UB if noreturn syscall actually returns

[rusty-snake]

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https://github.com/bytecodealliance/rustix/blob/cb01fbe4660844b67fdd4eee2a5f769518f6a655/src/backend/linux_raw/arch/x86_64.rs#L55-L63

https://github.com/bytecodealliance/rustix/blob/cb01fbe4660844b67fdd4eee2a5f769518f6a655/src/backend/linux_raw/runtime/syscalls.rs#L143-L146

Every syscall may return

Every syscall may return with any value because of seccomp-bpf.

This includes pure syscalls like getuid that may return the current uid as 4294967295_u32 (-EPERM) or exit/exit_group returning. While such seccomp-bpf filters will have huge compatibility problems and are unlikely in real-world, they must be handled in a safe way.

In the case of exit/exit_group there are two safe possibilities that maintain -> ! to the caller:

1. Infinity loop

   // Note that this is a pseudo example. If the `sys_exit` function is `-> !`,
   // Rust is free to remove the loop because returning from `-> !` is UB.
   // The loop must be implemented in assembly with a backward jump
   // if the `asm!` is marked `noreturn` as return from the `noreturn`-`asm!`
   // is already UB.
   loop {
       sys_exit(code)
   }
   

2. Segfault, i.e. ud2 (x86-64) after the syscall.