从64位汇编中调用C函数 [英] Call C functions from 64-bit assembly
问题描述
在Ubuntu 16.04上
On ubuntu 16.04
$ cat hola.asm
extern puts
global main
section .text
main:
mov rdi,message
call puts
ret
message:
db "Hola",0
$ nasm -f elf64 hola.asm
$ gcc hola.o
/usr/bin/ld:hola.o:针对符号重定位R_X86_64_PC32
创建共享库时，不能使用"puts @@ GLIBC_2.2.5"；
用-fPIC
重新编译
/usr/bin/ld:最终链接失败:错误的值
collect2:错误:ld返回1退出状态
/usr/bin/ld: hola.o: relocation R_X86_64_PC32 against symbol
`puts@@GLIBC_2.2.5' can not be used when making a shared object;
recompile with -fPIC
/usr/bin/ld: final link failed: Bad value
collect2: error: ld returned 1 exit status
使用:
$gcc -fPIC hola.o -o hola && ./hola
Hola
文档:
-fPIC如果目标计算机支持，则发出与位置无关的代码，适合动态链接并避免大小限制 全局偏移量表的值.此选项对 AArch64，m68k，PowerPC和SPARC.
-fPIC If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on AArch64, m68k, PowerPC and SPARC.
与位置无关的代码需要特殊的支持，因此 仅在某些机器上工作.设置此标志后，宏 " pic "和" PIC "定义为2.与位置无关的代码 需要特殊的支持，因此仅适用于某些情况 机器.
Position-independent code requires special support, and therefore works only on certain machines. When this flag is set, the macros "pic" and "PIC" are defined to 2. Position-independent code requires special support, and therefore works only on certain machines.
gcc的-static选项有效:
The -static option with gcc works:
使用-static完全避免外部调用动态库
use -static to completely avoid external calls to dynamic libraries
$nasm -f elf64 -l hola.lst hola.asm && gcc -m64 -static -o hola hola.o && ./hola
Hola
还有:
$nasm -f elf64 hello.asm && gcc -static -o hola hola.o && ./hola Hola
包括wrt ..plt也有效
Including wrt ..plt also worked
global main
extern puts
section .text
main:
mov rdi,message
call puts wrt ..plt
ret
message:
db "Hola", 0
$nasm -f elf64 hola.asm
$gcc -m64 -o hola hola.o && ./hola
Hola
来自 ..plt描述
.. plt 使用wrt ..plt引用过程名称会导致链接程序为该符号建立过程链接表条目，而引用将给出PLT条目的地址.您只能在会正常生成PC相对重定位(即作为CALL或JMP的目的地)的上下文中使用此功能，因为ELF不包含任何重定位类型来绝对引用PLT条目.
..plt Referring to a procedure name using wrt ..plt causes the linker to build a procedure linkage table entry for the symbol, and the reference gives the address of the PLT entry. You can only use this in contexts which would generate a PC-relative relocation normally (i.e. as the destination for CALL or JMP), since ELF contains no relocation type to refer to PLT entries absolutely.
推荐答案
我编写了此程序，使其与hi.c程序相同，而没有调用c lib.然后建议在hi.c上使用-S gcc选项，然后剖析生成的hi.s程序.
I wrote up this program to do the same as the hi.c program, without the c lib call. Then followed a suggestion to use the -S gcc option on hi.c then to dissect the resulting hi.s program.
$ cat hiasm.asm
section .text
global _start
_start:
mov dl, 5
mov esi, msg
xor di,di
xor al,al
inc di
inc al
syscall
xor rdi,rdi
mov al,60
syscall
msg: db "Hello"
$ nasm -f elf64 hiasm.asm&& ld -m elf_x86_64 hiasm.o -o hiasm&& ./hiasm
你好
$ echo $?
0
这很好用
再次，这是简单的hi.c
again, here's the simple hi.c
$ cat hi.c
#include <stdio.h>
int main(void)
{
puts("Hello");
return 0;
}
$ gcc -s hi.c&&嗨！
.file "hi.c"
.section .rodata
.LC0:
.string "Hello"
.text
.globl main
.type main, @function
main:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
leaq .LC0(%rip), %rdi
call puts@PLT
movl $0, %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size main, .-main
.ident "GCC: (Debian 6.3.0-18) 6.3.0 20170516"
.section .note.GNU-stack,"",@progbits
$ gcc hi.s -o hi&& ./hi
你好
.s文件中似乎未引用标签.LFB0和.LFE0 删除两个文件后，它们仍然可以按预期工作， 引用"as"汇编程序文档:
The labels .LFB0 and .LFE0 do not appear to be referenced within the .s file After removing both the file still works as expected, referencing 'as' assembler docs:
https://sourceware.org/binutils/docs/as/index.html
本地符号是在汇编程序中定义和使用的，但是它们是 通常不保存在目标文件中.因此，它们在以下情况下不可见 调试.您可以使用-L选项(请参阅包括本地符号)来 在目标文件中保留本地符号.
