{bp-13633} riscv_fork.c: Fix vfork() for kernel mode + SMP

[jerpelea]

Summary
There was an error in the fork() routine when system calls are in use: the child context is saved on the child's user stack, which is incorrect, the context must be saved on the kernel stack instead.

The result is a full system crash if (when) the child executes on a different CPU which does not have the same MMU mappings active.

Impact
RELEASE

Testing
CI

[nuttxpr]

[Experimental Bot, please feedback here]

NuttX PR Requirements Review

Missing Information:

• Summary:
  • Functional part of code being changed: Specify which part of the fork() routine and related data structures are modified.
  • How the change works: Detail the implementation fix: How is the child context saved on the kernel stack instead?
• Impact:
  • Existing feature changed: YES - fork() system call behavior corrected.
  • Impact on user: Potentially YES. While a bug fix, applications relying on the incorrect behavior (however unlikely) may break. Describe this potential impact and if any mitigation is needed.
  • Impact on hardware: Clarify if specific architectures are affected or if the fix is architecture-agnostic.
  • Impact on documentation: YES - If documentation exists for fork(), it should be reviewed and potentially updated to reflect the correct behavior.
  • Impact on security: Potentially YES. Exploiting the incorrect context saving could have security implications. Describe the potential vulnerability and how the fix addresses it.
• Testing:
  • Build Host(s): Provide details on the development environment used.
  • Target(s): Specify the architectures and boards where the fix was tested.
  • Testing logs: Include relevant logs demonstrating the issue before the change and the successful fix afterward.

Concise Assessment:

The PR addresses a critical issue in fork(), but lacks essential details in the Summary, Impact assessment, and Testing sections to meet NuttX's requirements fully. Provide the missing information to ensure thorough review and acceptance.

[lupyuen]

Oops arm-03 fails with the error below. examples/can calls ch_tcf, which comes from yesterday's PR: #13758

Hi @xiaoxiang781216: Wonder if we are building with the releases/12.7 of NuttX Apps? This error seems to be caused by the master branch?

https://github.com/apache/nuttx/actions/runs/11154095098/job/31002846380

Configuration/Tool: lx_cpu/nsh,CONFIG_ARM_TOOLCHAIN_GNU_EABI
Error: can_main.c:361:19: error: 'struct can_hdr_s' has no member named 'ch_tcf'
  361 |       txmsg.cm_hdr.ch_tcf    = 0;
      |   

[lupyuen]

@xiaoxiang781216 I think we have a bug in build.yml: https://github.com/apache/nuttx/pull/9289/files

echo "app_ref=$APPS_REF" >> $GITHUB_OUTPUT

app_ref should be be apps_ref? That's because build.yml refers to apps_ref later: https://github.com/apache/nuttx/blob/master/.github/workflows/build.yml#L100-L108

      - name: Checkout apps repo
        uses: actions/checkout@v4
        with:
          repository: apache/nuttx-apps
          ref: ${{ steps.gittargets.outputs.apps_ref }}

This is causing the incorrect version of NuttX Apps to be checked out.

Update: I have submitted the PR: #13806