根据vDSO--内核原理我们可以对一个syscall来进行vDSO的优化，再根据vDSO--示例之实现系统调用我们实现了一个自己的系统调用__do_sys_kylin，这里我们将__do_sys_kylin加入到vDSO中

一、vDSO的符号

我们从lds可以发现需要导出符号，所以我们添加一个__kernel_kylin的符号给应用，代码如下：

# git diff vdso/vdso.lds.S
diff --git a/arch/arm64/kernel/vdso/vdso.lds.S b/arch/arm64/kernel/vdso/vdso.lds.S
index b840ab1b705c..c766afed0ec8 100644
--- a/arch/arm64/kernel/vdso/vdso.lds.S
+++ b/arch/arm64/kernel/vdso/vdso.lds.S
@@ -84,6 +84,7 @@ VERSION
        global:
                __kernel_rt_sigreturn;
                __kernel_gettimeofday;
+               __kernel_kylin;
                __kernel_clock_gettime;
                __kernel_clock_getres;
        local: *;

二、实现vDSO的调用

为了完成vDSO的实验，我没有去故意设置vvar的值来获取，而是在vDSO中嵌套了一个syscall，这样方便大家理解，如下：

# git diff vdso/vgettimeofday.c
diff --git a/arch/arm64/kernel/vdso/vgettimeofday.c b/arch/arm64/kernel/vdso/vgettimeofday.c
index 4236cf34d7d9..0005f42565d9 100644
--- a/arch/arm64/kernel/vdso/vgettimeofday.c
+++ b/arch/arm64/kernel/vdso/vgettimeofday.c
@@ -18,6 +18,22 @@ int __kernel_gettimeofday(struct __kernel_old_timeval *tv,
        return __cvdso_gettimeofday(tv, tz);
 }
 
+int __kernel_kylin(char* words)
+{
+       register char *word asm("x0") = words;
+       register long ret asm("x0");
+#define __NR_sys_kylin 449
+        register long nr asm("x8") = __NR_sys_kylin;
+
+       asm volatile(
+       "       svc #0\n"
+       : "=r" (ret)
+       : "r" (word), "r" (nr)
+       : "memory");
+
+       return ret;
+}
+

三、生成vdso.so

至此vDSO的函数实现已经完成了，我们编译vdso.so即可。编译后文件如下：

arch/arm64/kernel/vdso/vdso.so

此时我们看看符号是否增加，如下：

# readelf -s arch/arm64/kernel/vdso/vdso.so
Symbol table '.dynsym' contains 7 entries:
 Num: Value Size Type Bind Vis Ndx Name
 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
 1: 0000000000000000 0 OBJECT GLOBAL DEFAULT ABS LINUX_2.6.39
 2: 0000000000000780 108 FUNC GLOBAL DEFAULT 7 __kernel_clock_getres@@LINUX_2.6.39
 3: 00000000000007f0 8 NOTYPE GLOBAL DEFAULT 7 __kernel_rt_sigreturn@@LINUX_2.6.39
 4: 00000000000005c0 424 FUNC GLOBAL DEFAULT 7 __kernel_gettimeofday@@LINUX_2.6.39
 5: 0000000000000770 12 FUNC GLOBAL DEFAULT 7 __kernel_kylin@@LINUX_2.6.39
 6: 0000000000000320 664 FUNC GLOBAL DEFAULT 7 __kernel_clock_gettime@@LINUX_2.6.39

我们看一下符号的代码段内容

# objdump --disassemble=__kernel_kylin arch/arm64/kernel/vdso/vdso.so
arch/arm64/kernel/vdso/vdso.so： 文件格式 elf64-littleaarch64

Disassembly of section .text:
0000000000000770 <__kernel_kylin@@LINUX_2.6.39>:
 770: d2803828 mov x8, #0x1c1 // #449
 774: d4000001 svc #0x0
 778: d65f03c0 ret

以上完全正确

四、测试vDSO调用

我们将arch/arm64/kernel/vdso/vdso.so作为标准动态库看待，直接复制到机器中，编写测试代码如下：

#include <sys/syscall.h>
#include <stdio.h>
 
extern int __kernel_kylin(char* words);
 
int main(int argc, char *argv[])
{
    int ret = 0;
    char* words = "Userspace say:hello kylin!";
 
    ret = __kernel_kylin(words);
    printf("vdso ret=%d \n", ret);
 
    return 0;
}

此时我们构建的时候应该把vdso当作标准动态库，如下编译

gcc test_kylin_vdso.c -o test_kylin_vdso -L. -lvdso

此时我们运行：

# ./test_kylin_vdso
vdso ret=0

然后查看日志

# dmesg
[ 5586.379813] kylin: Get sys_kylin call:[Userspace say:hello kylin!]. ret=0

可以发现能够正常的syscall到我的系统调用，我们ltrace看看

# ltrace ./test_kylin_vdso
__libc_start_main(0x5569b3083c, 1, 0x7fdfd074f8, 0x5569b30880 <unfinished ...>
__kernel_kylin(0x5569b30920, 0x7fdfd074f8, 0x7fdfd07508, 0x5569b3083c) = 0
printf("vdso ret=%d \n", 0vdso ret=0
) = 12
exit(0 <unfinished ...>
__cxa_finalize(0x5569b41008, 0x5569b307f0, 0x10d68, 1) = 0x7fba4bec70
+++ exited (status 0) +++

正常调用__kernel_kylin，至此我们可以正常通过vdso来调用syscall的内容。