问题描述
我正在尝试在我的 .text 部分中用汇编语言定义一个字节.我知道数据应该转到 .data 部分，但我想知道为什么在我这样做时它会给我一个分段错误.如果我在 .data 中定义字节，它不会给我任何错误，与 .text 不同.我正在使用运行 Mint 19.1 的 Linux 机器并使用 NASM + LD 来编译和链接可执行文件.
I'm trying to define a byte in Assembly language inside my .text section. I know data should go to the .data section but I was wondering why it gives me a segmentation fault when I do it. If I define the byte inside .data, it doesn't give me any errors, unlike .text. I am using a Linux machine running Mint 19.1 and using NASM + LD to compile and link the executable.
这在没有分段错误的情况下运行:
This runs without segmentation faults:
global _start
section .data
db 0x41
section .text
_start:
mov rax, 60 ; Exit(0) syscall
xor rdi, rdi
syscall
这给了我一个段错误:
global _start
section .text
_start:
db 0x41
mov rax, 60 ; Exit(0) syscall
xor rdi, rdi
syscall
我正在使用以下脚本来编译和链接它:
I'm using the following script to compile and link it:
nasm -felf64 main.s -o main.o
ld main.o -o main
我希望程序可以正常工作而不会出现任何分段错误，但是当我在 .text 中使用 DB 时却不会.我怀疑 .text 是只读的，这可能是这个问题的原因，我说得对吗?有人可以向我解释为什么我的第二个代码示例出现段错误吗?
I expect the program to work without any segmentation faults, but it doesn't when I use DB inside .text.I suspect that .text is readonly and that may be the reason of this problem, am I correct? Can someone explain to me why my second code example segfaults?
推荐答案
如果你告诉汇编器在某处汇编任意字节，它会的.db 是一个发出字节的伪指令，所以 mov eax, 60 和 db 0xb8, 0x3c, 0, 0, 0 是完全等价的就 NASM 而言.任何一个都会将这 5 个字节发送到当前位置的输出中.
If you tell the assembler to assemble arbitrary bytes somewhere, it will. db is a pseudo-instruction that emits bytes, so mov eax, 60 and db 0xb8, 0x3c, 0, 0, 0 are exactly equivalent as far as NASM is concerned. Either one will emit those 5 bytes into the output at the current position.
如果您不希望将数据解码为(部分)指令，请不要将其放在执行会到达的地方.
由于您使用的是 NASM，它会将 mov rax,60 优化为 mov eax,60，因此指令不会具有您期望从源中获得的 REX 前缀.
Since you're using NASM, it optimizes mov rax,60 into mov eax,60, so the instruction doesn't have the REX prefix you'd expect from the source.
您手动编码的 mov 的 REX 前缀将其更改为 mov 到 R8D 而不是 EAX:41 b8 3c 00 00 00 mov r8d,0x3c
Your manually-encoded REX prefix for mov changes it into a mov to R8D instead of EAX:41 b8 3c 00 00 00 mov r8d,0x3c
(我检查了 objdump -drwC -Mintel 而不是 寻找上哪位是 REX 前缀中的哪一个.我只记得 REX.W 是 0x48.但是 0x41 是 x86-64 中的 REX.B 前缀).
(I checked with objdump -drwC -Mintel instead of looking up which bit is which in the REX prefix. I only remember that REX.W is 0x48. But 0x41 is a REX.B prefix in x86-64).
因此，不是进行 sys_exit 系统调用，您的代码以 EAX=0 运行 syscall，即 __NR_read.(Linux 内核在进程启动之前将 RSP 以外的所有寄存器清零，并且在静态链接的可执行文件中，_start 是真正的入口点，没有首先运行动态链接器代码.因此 RAX 仍然为零).
So instead of making a sys_exit system call, your code runs syscall with EAX=0, which is __NR_read. (The Linux kernel zeros all the registers other than RSP before process startup, and in a statically-linked executable, _start is the true entry point with no dynamic linker code running first. So RAX is still zero).
$ strace ./rex
execve("./rex", ["./rex"], 0x7fffbbadad60 /* 54 vars */) = 0
read(0, NULL, 0) = 0
--- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_MAPERR, si_addr=NULL} ---
+++ killed by SIGSEGV (core dumped) +++
然后执行落入after syscall，在这种情况下是00 00 字节，解码为添加 [rax]、al，从而导致段错误.如果您在 GDB 中运行代码，您就会看到这一点.
And then execution falls through into whatever is after syscall, which in this case is 00 00 bytes that decode as add [rax], al, and thus segfault. You would have seen this if you'd run your code inside GDB.
脚注 1:如果您使用了未优化为 32 位操作数大小的 YASM:
Intel 的手册说在一条指令上有 2 个 REX 前缀是非法的.我预计会出现非法指令错误(#UD 机器异常 -> 内核提供 SIGILL)，但我的 Skylake CPU 忽略了第一个 REX 前缀并将其解码为 mov rax, sign_extended_imm32.
Intel's manuals say that it's illegal to have 2 REX prefixes on one instruction. I expected an illegal-instruction fault (#UD machine exception -> kernel delivers SIGILL), but my Skylake CPU ignores the first REX prefix and decodes it as mov rax, sign_extended_imm32.
单步执行，它被视为一个长指令，所以我猜Skylake选择像其他多个前缀的情况一样处理它，其中只有最后一个类型有效果.(但请记住，这不是面向未来的，其他 x86 CPU 可以以不同的方式处理它.)
Single-stepping, it's treated as one long instructions, so I guess Skylake chooses to handle it like other cases of multiple prefixes, where only the last one of a type has an effect. (But remember this is not future-proof, other x86 CPUs could handle it differently.)
其他情况下的相关/相同错误:
Related / same bug in other situations:
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