UBSAN with -fsanitize=implicit-integer-truncation reported a suspicious case:

testSetBlocking_overflow (test.test_socket.NonBlockingTCPTests) ... /scratch2/izbyshev/cpython/Modules/socketmodule.c:2688:33: runtime error: implicit conversion from type 'long' of value 4294967296 (64-bit, signed) to type 'int' changed the value to 0 (32-bit, signed)

It turned out that sock_setblocking() converts its (logically boolean) argument to long, but then passes it to internal_setblocking() which accepts int (https://github.com/python/cpython/blob/fd512d76456b65c529a5bc58d8cfe73e4a10de7a/Modules/socketmodule.c#L2688). This results in unexpected truncation on platforms with 64-bit long.

testSetBlocking_overflow() which is supposed to check this doesn't work because it only checks socket timeout which is updated correctly. However, the actual state of socket descriptor is changed to the opposite value (non-blocking) in this case.