Python/flowgraph.c:701: basicblock *push_except_block(struct _PyCfgExceptStack *, cfg_instr *): Assertion `stack->depth <= CO_MAXBLOCKS' failed

[alex]

Crash report

What happened?

Minimal reproducer

async def t():
 async with h,t,t,o,f,y,o,t,r,o,f,t,f,r,t,m,r,o,t,l:n

~/p/cpython ❯❯❯ ./python.exe -c 'async def t():
                 async with h,t,t,o,f,y,o,t,r,o,f,t,f,r,t,m,r,o,t,l:n'
Assertion failed: (stack->depth <= CO_MAXBLOCKS), function push_except_block, file flowgraph.c, line 707.
fish: Job 1, './python.exe -c 'async def t():' terminated by signal  async with h,t,t,o,f,y,o,t,r,o… (SIGABRT)
fish: Job Abort, '' terminated by signal  ()

Full ASAN:

fuzz_pycompile: Python/flowgraph.c:701: basicblock *push_except_block(struct _PyCfgExceptStack *, cfg_instr *): Assertion `stack->depth <= CO_MAXBLOCKS' failed.
--
  | AddressSanitizer:DEADLYSIGNAL
  | =================================================================
  | ==16858==ERROR: AddressSanitizer: ABRT on unknown address 0x0539000041da (pc 0x7a22fe4c600b bp 0x7a22fe63b588 sp 0x7ffe3dd3ff40 T0)
  | SCARINESS: 10 (signal)
  | #0 0x7a22fe4c600b in raise /build/glibc-SzIz7B/glibc-2.31/sysdeps/unix/sysv/linux/raise.c:51:1
  | #1 0x7a22fe4a5858 in abort /build/glibc-SzIz7B/glibc-2.31/stdlib/abort.c:79:7
  | #2 0x7a22fe4a5728 in __assert_fail_base /build/glibc-SzIz7B/glibc-2.31/assert/assert.c:92:3
  | #3 0x7a22fe4b6fd5 in __assert_fail /build/glibc-SzIz7B/glibc-2.31/assert/assert.c:101:3
  | #4 0x8f5084 in push_except_block cpython3/Python/flowgraph.c:701:5
  | #5 0x8f5084 in label_exception_targets cpython3/Python/flowgraph.c:896:27
  | #6 0x8e5db6 in _PyCfg_OptimizeCodeUnit cpython3/Python/flowgraph.c:2490:5
  | #7 0x8702e3 in optimize_and_assemble_code_unit cpython3/Python/compile.c:7598:9
  | #8 0x8702e3 in optimize_and_assemble cpython3/Python/compile.c:7640:12
  | #9 0x8948d7 in compiler_function_body cpython3/Python/compile.c:2309:24
  | #10 0x8948d7 in compiler_function cpython3/Python/compile.c:2410:9
  | #11 0x8770af in compiler_visit_stmt cpython3/Python/compile.c:0
  | #12 0x874f14 in compiler_body cpython3/Python/compile.c:1729:9
  | #13 0x86c2fc in compiler_codegen cpython3/Python/compile.c:1740:13
  | #14 0x869346 in compiler_mod cpython3/Python/compile.c:1781:9
  | #15 0x869346 in _PyAST_Compile cpython3/Python/compile.c:555:24
  | #16 0x9f3d07 in Py_CompileStringObject cpython3/Python/pythonrun.c:1449:10
  | #17 0x9f3dfc in Py_CompileStringExFlags cpython3/Python/pythonrun.c:1462:10
  | #18 0x4f8fcc in fuzz_pycompile cpython3/Modules/_xxtestfuzz/fuzzer.c:551:24
  | #19 0x4f8fcc in _run_fuzz cpython3/Modules/_xxtestfuzz/fuzzer.c:570:14
  | #20 0x4f8fcc in LLVMFuzzerTestOneInput cpython3/Modules/_xxtestfuzz/fuzzer.c:711:11
  | #21 0x4f99ed in ExecuteFilesOnyByOne /src/aflplusplus/utils/aflpp_driver/aflpp_driver.c:255:7
  | #22 0x4f97e8 in LLVMFuzzerRunDriver /src/aflplusplus/utils/aflpp_driver/aflpp_driver.c:0
  | #23 0x4f939d in main /src/aflplusplus/utils/aflpp_driver/aflpp_driver.c:311:10
  | #24 0x7a22fe4a7082 in __libc_start_main /build/glibc-SzIz7B/glibc-2.31/csu/libc-start.c:308:16
  | #25 0x43a0ad in _start

cc: @iritkatriel

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Output from running 'python -VV' on the command line:

No response

Linked PRs

• gh-116767: fix crash on 'async with' with many context managers #118348
• [3.12] gh-116767: fix crash on 'async with' with many context managers (GH-118348) #118477

[alex]

c31943a is the commit that OSS-Fuzz attributes this crash to (though obviously that just made it an assertion failure, instead of something else)

[alex]

@iritkatriel thanks for jumping on all of these

[encukou]

@iritkatriel do you need help with this bug?

[iritkatriel]

It's not complicated, I'm just prioritising things I need to get in before feature freeze.

[encukou]

This is a release blocker. Will you prioritize it in time for the release or should I look at it instead?

[iritkatriel]

It shouldn't be a release blocker. (1) It is an unrealistic edge case discovered by the fuzzer. (2) it is not new to 3.13.

[alex]

The reason I marked it as a blocker is that the RMs had previously marked all the fuzzer identified crashes as blockers.

[iritkatriel]

I'll make it a deferred blocker so we won't forget to fix it for 3.13 release.

[iritkatriel]

@encukou See #118348.

[encukou]

Thank you for getting to this!