Hacker News Reader

Comment by johnisgood | Hacker News Reader

Isn't "dsb nsh" or "isb" redundant for simple syscalls like write and exit?

Without them it'll segfault on QEMU, IIRC OpenBSD libc uses them invariably as well. I don't know how it fares on real hardware

Yeah, probably (although I am not sure) because QEMU doesn't fully emulate hardware behavior, but on real hardware some CPUs may internally handle these state transitions better. I suppose its inclusion is recommended to ensure correctness across all CPUs.

It may happen because without "dsb nsh" (Data Synchronization Barrier) and "isb" (Instruction Synchronization Barrier), QEMU may continue execution before the syscall fully completes, which causes a segfault or UB, but I am not entirely sure.

"dsb nsh" and "isb" ensures correct execution order which prevents speculative execution issues.

That doesn't sound right -- QEMU always works one instruction at a time, so we never start executing a following insn until the previous one is completed.

Yeah the segfault doesn't happen on other OSes, I believe it's an OpenBSD thing

So what causes QEMU to segfault without the synchronization? I have not tested it myself, however, so I cannot guarantee that it indeed does segfault.

The "dsb nsh; isb" sequence after "svc 0" is part of OpenBSD's mitigations for Spectre.

https://github.com/openbsd/src/commit/bbeaada4689520859307d5...

https://github.com/openbsd/src/commit/0c401ffc2a2550c32105ce...

https://github.com/openbsd/src/commit/5ecc9681133f1894e81c38...

If I'm reading the commits correctly, the OpenBSD kernel will skip two instructions after a "svc 0" when returning to userspace, on the assumption that any syscall comes from libc and therefore has "dsb nsh; isb" after it.

Reply on news.ycombinator.com