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Linux 内核系统调用 第三节

vsyscalls 和 vDSO

这是讲解 Linux 内核中系统调用章节的第三部分，前一节讨论了用户空间应用程序发起的系统调用的准备工作及系统调用的处理过程。在这一节将讨论两个与系统调用十分相似的概念，这两个概念是vsyscall 和 vdso。

我们已经了解什么是系统调用。这是 Linux 内核一种特殊的运行机制，使得用户空间的应用程序可以请求，像写入文件和打开套接字等特权级下的任务。正如你所了解的，在 Linux 内核中发起一个系统调用是特别昂贵的操作，因为处理器需要中断当前正在执行的任务，切换内核模式的上下文，在系统调用处理完毕后跳转至用户空间。以下的两种机制 - vsyscall 和d vdso 被设计用来加速系统调用的处理，在这一节我们将了解两种机制的工作原理。

vsyscalls 介绍

vsyscall 或 virtual system call 是第一种也是最古老的一种用于加快系统调用的机制。 vsyscall 的工作原则其实十分简单。Linux 内核在用户空间映射一个包含一些变量及一些系统调用的实现的内存页。 对于 X86_64 架构可以在 Linux 内核的 [文档] (https://github.com/torvalds/linux/blob/master/Documentation/x86/x86_64/mm.txt) 找到关于这一内存区域的信息：

ffffffffff600000 - ffffffffffdfffff (=8 MB) vsyscalls

或:

~$ sudo cat /proc/1/maps | grep vsyscall
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]

因此, 这些系统调用将在用户空间下执行，这意味着将不发生 上下文切换。 vsyscall 内存页的映射在 arch/x86/entry/vsyscall/vsyscall_64.c 源代码中定义的 map_vsyscall 函数中实现。这一函数在 Linux 内核初始化时被 arch/x86/kernel/setup.c 源代码中定义的函数setup_arch (我们在第五章 Linux 内核的初始化中讨论过该函数)。

注意 map_vsyscall 函数的实现依赖于内核配置选项 CONFIG_X86_VSYSCALL_EMULATION :

#ifdef CONFIG_X86_VSYSCALL_EMULATION
extern void map_vsyscall(void);
#else
static inline void map_vsyscall(void) {}
#endif

正如帮助文档中所描述的, CONFIG_X86_VSYSCALL_EMULATION 配置选项: 使能 vsyscall 模拟. 为何模拟 vsyscall? 事实上, vsyscall 由于安全原因是一种遗留 ABI 。虚拟系统调用具有绑定的地址, 意味着 vsyscall 的内存页的位置在任何时刻是相同，这一位置是在 map_vsyscall 函数中指定的。这一函数的实现如下: 

void __init map_vsyscall(void)
{
    extern char __vsyscall_page;
    unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
    ...
    ...
    ...
}

在 map_vsyscall 函数的开始，通过宏 __pa_symbol 获取了 vsyscall 内存页的物理地址(我们已在第四章 of the Linux kernel initialization process)讨论了该宏的实现）。__vsyscall_page 在 arch/x86/entry/vsyscall/vsyscall_emu_64.S 汇编源代码文件中定义， 具有如下的 虚拟地址:

ffffffff81881000 D __vsyscall_page

在 .data..page_aligned, aw 段 中 包含如下三中系统调用:

  • gettimeofday;
  • time;
  • getcpu.

或:

__vsyscall_page:
    mov $__NR_gettimeofday, %rax
    syscall
    ret

    .balign 1024, 0xcc
    mov $__NR_time, %rax
    syscall
    ret

    .balign 1024, 0xcc
    mov $__NR_getcpu, %rax
    syscall
    ret

回到 map_vsyscall 函数及 __vsyscall_page 的实现，在得到 __vsyscall_page 的物理地址之后，使用 __set_fixmap 为 vsyscall 内存页 检查设置 fix-mapped地址的变量vsyscall_mode:

if (vsyscall_mode != NONE)
    __set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
                 vsyscall_mode == NATIVE
                             ? PAGE_KERNEL_VSYSCALL
                             : PAGE_KERNEL_VVAR);

The __set_fixmap takes three arguments: The first is index of the fixed_addresses enum. In our case VSYSCALL_PAGE is the first element of the fixed_addresses enum for the x86_64 architecture:

enum fixed_addresses {
...
...
...
#ifdef CONFIG_X86_VSYSCALL_EMULATION
    VSYSCALL_PAGE = (FIXADDR_TOP - VSYSCALL_ADDR) >> PAGE_SHIFT,
#endif
...
...
...

