Invalid character '\' in mnemonic

Question

I'm asm/shellcode newbie and I have trouble with compile example shellcode included as template in ParrotOS.

I looked through following topics, but It couldn't solve my problem:

Assembly error: invalid character '$' in mnemonic

I tried use as command instead of gcc, however I got this same error.

char code[] = 

    "\xe9\x1e\x00\x00\x00"  //          jmp    8048083 <MESSAGE>
    "\xb8\x04\x00\x00\x00"  //          mov    $0x4,%eax
    "\xbb\x01\x00\x00\x00"  //          mov    $0x1,%ebx
    "\x59"                  //          pop    %ecx
    "\xba\x0f\x00\x00\x00"  //          mov    $0xf,%edx
    "\xcd\x80"              //          int    $0x80
    "\xb8\x01\x00\x00\x00"  //          mov    $0x1,%eax
    "\xbb\x00\x00\x00\x00"  //          mov    $0x0,%ebx
    "\xcd\x80"              //          int    $0x80
    "\xe8\xdd\xff\xff\xff"  //          call   8048065 <GOBACK>
    "give me a bottle of rum!\r\n";     // OR       "\x48\x65\x6c\x6c\x6f\x2c\x20\x57"
                            //          "\x6f\x72\x6c\x64\x21\x0d\x0a"


int main(int argc, char **argv)
{
    (*(void(*)())code)();

    return 0;
}

gcc shellcode.s

shellcode.s: Assembler messages:
shellcode.s:1: Error: no such instruction: `char code[]='
shellcode.s:3: Error: invalid character '\' in mnemonic
shellcode.s:4: Error: invalid character '\' in mnemonic
shellcode.s:5: Error: invalid character '\' in mnemonic
shellcode.s:6: Error: invalid character '\' in mnemonic
shellcode.s:7: Error: invalid character '\' in mnemonic
shellcode.s:8: Error: invalid character '\' in mnemonic
shellcode.s:9: Error: invalid character '\' in mnemonic
shellcode.s:10: Error: invalid character '\' in mnemonic
shellcode.s:11: Error: invalid character '\' in mnemonic
shellcode.s:12: Error: invalid character '\' in mnemonic
shellcode.s:13: Error: no such instruction: `give me a bottle of rum!\r\n"'
shellcode.s:17: Error: junk `(int argc,char **argv)' after expression
shellcode.s:17: Error: operand size mismatch for `int'
shellcode.s:18: Error: no such instruction: `{'
shellcode.s:19: Error: junk at end of line, first unrecognized character is `('
shellcode.s:21: Error: no such instruction: `return 0'
shellcode.s:22: Error: junk at end of line, first unrecognized character is `}'

score 7 · Accepted Answer · answered Feb 25 '20 at 12:14

7

This is C code, but it has a .s extension which indicates assembly. Rename your file to shellcode.c.

answered Feb 25 '20 at 12:14

John Kugelman

349,597
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Thank you, that was a little tricky to debug. I didn't expect that gcc recognize extensions, because I never felt that during programming in C/C++. I'm having segmentation fault currently, but I'll try solve that myself. – Innewit Feb 25 '20 at 12:33

Jean-François Fabre · Answer 2 · 2020-02-25T12:45:30.220

6

By default, gcc decides which language to compile the file in according to its extension.

Renaming the file properly according to its contents is the right thing to do (as already suggested)

If for some reason you can't rename the file, you can also force gcc to compile a file with any extension in a given (supported) language using -x option:

gcc -x c shellcode.s

This switch sets the language but doesn't set the default link options. Trying gcc on a C++ file to create an executable fails because gcc link driver doesn't add the C++ libraries by default.

edited Feb 25 '20 at 12:45

answered Feb 25 '20 at 12:23

Jean-François Fabre

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"*By default, gcc decides which language to compile the file in according to its extension.*" - Does that mean someone could compile a .cpp file with `gcc` instead of `g++`, like `gcc -o foo bar.cpp` without a problem? – RobertS supports Monica Cellio Feb 25 '20 at 12:38
1

actually it would compile but the link will fail unless you explicitly add the C++ libs (or use g++ to link) – Jean-François Fabre Feb 25 '20 at 12:44
Right, but I could use `gcc` to compile and `g++` to link? – RobertS supports Monica Cellio Feb 25 '20 at 12:46
1

`gcc -c foo.cpp` then `g++ -o foo foo.o` – Jean-François Fabre Feb 25 '20 at 12:50

Invalid character '\' in mnemonic

2 Answers2