x004115ec:455) = s0xfffffef8(0x004115c8:454) [] i0x004115ec:a5:8(free)
   0x004115ec:455: **
0x004115ec:468: s0xffffff04(0x004115ec:468) = s0xffffff04(0x004115c8:467) [] i0x004115ec:a5:8(free)
   0x004115ec:468: **
0x004115ec:47b: s0xffffffd0(0x004115ec:47b) = s0xffffffd0(0x004115c8:47a) [] i0x004115ec:a5:8(free)
   0x004115ec:47b: **
0x004115ec:48e: s0xffffffdc(0x004115ec:48e) = s0xffffffdc(0x004115c8:48d) [] i0x004115ec:a5:8(free)
   0x004115ec:48e: **
0x004115ec:4ed: s0xfffffffc(0x004115ec:4ed) = s0xfffffffc(0x004115c8:4ec) [] i0x004115ec:a5:8(free)
   0x004115ec:4ed: **

DEBUG 23: indirectcollapse
0x004115ec:384: s0xfffffe98(0x004115ec:384) = s0xfffffe98(0x004115c8:383) [] i0x004115ec:a5:8(free)
   0x004115ec:384: s0xfffffe98(0x004115ec:384) = s0xfffffe98(0x00411563:3d) [] i0x004115ec:a5:8(free)

DEBUG 24: propagatecopy
0x004115ec:a5: EAX(0x004115ec:a5) = call fCls3:8(free)(u0x00007a80(0x004115e6:a1))
   0x004115ec:a5: EAX(0x004115ec:a5) = call fCls3:8(free)(EAX(0x004115c8:83))

DEBUG 25: earlyremoval
0x004115ec:35e: s0xfffffe90(0x004115ec:35e) = s0xfffffe90(0x004115c8:35d) [] i0x004115ec:a5:8(free)
   0x004115ec:35e: **

DEBUG 26: earlyremoval
0x004115ec:371: s0xfffffe94(0x004115ec:371) = s0xfffffe94(0x004115ec:a4) [] i0x004115ec:a5:8(free)
   0x004115ec:371: **

DEBUG 27: indirectcollapse
0x004115ec:384: s0xfffffe98(0x004115ec:384) = s0xfffffe98(0x00411563:3d) [] i0x004115ec:a5:8(free)
   0x004115ec:384: **

DEBUG 28: propagatecopy
0x004115ec:41c: s0xfffffed4(0x004115ec:41c) = s0xfffffed4(0x004115d0:8f) [] i0x004115ec:a5:8(free)
   0x004115ec:41c: s0xfffffed4(0x004115ec:41c) = EAX(0x004115c8:83) [] i0x004115ec:a5:8(free)

DEBUG 29: earlyremoval
0x004115ec:a4: s0xfffffe94(0x004115ec:a4) = #0x4115f1
   0x004115ec:a4: **

DEBUG 30: setcasts
0x004115ec:50b: **
   0x004115ec:50b: EAX(0x004115ec:50b) = (cast) u0x10000129(0x004115ec:a5)
0x004115ec:a5: EAX(0x004115ec:a5) = call fCls3:8(free)(EAX(0x004115c8:50a))
   0x004115ec:a5: u0x10000129(0x004115ec:a5) = call fCls3:8(free)(EAX(0x004115c8:50a))

Decompilation complete
In my case, I wanted to know why the function call at 0x4115ec thinks that EAX is being passed as an argument instead of ECX.
Here is ChatGPT's explanation. And it's right! ECX will be equal to the value of EAX at the time of the call.
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Edward J. Schwartz

Computer Security Researcher

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