Local symbols are defined and used within the assembler, but they are normally not saved in object files. Thus, they are not visible when debugging. You may use the `-L' option (see Include Local Symbols) to retain the local symbols in the object files.
因此，作为不需要钟声的纯可执行文件，可以将它们切碎
So as a pure executable with no need for bells and whistles, they can be chopped
所以我摆脱了那些简单的事情
So I got rid of the easy ones
接下来，该函数要调用main，这没什么用，所以我将调用_start
Next the function wants to call main, there's not much use for this, so I'll call _start
对于ELF目标，.size指令的用法如下:
For ELF targets, the .size directive is used like this:
.size name , expression
此伪指令设置与符号名称关联的大小.尺寸 以字节为单位是根据可以使用标签的表达式计算得出的 算术.该指令通常用于设置 功能符号.
This directive sets the size associated with a symbol name. The size in bytes is computed from expression which can make use of label arithmetic. This directive is typically used to set the size of function symbols.
不需要功能符号的大小，摆脱了引用main底部的.size
Don't need function symbol sizes, got rid of the .size at the bottom that references main
$ cat hi.s
file "hi.c" ##tells 'as' that we are about to start a new logical file
.section .rodata ##assembles the following code into section '.rodata'
.LC0: ##.LC0, .LFB0, .LFE0 are just local labels; symbols that
## are guaranteed to be unique over the source code
## that allow the compiler to use names/simple notation
## to reference sections of code
##But here, only .LC0 is actually referenced in the code
.string "Hello" ##
.text
.globl _start
_start:
.cfi_startproc ##used at the beginning of each function that should have an
##entry in .eh_frame. It initializes some internal data
##structures. Don't forget to close by .cfi_endproc
pushq %rbp ##push base pointer onto stack
.cfi_def_cfa_offset 16 ##modifies a rule for computing CFA. Register remains the
##same, but offset is new. Note that it is the absolute
##offset that will be added to a defined register to
##compute CFA address
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
leaq .LC0(%rip), %rdi
call puts@PLT
movl $0, %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc ##close of .cfi_startproc
.ident "GCC: (Debian 6.3.0-18) 6.3.0 20170516"
.section .note.GNU-stack,"",@progbits
尝试:
$ gcc -o hi hi.s
/tmp/ccLxG1jh.o: In function `_start':
hi.c:(.text+0x0): multiple definition of `_start'
/usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/Scrt1.o:(.text+0x0): first defined here
/usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
$嗨
linux-vdso.so.1 (0x00007fffb6569000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fe7456e7000)
/lib64/ld-linux-x86-64.so.2 (0x000055edc8bc8000)
肯定是在使用libc，它解释了_start的多个定义 因此，我将尝试使用-nostdlib gcc选项摆脱std lib
It's definitely using libc, which explains our multiple definitions of _start So I'll try getting rid of std lib with the -nostdlib gcc option
$ gcc -nostdlib -o hi hi.s
/tmp/ccV5QYaT.o: In function `_start':
hi.c:(.text+0xc): undefined reference to puts'
collect2: error: ld returned 1 exit status
对，仍然需要C进行看跌期权，摆脱掉看跌期权
Right, still need C for puts, getting rid of puts
.file "hi.c" ##tells 'as' that we are about to start a new logical file
.section .rodata ##assembles the following code into section '.rodata'
.LC0: ##.LC0, .LFB0, .LFE0 are just local labels; symbols that
## are guaranteed to be unique over the source code
## that allow the compiler to use names/simple notation
## to reference sections of code
##But here, only .LC0 is actually referenced in the code
.string "Hello" ##
.text
.globl _start
_start:
.cfi_startproc ##used at the beginning of each function that should have an
##entry in .eh_frame. It initializes some internal data
##structures. Don't forget to close by .cfi_endproc
pushq %rbp ##push base pointer onto stack
.cfi_def_cfa_offset 16 ##modifies a rule for computing CFA. Register remains the
##same, but offset is new. Note that it is the absolute
##offset that will be added to a defined register to
##compute CFA address
