Stack overflow in Rust does abort the process, but not immediately: the signal handler first distinguishes stack overflow from other sources of SIGSEGV/SIGBUS,

Stack overflow in Rust does abort the process, but not immediately: the signal handler first distinguishes stack overflow from other sources of SIGSEGV/SIGBUS, and then prints a message containing the name of the current thread. Both of these require accessing TLS, which in turn may panic, or invoke other non-async-signal safe functions. That is UB. I don't know what the definition of a "soundness issue" is, but I think there are some nasal demons to be found here; it will be fun to try at least. edit the linked issue missed a case! Rust aborts by using libc::abort(), which is not async signal-safe in glibc prior to 2.26.