zlacker

>>rvz+(OP)
As usual HN comments react to the headline, without reading the content.

A lot of modern userspace code, including Rust code in the standard library, thinks that invariant failures (AKA "programmer errors") should cause some sort of assertion failure or crash (Rust or Go `panic`, C/C++ `assert`, etc). In the kernel, claims Linus, failing loudly is worse than trying to keep going because failing would also kill the failure reporting mechanisms.

He advocates for a sort of soft-failure, where the code tells you you're entering unknown territory and then goes ahead and does whatever. Maybe it crashes later, maybe it returns the wrong answer, who knows, the only thing it won't do is halt the kernel at the point the error was detected.

Think of the following Rust API for an array, which needs to be able to handle the case of a user reading an index outside its bounds:

  struct Array<T> { ... }
  impl<T> Array<T> {
    fn len(&self) -> usize;

    // if idx >= len, panic
    fn get_or_panic(&self, idx: usize) -> T;

    // if idx >= len, return None
    fn get_or_none(&self, idx: usize) -> Option<T>;

    // if idx >= len, print a stack trace and return
    // who knows what
    unsafe fn get_or_undefined(&self, idx: usize) -> T;
  }

The first two are safe by the Rust definition, because they can't cause memory-unsafe behavior. The second two are safe by the Linus/Linux definition, because they won't cause a kernel panic. If you have to choose between #1 and #3, Linus is putting his foot down and saying that the kernel's answer is #3.

>>jmilli+Fb
The policy of ‘oopsing’ and limping on is, in my opinion, literally one of Linux’s worst features. It has bitten me in various cases:

- Remember when Linux had that caused the kernel to partially crash and eat 100% CPU due to some bug in the leap second application code? That caused a >1MW spike in power usage at Hetzner at the time. That must have been >1GW globally. Many people didn’t notice it immediately, so it must have taken weeks before everyone rebooted.

- I’ve personally run into issues where not crashing caused Linux to go on and eat my file system.

On any Linux server I maintain, I always toggle those sysctls that cause the kernel to panic on oops, and reboot on panic.

>>EdScho+Vf
So instead of a power spike, we'd have had a major internet outage across the world, across the entire industry and beyond, probably, if everyone had panicked on oops. The blame really lies with people not monitoring their systems.

As you said, you have the option to reboot on panic, but Linus is absolutely not wrong that this size does not fit all.

What about a medical procedure that WILL kill the patient if interrupted? What about life support in space? Hitting an assert in those kinds of systems is a very bad place to be, but an automatic halt is worse than at least giving the people involved a CHANCE to try and get to a state where it's safe to take the system offline and restart it.

>>mike_h+7k
> What about a medical procedure that WILL kill the patient if interrupted? What about life support in space?

The proper answer to those is redundancy, not continuing in an unknown and quite likely harmful state.

>>notaco+iO
The leap second bug would have crashed all nodes of a redundant system, at the same time...

>>yencab+m21
Perhaps. On the other hand, letting a medical device continue moving an actuator or dispensing a medication when it's known to be in a bad "never happen" state could also be fatal. Ditto for the "life support in space" example. Ditto for anything reliant on position, where the system suddenly realizes it has no idea whether its position is correct. Imagine that e.g. on a warship. Limiting responses to external inputs (including time adjustments) can ameliorate such problems. So can software diversity. Many safety-critical systems require one or both, and other measures as well. Picking one black-swan event while ignoring literally every day scenarios doesn't seem very helpful. That's especially true when the thing you're advocating is what actually happened and led to its own bad outcomes.

>>notaco+q51
Picking medical devices and warships is also quite the cherry picking. Most Linuxes aren't like that. Critical embedded systems tend to have a hard realtime component, and if Linux is on the system it sits under e.g. seL4, or on a different CPU.

At the end of the day, what Linux does is what Linus wants out of it. He's stated, often, that halting the CPU at the exact moment something goes wrong is not the goal. If your goal is to do that, you might not be able to use Linux. If your goal is to put Rust in the Linux kernel, you might have to let go of your goal.