GCC/X86, Problems with relative jumps
- by Ian Kelly
I'm trying to do a relative jump in x86 assembly, however I can not get it to work. It seems that for some reason my jump keeps getting rewritten as an absolute jump or something.
A simple example program for what I'm trying to do is this:
.global main
main:
jmp 0x4
ret
Since the jmp instruction is 4 bytes long and a relative jump is offset from the address of the jump + 1, this should be a fancy no-op. However, compiling and running this code will cause a segmentation fault.
The real puzzler for me is that compiling it to the object level and then disassembling the object file shows that it looks like the assembler is correctly doing a relative jump, but after the file gets compiled the linker is changing it into another type of jump.
For example if the above code was in a file called asmtest.s:
$gcc -c asmtest.s
$objdump -D asmtest.o
... Some info from objdump
00000000 <main>:
0: e9 00 00 00 00 jmp 5 <main+0x5>
5: c3 ret
This looks like the assembler correctly made a relative jump, although it's suspicious that the jmp instruction is filled with 0s.
I then used gcc to link it then disassembled it and got this:
$gcc -o asmtest asmtest.o
$objdump -d asmtest
...Extra info and other disassembled functions
08048394 <main>:
8048394: e9 6b 7c fb f7 jmp 4 <_init-0x8048274>
8048399: c3 ret
This to me looks like the linker rewrote the jmp statement, or substituted the 5 in for another address.
So my question comes down to, what am I doing wrong?
Am I specifying the offset incorrectly? Am I misunderstanding how relative jumps work? Is gcc trying to make sure I don't do dangerous things in my code?