该变量值为 511。第二个参数为映射内存页的物理地址，第三个参数为内存页的标志位。注意 VSYSCALL_PAGE 标志位依赖于变量 vsyscall_mode 。当 vsyscall_mode 变量为 NATIVE 时， 标志位为 PAGE_KERNEL_VSYSCALL，其他情况则是PAGE_KERNEL_VVAR 。两个宏 ( PAGE_KERNEL_VSYSCALL 及 PAGE_KERNEL_VVAR) 都将被扩展以下标志:

#define __PAGE_KERNEL_VSYSCALL          (__PAGE_KERNEL_RX | _PAGE_USER)
#define __PAGE_KERNEL_VVAR              (__PAGE_KERNEL_RO | _PAGE_USER)

标志反映了 vsyscall 内存页的访问权限。两个标志都带有 _PAGE_USER 标志， 这意味着内存页可被运行于低特权级的用户模式进程访问。第二个标志位取决于 vsyscall_mode 变量的值。第一个标志 (__PAGE_KERNEL_VSYSCALL) 在 vsyscall_mode 为 NATIVE 时被设定。这意味着虚拟系统调用将以本地 syscall 指令的方式执行。另一情况下，在 vsyscall_mode 为 emulate 时 vsyscall 为 PAGE_KERNEL_VVAR，此时系统调用将被置于陷阱并被合理的模拟。 vsyscall_mode 变量通过 vsyscall_setup 获取值： 

static int __init vsyscall_setup(char *str)
{
    if (str) {
        if (!strcmp("emulate", str))
            vsyscall_mode = EMULATE;
        else if (!strcmp("native", str))
            vsyscall_mode = NATIVE;
        else if (!strcmp("none", str))
            vsyscall_mode = NONE;
        else
            return -EINVAL;

        return 0;
    }

    return -EINVAL;
}

函数将在早期的内核分析时被调用：

early_param("vsyscall", vsyscall_setup);

关于 early_param 宏的更多信息可以在第六章 Linux 内核初始化中找到。

在函数 vsyscall_map 的最后仅通过 BUILD_BUG_ON 宏检查 vsyscall 内存页的虚拟地址是否等于变量 VSYSCALL_ADDR :

BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
             (unsigned long)VSYSCALL_ADDR);

就这样vsyscall 内存页设置完毕。上述的结果如下： 若设置 vsyscall=native 内核命令行参数，虚拟内存调用将以 arch/x86/entry/vsyscall/vsyscall_emu_64.S 文件中本地 系统调用 指令的方式执行。 glibc 知道虚拟系统调用处理器的地址。注意虚拟系统调用的地址以 1024 (或 0x400) 比特对齐。

__vsyscall_page:
    mov $__NR_gettimeofday, %rax
    syscall
    ret

    .balign 1024, 0xcc
    mov $__NR_time, %rax
    syscall
    ret

    .balign 1024, 0xcc
    mov $__NR_getcpu, %rax
    syscall
    ret

vsyscall 内存页的起始地址为 ffffffffff600000 。因此, glibc 知道所有虚拟系统调用处理器的地址。可以在 glibc 源码中找到这些地址的定义：

#define VSYSCALL_ADDR_vgettimeofday   0xffffffffff600000
#define VSYSCALL_ADDR_vtime           0xffffffffff600400
#define VSYSCALL_ADDR_vgetcpu         0xffffffffff600800

所有的虚拟系统调用请求都将映射至 __vsyscall_page + VSYSCALL_ADDR_vsyscall_name 偏置, 将虚拟内存系统调用的编号置于通用目的寄存器，本地的 x86_64 系统调用指令将被执行。

