编写一个像 printf 这样的函数，它可以清理第一个参数之后的所有参数，使其摆脱 `%` 符号

Question

user_input = "%s%s%s%s%s%s%s%s";
printf("user input is: %s", user_input);

... crash!

The above lines cause an error. I want to write a function which can be used like printf but can sanitize all arguments after the first to make sure they do not contain the % symbol. The function should be used like 'printf' in that it can take any number of arguments and it print outs a string in the same manner. If the other arguments contain the % symbol, I just want that symbol taken out before it is put in the format string.

If this new function were called safe_printf , I would want the behavior to be like this:

user_input = "%s%s%s%s%s%s%s%s";
safe_printf("user input is: %s, user_input);

user input is: ssssssss

It seems like writing a function like this may not be possible, (I can't figure out how to preprocess the char *s in the va_list without knowing how many there are) if that's the case please let me know. Thanks!

Answer 1

The string formatting vulnerability exists if user provided input is passed to the printf(3) function. You can prevent this by not allowing that happen, ever . There is absolutely not reason to have a dynamic format string because you should know ahead of time the required format, and therefore you can put the format string in read only memory through the use of a const char *