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
leaq .LC0(%rip), %rsi ##this reg value and others were changed for write call
movq $1, %rax
movq $1, %rdi
movq $5, %rdx
syscall
movl $0, %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc ##close of .cfi_startproc
$ gcc -nostdlib -o hi.s&& ./hi
HelloSegmentation错误
有前途的
.file "hi.c" ##tells 'as' that we are about to start a new logical file
.section .rodata ##assembles the following code into section '.rodata'
.LC0: ##.LC0, .LFB0, .LFE0 are just local labels; symbols that
## are guaranteed to be unique over the source code
## that allow the compiler to use names/simple notation
## to reference sections of code
##But here, only .LC0 is actually referenced in the code
.string "Hello"
.text
.globl _start
_start:
.cfi_startproc ##used at the beginning of each function that should have an
##entry in .eh_frame. It initializes some internal data
##structures. Don't forget to close by .cfi_endproc
##deleted the base pointer push and pops from stack, don't need stack
.cfi_def_cfa_offset 16 ##modifies a rule for computing CFA. Register remains the
##same, but offset is new. Note that it is the absolute
##offset that will be added to a defined register to
##compute CFA address
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
leaq .LC0(%rip), %rsi
movq $1, %rax
movq $1, %rdi
movq $5, %rdx
syscall
xor %rdi,%rdi
mov $60, %rax
.cfi_def_cfa 7, 8
syscall
.cfi_endproc ##close of .cfi_startproc
$ gcc -g -nostdlib -o hi hi.s&& ./hi 你好
知道了！ 试图弄清楚CFA是什么 http://dwarfstd.org/doc/DWARF4.pdf 6.4
Got it! Trying to figure out what a CFA is http://dwarfstd.org/doc/DWARF4.pdf Section 6.4
在堆栈上分配的内存区域，称为调用帧". 调用帧由堆栈上的地址标识.我们指的是 此地址为规范框架地址或CFA.通常， CFA被定义为调用时堆栈指针的值 前一帧中的网站(可能与其在 进入当前帧)
An area of memory that is allocated on a stack called a "call frame." The call frame is identified by an address on the stack. We refer to this address as the Canonical Frame Address or CFA. Typically, the CFA is defined to be the value of th e stack pointer at the call site in the previous frame (which may be different from its value on entry to the current frame)
因此，所有.cfi_def_cfa_offset，.cfi_offset和.cfi_def_cfa_register都在进行计算， 和操作堆栈.但是该程序根本不需要堆栈，因此最好将其删除
So then all .cfi_def_cfa_offset, .cfi_offset and .cfi_def_cfa_register are doing is computing, and manipulating the stack. But this program doesn't need the stack at all, so might as well delete that too
$ cat hi.s
.file "hi.c" ##tells 'as' that we are about to start a new logical file
.section .rodata ##assembles the following code into section '.rodata'
.LC0: ##.LC0, .LFB0, .LFE0 are just local labels; symbols that
## are guaranteed to be unique over the source code
## that allow the compiler to use names/simple notation
## to reference sections of code
##But here, only .LC0 is actually referenced in the code
.string "Hello"
.text
.globl _start
_start:
.cfi_startproc ##used at the beginning of each function that should have an
##entry in .eh_frame. It initializes some internal data
##structures. Don't forget to close by .cfi_endproc
leaq .LC0(%rip), %rsi
movq $1, %rax
movq $1, %rdi
movq $5, %rdx
syscall
xor %rdi,%rdi
mov $60, %rax
syscall
.cfi_endproc ##close of .cfi_startproc
.cfi_startproc:
.cfi_startproc :
在每个应该在其中添加条目的函数的开头使用 .eh_frame
Used at the beginning of each function that should have an entry in the .eh_frame
What is eh_frame "When using languages that support exceptions, such as C++, additional information must be provided to the runtime environment that describes the call frames that much be unwound during the processing of an exception. This information is contained in the special sections .eh_frame and .eh_framehdr."
不需要异常处理，不使用C ++
Don't need exception handling, not using C++
$ cat hi.s
$ cat hi.s
.section .rodata
.LC0:
.string "Hello"
.text
.globl _start
_start:
leaq .LC0(%rip), %rsi
movq $1, %rax
movq $1, %rdi
movq $5, %rdx
syscall
xor %rdi,%rdi
mov $60, %rax
syscall
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