toStringz and toUTFz potentially unsafe

[Jonathan M Davis]

On Sunday 24 July 2011 20:41:47 Johann MacDonagh wrote:
> Both toStringz and toUTFz do something potentially unsafe. Both check
> whether the character after the end of the string is NULL. If so, then
> it simply returns a pointer to the original string. This is a good
> optimization in theory because this code:
> 
> string s = "abc";
> 
> will be a slice to a read-only section of the executable. The compiler
> will insert a NULL after the string in the read-only section. So this:
> 
> auto x = toStringz("abc");
> 
> is efficient. No relocations.
> 
> As @AndrejMitrovic commented in Phobos pull request 123
> https://github.com/D-Programming-Language/phobos/pull/123, this has
> potential issues:
> 
> import std.string;
> import std.stdio;
> 
> struct A
> {
>      immutable char[2] foo;
>      char[2] bar;
> }
> 
> void main()
> {
>      auto a = A("aa", "\0b");
>      auto charptr = toStringz(a.foo[]);
> 
>      a.bar = "bo";
>      printf(charptr);   // two chars, then garbage
> }
> 
> Another issue not mentioned is with slices. If I do...
> 
> string s = "abc";
> string y = s[];
> string z = y[];
> 
> z ~= '\0';
> 
> auto c = toStringz(y);
> 
> assert(c.ptr == y.ptr);
> 
> ... what happens if I change that last character of z before I pass c to
> the C routine? Bad news. I think this optimization is great, but doesn't
> it go against D's motto of doing "the right thing by default"?
> 
> The question is, how can we keep this optimization so that:
> 
> toStringz("abc");
> 
> remains efficient?
> 
> The capacity field is 0 if the string is in a read-only section *or* if
> the string is on the stack:
> 
> auto x = "abc";
> assert(x.capacity == 0);
> char[3] y = "abc";
> assert(x.capacity == 0);
> 
> So, this isn't safe either. This code:
> 
>      char[3] x = "abc";
>      char y = '\0';
> 
> will put y right after x, so changing y after calling toStringz will
> cause issues.
> 
> In reality, the only time it's safe to do the "peek after end" is if the
> string is in the read-only section. Otherwise, there are potential
> issues (even if they are edge cases).
> 
> Do we care about this? Is there something we can add to druntime arrays
> that will tell whether or not the data backing a slice is in read-only
> memory (or perhaps an enum: read-only, stack, heap, other)? In reality,
> the only time this changes is when a read-only / stack heap is appended
> to, so performance issues are minimal.
> 
> Comments? Ideas?

The _only_ time that this matters is if you actually keep the result of 
toStringz or toUTFz around. If you do what is almost always done - that is 
pass the the result of toStringz/toUTFz directly to a function and not save it 
at all - then there is _zero_ problem.

If you want to save the char* instead, then you can simply choose to append 
'\0' instead of calling toStringz/toUTFz. toStringz has pretty much 0 value if 
it's forced to copy the string in all cases, and unless checking past the end 
of the string _and_ guaranteeing that the character one past the end won't 
change can be done and done efficiently, then the only way to guarantee that 
then value one past the end won't change is to make a copy, in which case 
toStringz is essentially pointless.

The documentation for toUTFz has an appropriate warning on it, so the issue 
should be clear. If you want to avoid it, simply append '\0' instead of 
calling toStringz or toUTFz. In 99.99% of cases, the char* is passed directly 
to a C function anyway, in which case there is not problem.

So, I really don't think that this is really an issue. If we can find an 
efficient way to make better guarantees, then that would be great, but the risk 
at this point is _very_ minimal, and there's an easy way to get around it in 
the _rare_ case where it could be a problem (simply append '\0' instead of 
calling toStringz/toUTFz).

- Jonathan M Davis