在第二种情况中, 若将 vsyscall=emulate 参数传递给内核命令行, 提升虚拟系统调用处理器的尝试导致一个 page fault 异常。 谨记, vsyscall 内存页 具有 __PAGE_KERNEL_VVAR 的访问权限，这将禁止执行。 do_page_fault 函数是 #PF 或 page fault 的处理器。它将尝试了解最后一次 page fault 的原因。一种可能的场景是 vsyscall 模式为 emulate 情况下的虚拟系统调用。此时 vsyscall 将被 arch/x86/entry/vsyscall/vsyscall_64.c 源码中定义的 emulate_vsyscall 函数处理。

The emulate_vsyscall function gets the number of a virtual system call, checks it, prints error and sends segementation fault single:

...
...
...
vsyscall_nr = addr_to_vsyscall_nr(address);
if (vsyscall_nr < 0) {
    warn_bad_vsyscall(KERN_WARNING, regs, "misaligned vsyscall...);
    goto sigsegv;
}
...
...
...
sigsegv:
    force_sig(SIGSEGV, current);
    reutrn true;

As it checked number of a virtual system call, it does some yet another checks like access_ok violations and execute system call function depends on the number of a virtual system call:

switch (vsyscall_nr) {
    case 0:
        ret = sys_gettimeofday(
            (struct timeval __user *)regs->di,
            (struct timezone __user *)regs->si);
        break;
    ...
    ...
    ...
}

In the end we put the result of the sys_gettimeofday or another virtual system call handler to the ax general purpose register, as we did it with the normal system calls and restore the instruction pointer register and add 8 bytes to the stack pointer register. This operation emulates ret instruction.

    regs->ax = ret;

do_ret:
    regs->ip = caller;
    regs->sp += 8;
    return true;

That's all. Now let's look on the modern concept - vDSO.

Introduction to vDSO

As I already wrote above, vsyscall is an obsolete concept and replaced by the vDSO or virtual dynamic shared object. The main difference between the vsyscall and vDSO mechanisms is that vDSO maps memory pages into each process in a shared object form, but vsyscall is static in memory and has the same address every time. For the x86_64 architecture it is called -linux-vdso.so.1. All userspace applications linked with this shared library via the glibc. For example:

~$ ldd /bin/uname
    linux-vdso.so.1 (0x00007ffe014b7000)
    libc.so.6 => /lib64/libc.so.6 (0x00007fbfee2fe000)
    /lib64/ld-linux-x86-64.so.2 (0x00005559aab7c000)

Or:

~$ sudo cat /proc/1/maps | grep vdso
7fff39f73000-7fff39f75000 r-xp 00000000 00:00 0       [vdso]

Here we can see that uname util was linked with the three libraries:

  • linux-vdso.so.1;
  • libc.so.6;
  • ld-linux-x86-64.so.2.

The first provides vDSO functionality, the second is C standard library and the third is the program interpreter (more about this you can read in the part that describes linkers). So, the vDSO solves limitations of the vsyscall. Implementation of the vDSO is similar to vsyscall.

Initialization of the vDSO occurs in the init_vdso function that defined in the arch/x86/entry/vdso/vma.c source code file. This function starts from the initialization of the vDSO images for 32-bits and 64-bits depends on the CONFIG_X86_X32_ABI kernel configuration option:

static int __init init_vdso(void)
{
    init_vdso_image(&vdso_image_64);

#ifdef CONFIG_X86_X32_ABI
    init_vdso_image(&vdso_image_x32);
#endif

Both function initialize the vdso_image structure. This structure is defined in the two generated source code files: the arch/x86/entry/vdso/vdso-image-64.c and the arch/x86/entry/vdso/vdso-image-64.c. These source code files generated by the vdso2c program from the different source code files, represent different approaches to call a system call like int 0x80, sysenter and etc. The full set of the images depends on the kernel configuration.

