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tests: add an MPU test that circumvents libc
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‎examples/tests/mpu/mpu_no_libc/Makefile

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# Makefile for user application
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# Specify this directory relative to the current application.
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TOCK_USERLAND_BASE_DIR = ../../../..
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# Which files to compile.
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C_SRCS := $(wildcard *.c)
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STACK_SIZE := 1024
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# Include userland master makefile. Contains rules and flags for actually
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# building the application.
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include $(TOCK_USERLAND_BASE_DIR)/AppMakefile.mk

‎examples/tests/mpu/mpu_no_libc/README.md

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MPU No LibC
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===========
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This test verifies that an application can read and write all of the memory it
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has been allocated in ram, and can read all of its memory in flash.
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This test is raw assembly, and circumvents the usual libtock-c crt0 and any
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other userspace startup. Of note, the application will never call `brk` or
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`sbrk` or any other `memop` which would change the application memory layout
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beyond the initial configuration provided by the kernel.
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The application uses low-level debug to print its progress. A successful run
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will see debug numbers [0,10], and then terminate the process with a successful
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exit code.

‎examples/tests/mpu/mpu_no_libc/main.c

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/* vim: set sw=2 expandtab tw=80: */
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// Allow _start to go undeclared
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#pragma GCC diagnostic ignored "-Wmissing-declarations"
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#pragma GCC diagnostic ignored "-Wmissing-prototypes"
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__attribute__ ((section(".start"), used))
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__attribute__ ((naked))
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__attribute__ ((noreturn))
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void _start(void* app_start __attribute__((unused)),
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void* mem_start __attribute__((unused)),
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void* memory_len __attribute__((unused)),
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void* app_heap_break __attribute__((unused))) {
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__asm__ volatile (
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// This minimal test evaluates whether memory protection is configured
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// as-expected. It uses the low-level debug (LLD) mechanism to print
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// incremental numbers as it progresses, but provides no other output as it is
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// all assembly to precisely control behavior (and never sets up a stack
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// beyond the minimal one provided by the kernel).
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// First, save all the variables in hardware-stacked, callee-save registers
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#if defined(__thumb__)
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"movs r4, r0\n" // r4 = app_start
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"movs r5, r1\n" // r5 = mem_start
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"movs r6, r2\n" // r6 = memory_len
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"movs r7, r3\n" // r7 = app_heap_break
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"mov r8, sp\n" // r8 = stack pointer
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#elif defined(__riscv)
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"mv s0, a0\n" // s0 = app_start
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"mv s1, a1\n" // s1 = mem_start
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"mv s2, a2\n" // s2 = memory_len
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"mv s3, a3\n" // s3 = app_heap_break
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"mv s4, sp\n" // s4 = stack pointer
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#else
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#error Unknown arch. Note: Subsequent code assumes only two archs.
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#endif
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// Note that the test has commenced
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#if defined(__thumb__)
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"movs r2, #0\n" // LLD Command 2, Arg 1: Number to print (0)
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"bl lld_print_r2\n"
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#else
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"li a2, 0\n"
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"jal ra, lld_print_a2\n"
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#endif
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// Print all of the interesting starting values
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// 1. app_start: where app starts in flash
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// 2. mem_start: where app-accesible RAM starts
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// 3. memory_len: *total* length of app allocation (incl grant)
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// 4. app_heap_break: max app-accesible address
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// 5. the initial stack pointer when process was created
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// 6. the current stack pointer (should be same as 5)
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#if defined(__thumb__)
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"movs r2, #1\n"
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"movs r3, r4\n"
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"bl lld_print_r2r3\n"
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"movs r2, #2\n"
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"movs r3, r5\n"
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"bl lld_print_r2r3\n"
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"movs r2, #3\n"
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"movs r3, r6\n"
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"bl lld_print_r2r3\n"
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"movs r2, #4\n"
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"movs r3, r7\n"
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"bl lld_print_r2r3\n"
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"movs r2, #5\n"
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"movs r3, r8\n"
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"bl lld_print_r2r3\n"
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"movs r2, #6\n"
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"mov r3, sp\n"
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#else
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"li a2, 1\n"
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"mv a3, s0\n"
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"jal ra, lld_print_a2a3\n"
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"li a2, 2\n"
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"mv a3, s1\n"
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"jal ra, lld_print_a2a3\n"
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"li a2, 3\n"
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"mv a3, s2\n"
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"jal ra, lld_print_a2a3\n"
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"li a2, 4\n"
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"mv a3, s3\n"
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"jal ra, lld_print_a2a3\n"
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"li a2, 5\n"
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"mv a3, s4\n"
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"jal ra, lld_print_a2a3\n"
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"li a2, 6\n"
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"mv a3, sp\n"
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"jal ra, lld_print_a2a3\n"
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#endif
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// 7. Read all app-accessible memory: [mem_start , app_heap_break)
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// Assumed invariant: app_heap_break >= [mem_start + 4]
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#if defined(__thumb__)
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"movs r2, #7\n"
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"bl lld_print_r2\n"
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"movs r3, r5\n" // r3 (loop var) = mem_start
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"test_7_loop:\n"
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"ldr r2, [r3]\n" // read *r3
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"add r3, r3, #4\n" // increment r3=r3+4
