Add basic U-mode support with ecall-based syscalls

[HeatCrab]

This PR implements basic U-mode (User Mode) execution for tasks, addressing Issue #19 where tasks executing in M-mode could bypass isolation by modifying privileged CSRs. This change is essential for PMP (Physical Memory Protection) to function correctly, as PMP only affects U-mode and S-mode.

Tasks now start in user mode and interact with the kernel through a syscall interface based on the ecall trap mechanism. The kernel dispatcher has been refactored to support architecture-specific implementations through weak linking, while the RISC-V backend provides an ecall wrapper following standard calling conventions. Exception handling has been extended to recognize and service user mode traps, maintaining proper privilege boundaries throughout task execution.

Related to #19

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Summary by cubic

Enables user-mode execution on RISC-V with an ecall-based syscall path to enforce privilege separation and make PMP effective. Addresses the isolation issue where tasks could modify privileged CSRs in M-mode (Issue #19).

• New Features

  • Tasks start in U-mode (MPP set to USER).
  • Syscalls use ecall following RISC-V ABI; arch-specific implementation added in arch/riscv/entry.c.
  • Trap handler services U-mode ecalls: advances mepc, extracts args, dispatches, and returns via a0.

• Refactors

  • Syscall dispatcher marked as weak to allow architecture-specific overrides.
  • Build updated to include entry.o in HAL objects.

^{Written for commit e2eec20. Summary will update automatically on new commits.}

[cubic-dev-ai]

1 issue found across 4 files

Prompt for AI agents (all 1 issues)

Understand the root cause of the following 1 issues and fix them.


<file name="arch/riscv/hal.c">

<violation number="1" location="arch/riscv/hal.c:334">
Syscall handler reads the wrong registers from the ISR frame, so the kernel receives an incorrect syscall number and arguments.</violation>
</file>

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[cubic-dev-ai]

Syscall handler reads the wrong registers from the ISR frame, so the kernel receives an incorrect syscall number and arguments.

Prompt for AI agents

Address the following comment on arch/riscv/hal.c at line 334:

<comment>Syscall handler reads the wrong registers from the ISR frame, so the kernel receives an incorrect syscall number and arguments.</comment>

<file context>
@@ -321,6 +321,31 @@ uint32_t do_trap(uint32_t cause, uint32_t epc, uint32_t isr_sp)
+            isr_frame[31] = new_epc;
+
+            /* Extract syscall arguments from ISR frame */
+            int syscall_num = isr_frame[17];
+            void *arg1 = (void *) isr_frame[10];
+            void *arg2 = (void *) isr_frame[11];
</file context>

[HeatCrab]

Fixed! Got the ISR frame offsets wrong - checked boot.c and corrected them.

[cubic-dev-ai]

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