Lab5: User Library

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User library

user/ulib.h

user library 因為大部份的函式 kernel 也需要，所以在 kernel file 內也 include 不少

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#include "include/syscall.h"
#include "include/stdlib.h"
#include "include/stdio.h"
#include "include/string.h"
#include "include/signal.h"
#include "include/types.h"

user/main.c

簡易的 main function

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#include "user/ulib.h"

void delay(int period)
{
    while (period--)
        ;
}

int main()
{
    int count = 0;
    while (1) {
        printf("[PID %d] Hello world x%d\n", get_taskid(), count++);
        delay(10000000);
    }
    return 0;
}

user/crt0.S

以 0x0 作為 user program 的起點，跳轉到 main 並在 return 時呼叫 exit 回收資源

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.section ".text.entry"

.global _user_entry
_user_entry:
    bl main
    bl exit // syscall

user/linker.ld

放上 .text.entry symbol 以便在 objdmup 時清楚看到 crt0

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SECTIONS
{
    . = 0x0;
    .text.entry : {
        *(.text.entry)
    }
    .text : {
        *(.text)
    }
    .data : {
        *(.data)
    }
    .bss : {
        . = ALIGN(16);
        *(.bss .bss.*)
    }
}

Embed compiled binary to kernel

將 user program 嵌入到 kernel

  1. 生成 elf

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    $(LD) -T $(USER_LINKER_FILE) $^ -o $@

  2. 轉成 raw binary

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    $(OBJCPY) -O binary $< $@

  3. 轉成 allocatable binary ，user program loader 得以透過下列 symbol 定位 user program 及取得大小

    • _binary_<input name>_start
    • _binary_<input name>_end
    • _binary_<input name>_size
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    $(LD) -r -b binary $< -o $@

kernel/Makefile

加上 -fno-zero-initialized-in-bss 來讓 zero-initialized global variable 放在 .data 而非 .bss section，參考 .bss section：C 語言所種下的因

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USER_BIN = user/user.bin

# library
lib/%.o: lib/%.c
	$(CC) $(CFLAGS) $< -c -o $@

# user
user/%.o: user/%.c
	$(CC) $(CFLAGS) -fno-zero-initialized-in-bss $< -c -o $@

user/%.asm.o: user/%.S
	$(CC) $(CFLAGS) -fno-zero-initialized-in-bss $< -c -o user/$*.asm.o

user/user.elf: $(LIB_OBJ) $(USER_OBJS)
	$(LD) -T $(USER_LINKER_FILE) $^ -o $@

user/user.img: user/user.elf
	$(OBJCPY) -O binary $< $@

user/user.bin: user/user.img
	$(LD) -r -b binary $< -o $@

#kernel
kernel/kernel8.elf: $(KERNEL_OBJS) $(LIB_OBJ) $(USER_BIN)
	$(LD) -T $(KERNEL_LINKER_FILE) $^ -o $@

Raw binary loading and mapping

kernel/demo.c

從 crt0 的起始位置 0x0 開始執行

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// kernel task
void required_3_1()
{
    extern char _binary_user_user_img_start;
    extern char _binary_user_user_img_size;
    void *start = (void *) &_binary_user_user_img_start;
    ssize_t size = (ssize_t) &_binary_user_user_img_size;
    do_exec(start, size, (uint64_t) 0x0);
}

Demo

剛開始執行的時候，一直跳以下錯誤

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Exception return address 0x01010
Exception class (EC) 0x20
Instruction specific synfrome (ISS) 0xC

檢查 page table 許久找不到原因。後來看到 main 的地址 0x1010 想到是 user image 太大導致 do_exec 複製超出預先分配的 4KB page (連結的 user library 超級大)，main 自然無法存取到。至於 EC 和 ISS 為何查 ARM manual 結果是 level 0 permission denied 就不曉得…

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0000000000001362 D _binary_user_user_img_end
0000000000001362 A _binary_user_user_img_size
0000000000000000 D _binary_user_user_img_start
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0000000000000000 <_user_entry>:
.section ".text.entry"

.global _user_entry
_user_entry:
    bl main
   0:   94000404        bl      1010 <main>

解決的方法是多分配一點 page

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void do_exec(void (*func)(), size_t size, uint64_t pc)
{
    /*
     * If the task was created by privilege_task_create() and hasn't initlized
     * page table, the default value of TTBR0_EL1 is 0x0
     */
    const task_t *task = get_current();
    uintptr_t sp =
        USER_VIRT_TOP -
        sizeof(uintptr_t);  // stack address grows towards lower memory address

    // allocate pages for user program
    size_t upper_bound = ROUNDUP(size, PAGE_SIZE);
    for (size_t offset = 0; offset < upper_bound;
         offset += PAGE_SIZE, size -= PAGE_SIZE) {
        void *utext = map_addr_user((void *) (pc + offset));
        if (!utext) {
            goto _do_exec_err;
        }
        memcpy(utext, (char *) func + offset, MIN(size, PAGE_SIZE));
    }

    // assume stack size < 4KB
    void *ustack = map_addr_user((void *) sp);
    if (!ustack) {
        goto _do_exec_err;
    }

    // switch to el0
    update_pgd(task->mm.pgd);
    asm volatile(
        "msr     sp_el0, %0\n\t"
        "msr     elr_el1, %1\n\t"
        "msr     spsr_el1, %2\n\t"
        "eret" ::"r"(sp),
        "r"(pc), "r"(SPSR_EL1_VALUE));

_do_exec_err:
    // TODO: reclaim all pages

    return;
}

跑起來畫面如下

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[PID 2] Hello world x35
[PID 2] Hello world x36
[PID 2] Hello world x37
[PID 2] Hello world x38
[PID 2] Hello world x39
[PID 2] Hello world x40
[PID 2] Hello world x41
[PID 2] Hello world x42
[PID 2] Hello world x43
[PID 2] Hello world x44
[PID 2] Hello world x45
[PID 2] Hello world x46
[PID 2] Hello world x47
[PID 2] Hello world x48
[PID 2] Hello world x49
[PID 2] Hello world x50
[PID 2] Hello world x51

Reference