ud2是什么指令？

netentsec 发表于 2007-06-20 20:58

#define BUG() \
__asm__ __volatile__( "ud2\n" \
"\t.word %c0\n" \
"\t.long %c1\n" \
: : "i" (__LINE__), "i" (__FILE__))

ud2是什么指令？机器码好像是0f 0b ？
BUG()能造成系统停机么？

%0、%1是占位符号，那么前面的修饰符'c'是什么含意？（%c0、%c1）
除了'c'还有哪些修饰符？在哪里可以查到这些修饰符的信息？

albcamus 发表于 2007-06-21 10:06

undefined，故意让CPU出错的 -- 记得是这样，FIXME

xiaozhaoz 发表于 2007-06-22 09:46

albcamus 说得没错,

查了一下手册, UD2是一种让CPU产生invalid opcode exception的软件指令.内核发现CPU出现这个异常, 会立即停止运行.

'c' 在gcc中, 叫做operand code, c只能用在常量变量(就是constraint是'i')和条件判断指令中. 作用是将这个常量值打印在指令中.
这段代码:

#define BUG() \
do { \
asm volatile("1:\tud2\n" \
      ".pushsection __bug_table,\"a\"\n" \
      "2:\t.long 1b, %c0\n" \
      "\t.word %c1, 0\n" \
      "\t.org 2b+%c2\n" \
      ".popsection" \
      : : "i" (__FILE__), "i" (__LINE__), \
      "i" (sizeof(struct bug_entry))); \
for(;;) ; \
} while(0)

编译后的代码为:

.LC0:
   .string "asm.c"
   .text
.....

#APP
   1:    ud2
.pushsection __bug_table,"a"
2:    .long 1b, .LC0
   .word 18, 0
   .org 2b+4
.popsection
#NO_APP

其实除了ud2这条指令, 其他都是assembler directives.

如果没有'c'这个operand code, 产生的指令是:

.LC0:
   .string "asm.c"
   .text

#APP
   1:    ud2
.pushsection __bug_table,"a"
2:    .long 1b, $.LC0
   .word $18, 0
   .org 2b+$4
.popsection
#NO_APP

这段代码对于assembler directives(GAS的输入)来说, 可能是不能正常工作的.

i386的operand code可以在gcc源代码的gcc/config/i386.c中查询到, 我也是很久以前移植过一种CPU ARCH才记得有这么个东西.
L,W,B,Q,S,T -- print the opcode suffix for specified size of operand.
C -- print opcode suffix for set/cmov insn.
c -- like C, but print reversed condition
F,f -- likewise, but for floating-point.
O -- if HAVE_AS_IX86_CMOV_SUN_SYNTAX, expand to "w.", "l." or "q.",
   otherwise nothing
R -- print the prefix for register names.
z -- print the opcode suffix for the size of the current operand.
* -- print a star (in certain assembler syntax)
A -- print an absolute memory reference.
w -- print the operand as if it's a "word" (HImode) even if it isn't.
s -- print a shift double count, followed by the assemblers argument
   delimiter.
b -- print the QImode name of the register for the indicated operand.
   %b0 would print %al if operands is reg 0.
w --likewise, print the HImode name of the register.
k --likewise, print the SImode name of the register.
q --likewise, print the DImode name of the register.
h -- print the QImode name for a "high" register, either ah, bh, ch or dh.
y -- print "st(0)" instead of "st" as a register.
D -- print condition for SSE cmp instruction.
P -- if PIC, print an @PLT suffix.
X -- don't print any sort of PIC '@' suffix for a symbol.
& -- print some in-use local-dynamic symbol name.

ps. 较新的内核代码都已经将BUG()的实现做了一些修改, 改成了我上面提到的那种, 这可以让出错的指令直接在栈顶, 便于调试.

nswcfd 发表于 2015-12-04 10:13

终于找到这个老帖子了。

https://gcc.gnu.org/onlinedocs/gcc-5.2.0/gcc/Extended-Asm.html
6.43.2.7 x86 Operand Modifiers
cRequire a constant operand and print the constant expression with no punctuation

void f1()
{
asm("nop");
asm (".word %0"::"i"(0xff)); => 生成0x00 0x00 => link error: undefined reference to `$255'
asm("nop");
asm (".word %c0"::"i"(0xff));=> 生成0xff 0x00
}

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ud2是什么指令？