Address Sanitizer Internals

He’s saying that with a compiler flag you can get the same kind of safety as in Ada or Rust. Later downthread he clarifies to say that you only get this safety if you stop using pointers, which he considers “trivial” to do.

This is the same guy who commented a few months back saying the following:

“All decent C compilers have compilation options so that at run-time any undefined actions, including integer overflow and out-of-bounds accesses, will be trapped.”

There is, put simply, no flag that will trap on “any undefined actions”, period. That’s not what sanitizers are capable of. That’s the kind of overhyped comment that isn’t helpful when trying to understand what sanitizers are actually useful for.

    #include <stdio.h>

    int main(int argc, char **argv) {
        printf("%s\n", argv[-8]);
    }

Oh, but maybe that's because the compiler has no model of how `argv` works. Fine, try this?

    #include <stdio.h>
    
    int foo(volatile char *x) {
        return x[0] + x[-64];
    }
    
    int main(int argc, char **argv) {
        char x[1] = {42};
        printf("%d\n", foo(x));
    }

Happily loads way out of bounds, no problem whatsoever - no runtime error, no sanitizer complaints, nothing.

Sanitizers are great, but they are not perfect. They are not a replacement for real array bounds checking; other languages that do real bounds checking do so by carrying the bounds with the object which is categorically impossible under the standard C ABIs.