Print bytes into file using printf shell command

Question

I am trying to print the following data into file, but i get the following error: "/bin/sh: 1: Syntax error: Unterminated quoted string". Here is my program:

int main(void)
{
char* argv[] =  {"/bin/sh", "-c","printf '%b' '\x7f\x45\x4c\x46\x01\x00\x02\x03' > file.bin", NULL };
execve("/bin/sh", argv, NULL);

  return 1;
}

The problem is in the null byte in the middle of above text: \x7f\x45\x4c\x46\x01\x00\x02\x03" Is there any way to do this like above ?

FYI you are returning an error by default (return nonzero usually indicates a problem). — Jacob Faib
– Jacob Faib, Commented Feb 16, 2021 at 16:38

user202729 · Accepted Answer · 2021-02-16 16:21:44Z

4

The problem with your code is that the backslashes are interpreted in the C string, while you want to pass it as argument for the sh command.

Therefore you must double escape each backslash character.

int main(void)
{
    char* argv[] =  {"/bin/sh", "-c","printf '%b' '\\x7f\\x45\\x4c\\x46\\x01\\x00\\x02\\x03' > file.bin", NULL };
    execve("/bin/sh", argv, NULL);
    return 1;
}

There's a compiler extension that allows R"(printf '%b' '\x7f\x45\x4c\x46\x01\x00\x02\x03' > file.bin)" -- see Does C support raw string literals? , but for portability don't use it.

Also, it's not possible to pass raw null byte into execve, see those questions

answered Feb 16, 2021 at 16:21

user202729

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