RFR: 8345296: AArch64: VM crashes with SIGILL when prctl is disallowed

[Evgeny Astigeevich]

On Mon, 2 Dec 2024 12:17:26 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> We have caught this in some prod environments, where `prctl` is forbidden by the sandboxing mechanism. This fails the JVM, because we have the following code to check for SVE vector length:
> 
> 
> int VM_Version::get_current_sve_vector_length() {
>   assert(VM_Version::supports_sve(), "should not call this");
>   return prctl(PR_SVE_GET_VL);
> }
> 
> 
> That code returns `-1` when `prctl` is disallowed, which JVM then blindly interprets as vector length, leading to `SIGILL`. I looked around other uses of `prctl` around Hotspot, and they all seem to handle the errors correctly.
> 
> Additional testing:
>  - [x] Linux AArch64 server fastdebug, with seccomp reproducer
>  - [ ] Linux AArch64 server fastdebug, `all`

Changes requested by eastigeevich (Committer).

src/hotspot/cpu/aarch64/vm_version_aarch64.cpp line 447:

> 445: 
> 446:   if (UseSVE > 0) {
> 447:     _initial_sve_vector_length = get_current_sve_vector_length();

We need an assert checking `_initial_sve_vector_length > 0`

src/hotspot/cpu/aarch64/vm_version_aarch64.cpp line 453:

> 451:     } else {
> 452:       _initial_sve_vector_length = vl;
> 453:     }

I think we need to disable SVE in `VM_Version::get_os_cpu_info` in `src/hotspot/os_cpu/linux_aarch64/vm_version_linux_aarch64.cpp`:


  if (auxv2 & HWCAP2_SVE2) _features |= CPU_SVE2;
  if (auxv2 & HWCAP2_SVEBITPERM) _features |= CPU_SVEBITPERM;

  if (prctl(PR_SVE_GET_VL) == -1) {
    warning("Unable to get SVE vector length on this system. Disabling SVE.");
    _features &= ~CPU_SVE;
  }

-------------

PR Review: https://git.openjdk.org/jdk/pull/22479#pullrequestreview-2472663905
PR Review Comment: https://git.openjdk.org/jdk/pull/22479#discussion_r1865833797
PR Review Comment: https://git.openjdk.org/jdk/pull/22479#discussion_r1865828571