ARM64汇编打印hello

不用链接C库的裸程序，向标准输出打印hello并调用exit退出：

.section .rodata

hellostr:
.ascii "hello\n"


.section .text
.global _start

_start:
        mov x0, #1
        ldr x1, =hellostr
        mov x2, #6
        mov x8, #64  //syscall write
        svc #0


        mov x0, #0
        mov x8, #93  //syscall exit
        svc #0

all:
        as main.s -o main.o
        ld main.o -o a.out
        rm main.o

链接C库，调用printf打印hello并退出：

.section .rodata

hellostr:
.ascii "hello\n"


.section .text
.global _start

_start:
        ldr x0, =hellostr
        bl printf

        mov x0, #0
        bl exit

all:
        as main.s -o main.o
        ld -dynamic-linker /lib/ld-linux-aarch64.so.1 -lc  main.o -o a.out
        rm main.o

$ ldd a.out
        linux-vdso.so.1 (0x0000007fa9beb000)
        libc.so.6 => /lib/aarch64-linux-gnu/libc.so.6 (0x0000007fa9a33000)
        /lib/ld-linux-aarch64.so.1 (0x0000007fa9bbb000)

ldd 可以看到依赖了C库， vdso及ld解释器(loader)。

关于vdso，为了提高时间相关的系统调用效率，内核会把时间及时刷新到一块内存上，把这段内存再映射到用户空间，获取这这些数据相关的代码就是在vdso中实现的，映射到用户空间的内存数据就是vvar。找一个运行中的程序，内存映射如下，vvar即是存储了时间的只读数据段，vdso即是访问它的代码段：

55a483a000-55a485b000 rw-p 00000000 00:00 0                              [heap]
7fa8154000-7fa873d000 r--p 00000000 b3:02 660                            /usr/lib/locale/locale-archive
7fa873d000-7fa889a000 r-xp 00000000 b3:02 7015                           /usr/lib/aarch64-linux-gnu/libc-2.31.so
7fa889a000-7fa88aa000 ---p 0015d000 b3:02 7015                           /usr/lib/aarch64-linux-gnu/libc-2.31.so
7fa88aa000-7fa88ad000 r--p 0015d000 b3:02 7015                           /usr/lib/aarch64-linux-gnu/libc-2.31.so
7fa88ad000-7fa88b0000 rw-p 00160000 b3:02 7015                           /usr/lib/aarch64-linux-gnu/libc-2.31.so
7fa88b0000-7fa88b3000 rw-p 00000000 00:00 0
7fa88c5000-7fa88e7000 r-xp 00000000 b3:02 6839                           /usr/lib/aarch64-linux-gnu/ld-2.31.so
7fa88f1000-7fa88f3000 rw-p 00000000 00:00 0
7fa88f3000-7fa88f5000 r--p 00000000 00:00 0                              [vvar]
7fa88f5000-7fa88f6000 r-xp 00000000 00:00 0                              [vdso]
7fa88f6000-7fa88f7000 r--p 00021000 b3:02 6839                           /usr/lib/aarch64-linux-gnu/ld-2.31.so
7fa88f7000-7fa88f9000 rw-p 00022000 b3:02 6839                           /usr/lib/aarch64-linux-gnu/ld-2.31.so
7fe3763000-7fe3784000 rw-p 00000000 00:00 0                              [stack]

ld-linux-aarch64.so 是一个加载器，在命令行上运行程序时，bash终端执行fork, exec打开a.out，a.out需要解释器，内核就将ld-linux-aarch64.so映射进来，接下来ld-linux-aarch64.so 将a.out依赖的动态库加载到进程地址空间（代码段，数据段等映射进来），准备好相关工作后，调用a.out入口点_start 。

用strace 查看a.out执行相关的系统调用，没有ld-linux-aarch64.so，是因为execve执行时，内核把它加载了。

$ strace ./a.out
execve("./a.out", ["./a.out"], 0x7fd3127fb0 /* 24 vars */) = 0
brk(NULL)                               = 0x33b33000
faccessat(AT_FDCWD, "/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=70321, ...}) = 0
mmap(NULL, 70321, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fa50e4000
close(3)                                = 0
openat(AT_FDCWD, "/lib/aarch64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0\267\0\1\0\0\0`C\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1458480, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fa5122000
mmap(NULL, 1531032, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fa4f6e000
mprotect(0x7fa50cb000, 65536, PROT_NONE) = 0
mmap(0x7fa50db000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x15d000) = 0x7fa50db000
mmap(0x7fa50e1000, 11416, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fa50e1000
close(3)                                = 0
mprotect(0x7fa50db000, 12288, PROT_READ) = 0
mprotect(0x410000, 4096, PROT_READ)     = 0
mprotect(0x7fa5127000, 4096, PROT_READ) = 0
munmap(0x7fa50e4000, 70321)             = 0
write(1, "hello\n", 6hello
)                  = 6
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(0x88, 0), ...}) = 0
brk(NULL)                               = 0x33b33000
brk(0x33b54000)                         = 0x33b54000
write(1, "hello\n", 6hello
)                  = 6
exit_group(0)                           = ?
+++ exited with 0 +++

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