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"cmp r3, r7\n"
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"ble test_7_loop\n" // if r3 < app_heap_break[r7], loop
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#else
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"li a2, 7\n"
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"jal ra, lld_print_a2\n"
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"mv t1, s1\n"
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"test_7_loop:\n"
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"lw t0, 0(t1)\n"
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"addi t1, t1, 4\n"
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"blt t1, s3, test_7_loop\n"
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#endif
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// 8. Write all app-accessible memory: [mem_start , app_heap_break)
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// Assumed invariant: app_heap_break >= [mem_start + 4]
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#if defined(__thumb__)
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"movs r2, #8\n"
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"bl lld_print_r2\n"
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"movs r3, r5\n" // r3 (loop var) = mem_start
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"test_8_loop:\n"
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"str r2, [r3]\n" // read *r3
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"add r3, r3, #4\n" // increment r3=r3+4
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"cmp r3, r7\n"
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"ble test_8_loop\n" // if r3 < app_heap_break[r7], loop
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#else
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"li a2, 8\n"
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"jal ra, lld_print_a2\n"
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"mv t1, s1\n"
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"test_8_loop:\n"
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"sw t0, 0(t1)\n"
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"addi t1, t1, 4\n"
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"blt t1, s3, test_8_loop\n"
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#endif
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// 9. Read all app-accessible flash: [app_start , memop(6))
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// Flash length is not a startup argument, so we must use
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// memop(6) to get the end of app flash first.
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#if defined(__thumb__)
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"movs r2, #9\n"
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"bl lld_print_r2\n"
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"movs r0, #6\n"
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"movs r1, #0\n"
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"svc 5\n"
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"movs r11, r1\n"
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"movs r2, #9\n"
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"movs r3, r11\n"
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"bl lld_print_r2r3\n"
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"movs r3, r4\n" // r3 (loop var) = app_start
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"test_9_loop:\n"
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"ldr r2, [r3]\n" // read *r3
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"add r3, r3, #4\n" // increment r3=r3+4
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"cmp r3, r11\n"
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"ble test_9_loop\n" // if r3 < memop(6)[r11], loop
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#else
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"li a2, 9\n"
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"jal ra, lld_print_a2\n"
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"li a0, 6\n"
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"li a1, 0\n"
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"li a4, 5\n"
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"ecall\n"
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"mv t2, a1\n"
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"li a2, 9\n"
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"mv a3, t2\n"
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"jal ra, lld_print_a2a3\n"
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"mv t1, s0\n"
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"test_9_loop:\n"
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"lw t0, 0(t1)\n"
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"addi t1, t1, 4\n"
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"blt t1, t2, test_9_loop\n"
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#endif
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// 10. Return success
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// Indicate all tests are done and exit.
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#if defined(__thumb__)
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"movs r2, #10\n"
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"bl lld_print_r2\n"
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"movs r0, #0\n" // exit-terminate
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"movs r1, #0\n" // completion: sucess
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"svc 6\n" // Exit syscall
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#else
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"li a2, 10\n"
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"jal ra, lld_print_a2\n"
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"li a0, 0\n"
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"li a1, 0\n"
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"li a4, 6\n"
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"ecall\n"
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#endif
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// -1. Unreachable/error
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#if defined(__thumb__)
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"loop_error:\n"
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"movs r2, #0\n"
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"sub r2, r2, #1\n"
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"bl lld_print_r2\n"
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"b loop_error\n"
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#else
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"loop_error:\n"
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"li a2, -1\n"
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"jal ra, lld_print_a2\n"
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"j loop_error\n"
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#endif
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// Function to invoke LLD print of one number
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// r2/a2 must be set to desired number
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// may clobber any caller-save b/c of syscall rules
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#if defined(__thumb__)
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"lld_print_r2:\n"
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"movs r0, #8\n" // LLD is driver number 8
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"movs r1, #2\n" // LLD Command 2: Print one number
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"svc 2\n" // Invoke Command syscall
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"bx lr\n" // Return.
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#else
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"lld_print_a2:\n"
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"li a0, 8\n" // LLD is driver number 8
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"li a1, 2\n" // LLD Command 2: Print one number
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"li a4, 2\n" // Command is syscall 2
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"ecall\n" // Invoke Command syscall
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"jr ra\n" // Return.
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#endif
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// Function to invoke LLD print of two numbers
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// r2,r3/a2,a3 must be set to desired number
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// may clobber any caller-save b/c of syscall rules
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#if defined(__thumb__)
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"lld_print_r2r3:\n"
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"movs r0, #8\n" // LLD is driver number 8
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"movs r1, #3\n" // LLD Command 3: Print one number
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"svc 2\n" // Invoke Command syscall
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"bx lr\n" // Return.
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#else
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"lld_print_a2a3:\n"
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"li a0, 8\n" // LLD is driver number 8
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"li a1, 3\n" // LLD Command 3: Print one number
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"li a4, 2\n" // Command is syscall 2
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"ecall\n" // Invoke Command syscall
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"jr ra\n" // Return.
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#endif
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);
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}

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