[lldb][RFC] hot reload C/C++ functions from lldb #168799
Description
I created a Python scripting bridge script to hot reload functions while in an lldb session. I wanted to see what folks thought of it, and potentially integrating it into lldb. You can try it as described here https://github.com/cs01/lldbhotreload. To be clear it is basically a proof of concept in its current form.
You can use it like so:
$ curl -O https://raw.githubusercontent.com/cs01/lldbhotreload/main/src/hotreload.py
$ lldb ./your_program
(lldb) command script import hotreload.py
(lldb) b your_file.cpp:50
(lldb) run
# Edit your_file.cpp in your editor and save
(lldb) hotreload your_file.cpp
(lldb) continue
This is what the output looks like. It's kind of verbose but it does describe what it's doing.
What it does is
- generate a temporary C++ file with the modified code
- call clang to build an .so out of it
- dlload the .so
- get the address the .so was loaded at
- patch the original function to jump to the new function in the new .so.
More details are available in the github repo.
hotreload example.cpp
Hot reloading: example.cpp
Found 1 functions: ['addOne(int)']
Compiling .so...
→ (int)dlclose((void*)0x417300)
→ dlclose() succeeded
→ g++ -std=c++17 -g -O0 -fPIC -shared -o /tmp/lldb_hotreload/hotreload_example_de7d20e4.so /tmp/lldb_hotreload/hotreload_example_de7d20e4.cpp
Compiled to /tmp/lldb_hotreload/hotreload_example_de7d20e4.so
→ (void*)dlopen("/tmp/lldb_hotreload/hotreload_example_de7d20e4.so", 2 | 256)
→ dlopen() returned handle 0x417300
→ LLDB auto-detected module: hotreload_example_de7d20e4.so
Processing addOne(int) @ 0x401176
→ ((void*(*)())dlsym((void*)0x417300, "__addOne_hotreload_de7d20e4_ptr"))())
→ Resolved addOne(int) to 0x7ffff7fb5169
Re-patching addOne(int): 0x401176 → 0x7ffff7fb5169
→ WriteMemory(0x401176, 21 bytes)
[48 b8 69 51 fb f7 ff 7f 00 00 ff e0] + 9 NOPs
Disassembly: movabsq $0x7ffff7fb5169, %rax; jmp *%rax; nop×9
Deleted 1 old breakpoint(s) from previous hot reloads
✓ Patched 1/1 functions
Auto-breakpoint: addOne(int) at hotreload_example_de7d20e4.cpp:23 (addr 0x7ffff7fb5169)
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✓ Created 1 auto-breakpoint(s) in hot-reloaded code
Next 'continue' will hit breakpoints in new code!
Manual breakpoints: b /tmp/lldb_hotreload/hotreload_example_de7d20e4.cpp:<line>
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