For example for the x86_64 Linux kernel it will contain vdso_image_64:

#ifdef CONFIG_X86_64
extern const struct vdso_image vdso_image_64;
#endif

But for the x86 - vdso_image_32:

#ifdef CONFIG_X86_X32
extern const struct vdso_image vdso_image_x32;
#endif

If our kernel is configured for the x86 architecture or for the x86_64 and compability mode, we will have ability to call a system call with the int 0x80 interrupt, if compability mode is enabled, we will be able to call a system call with the native syscall instruction or sysenter instruction in other way:

#if defined CONFIG_X86_32 || defined CONFIG_COMPAT
  extern const struct vdso_image vdso_image_32_int80;
#ifdef CONFIG_COMPAT
  extern const struct vdso_image vdso_image_32_syscall;
#endif
 extern const struct vdso_image vdso_image_32_sysenter;
#endif

As we can understand from the name of the vdso_image structure, it represents image of the vDSO for the certain mode of the system call entry. This structure contains information about size in bytes of the vDSO area that always a multiple of PAGE_SIZE (4096 bytes), pointer to the text mapping, start and end address of the alternatives (set of instructions with better alternatives for the certain type of the processor) and etc. For example vdso_image_64 looks like this:

const struct vdso_image vdso_image_64 = {
    .data = raw_data,
    .size = 8192,
    .text_mapping = {
        .name = "[vdso]",
        .pages = pages,
    },
    .alt = 3145,
    .alt_len = 26,
    .sym_vvar_start = -8192,
    .sym_vvar_page = -8192,
    .sym_hpet_page = -4096,
};

Where the raw_data contains raw binary code of the 64-bit vDSO system calls which are 2 page size:

static struct page *pages[2];

or 8 Kilobytes.

The init_vdso_image function is defined in the same source code file and just initializes the vdso_image.text_mapping.pages. First of all this function calculates the number of pages and initializes each vdso_image.text_mapping.pages[number_of_page] with the virt_to_page macro that converts given address to the page structure:

void __init init_vdso_image(const struct vdso_image *image)
{
    int i;
    int npages = (image->size) / PAGE_SIZE;

    for (i = 0; i < npages; i++)
        image->text_mapping.pages[i] =
            virt_to_page(image->data + i*PAGE_SIZE);
    ...
    ...
    ...
}

The init_vdso function passed to the subsys_initcall macro adds the given function to the initcalls list. All functions from this list will be called in the do_initcalls function from the init/main.c source code file:

subsys_initcall(init_vdso);

Ok, we just saw initialization of the vDSO and initialization of page structures that are related to the memory pages that contain vDSO system calls. But to where do their pages map? Actually they are mapped by the kernel, when it loads binary to the memory. The Linux kernel calls the arch_setup_additional_pages function from the arch/x86/entry/vdso/vma.c source code file that checks that vDSO enabled for the x86_64 and calls the map_vdso function:

int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
{
    if (!vdso64_enabled)
        return 0;

    return map_vdso(&vdso_image_64, true);
}

The map_vdso function is defined in the same source code file and maps pages for the vDSO and for the shared vDSO variables. That's all. The main differences between the vsyscall and the vDSO concepts is that vsyscal has a static address of ffffffffff600000 and implements 3 system calls, whereas the vDSO loads dynamically and implements four system calls:

  • __vdso_clock_gettime;
  • __vdso_getcpu;
  • __vdso_gettimeofday;
  • __vdso_time.

That's all.

Conclusion

This is the end of the third part about the system calls concept in the Linux kernel. In the previous part we discussed the implementation of the preparation from the Linux kernel side, before a system call will be handled and implementation of the exit process from a system call handler. In this part we continued to dive into the stuff which is related to the system call concept and learned two new concepts that are very similar to the system call - the vsyscall and the vDSO.

After all of these three parts, we know almost all things that are related to system calls, we know what system call is and why user applications need them. We also know what occurs when a user application calls a system call and how the kernel handles system calls.

The next part will be the last part in this chapter and we will see what occurs when a user runs the program.

If you have questions or suggestions, feel free to ping me in twitter 0xAX, drop me email or just create issue.

Please note that English is not my first language and I am really sorry for any inconvenience. If you found any mistakes please send me PR to linux-insides.

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