Attention Hackers: Filling Drop

Attention Hackers: Filling Drop

The zeroes you are reading today may not be there tomorrow.

As part of a future-proofing effort to pave the way for non-zeroing drop / RFC 320, pnkfelix plans to put together a Pull Request that replacing today’s zeroing drop with a so-called “filling drop.” He hopes to get the Pull Request together in time for the 1.0 beta.

This future-proofing effort is a relatively short-term step towards implementing a feature whose RFC has already been accepted. Therefore, pnkfelix (with other members of the team) decided it did not warrant an RFC (meeting notes). But it is a big enough change to warrant discussion in a forum less transient than IRC; thus, here we are.

The problem

(If you do not know what “zeroing drop” is, you might consider reading the how dynamic drop semantics works appendix of RFC 320.)

RFC 320 describes a long-term strategy for replacing zeroing-based dynamic-drop with a non-zeroing variant.

However, community members have raised concerns that if we ship 1.0 with the dynamic drop semantics as implemented today, developers will end up implicitly relying on the zeroing drop semantics.

For example, a developer who wants to cause a certain set of objects to be forgotten (i.e. not have their destructors run) may think that it suffices to simply overwrite the entirety of the memory representing those objects with zero bits.

pnkfelix does not know when the full implementation of non-zeroing dynamic drop will land; namely, it is possible it will happen for 1.0. (He hopes to put effort into such during the six weeks between 1.0 beta and 1.0.)

But non-zeroing drop certainly is not going to happen for 1.0 beta, and pnkfelix would prefer to try to put up a better defense against the scenario outlined in the previous paragraph.

Short-term proposal: Filling Drop

Currently Rust initializes the drop-flag of an object with a nonzero byte (it turns out to be a 0x01) whenever the object itself is initialized. When the object and its contents are moved or destroyed, we zero its memory entirely (including its drop flag). Likewise, when a Box<T> is dropped and deallocated, we can just zero out the memory where the pointer was stored (no separate drop flag needed).

The core idea of filling-drop is simple:

  • Continue using some nonzero byte (today it is 0x01) for the drop-flag when initializing an object. (Let us name this marker byte, whatever it is, NEED_DROP.)

  • Instead of overwriting the object’s memory with a zero byte, use some other marker byte, such as 0x1d. (Let us name this marker byte DONE_DROP)

  • When checking the drop-flag to decide whether a value needs to have its drop glue run, change the existing logic so that it skips when it sees a DONE_DROP (rather than conditionalizing on zero specifically).

  • In addition to the previous bullet, when checking the drop-flag on debug builds, invoke an llvm::debugtrap if the drop flag is equal to neither NEED_DROP nor DONE_DROP.

    • We issue llvm::debugtrap rather than panicking because debugtrap is cheap in code size (at least on Intel). However, pnkfelix is open to alternative ideas on this point.

(Note that much of the code for the above could be implemented today as a refactoring of the existing code, with no observable change apart from the debug build checking, by just assigning the values NEED_DROP = 0x01 and DONE_DROP = 0x00.)

  • Code using the unstable attribute #[unsafe_no_drop_flag] and relying on zeroing for their destructor logic will need to do something else

  • std::mem will expose (unstable) identifiers that provide value for DONE_DROP at various bit-widths, just so such libraries can get something running again with relatively little fuss. But obviously it would be better (and in the long-term, outright necessary) for those libraries to stop relying on such logic.

  • The init intrinsic will be replaced with two distinct intrinsics: init_zeroed and init_dropped. (We still provide init_zero rather than just change the semantics of init because there are many pieces of code that rely on zeroing on initialization, e.g. for FFI interactions, and have nothing to do with Rust’s destructor semantics.)


The main goal of filling-drop is to catch libraries that are today relying on memory zeroing.

Our primary interest is to identify libraries that is today writing zeroes themselves to cause destructors to be skipped, and tell them to stop doing this. Likewise, libraries that use unsafe_no_drop_flag will be flagged (see above note about std::mem accommodations).

A secondary concern is about libraries that assume that other clients that read from dropped memory will always only see zeroes (a ficticious example: a library that passed pointers to dropped memory into a C foreign function that wants null-terminated strings).

  • The current filling-drop plan does not future-proof things as much as one might desire for this case, since such a client might only be relying on the fact that the memory gets overwritten by some garbage data, and the fact that it is zero data is irrelevant. But, one cannot please everybody; that library is simply going to break when non-zeroing drop is actually implemented.


Is this strategy dangerous? Of course it is, in many ways!

Here is one example problem: the reason that zero was a particularly good choice for the role of was because a pointer-sized value filled with zeroes is (for our target architectures) the same as the null pointer; i.e. values after zeroing never corresponds to valid memory addresses. This property might not hold for a filling-drop implementation.

  • Since we control our allocator, we should be able to ensure for the short-term that at least no Box<_> will never be mistaken for DONE_DROP, so this may not be that big of a problem.
  • In any case, pnkfelix suspects the easiest way around this will be via selecting an appropriate value for DONE_DROP; 0x1d1d1d1d might alias some valid address on a 32-bit system, but 0xfdfdfdfd probably does not.

Does this accomplish anything?

One might ask: Won’t these libraries that were relying on zeroing-drop just start relying on filling-drop? In particular, couldn’t they just start filling in the memory with the 0x1d byte or whatever DONE_DROP is?

Its a reasonable question, and of couse, yes, we cannot prevent libraries from actively shooting themselves in the foot. However, it seems less likely that someone would start relying on the filling-drop semantics by accident.


Won’t this slow things down?


Probably not by much, but many code sequences can do tricks with zero that are not available when one is using non-trivial values for DONE_DROP.

pnkfelix plans to measure the cost and make sure that code size is not being blown up by too much.

But the more important thing to note is this:

This is a short term plan.

It is just a future-proofing placeholder for a proper non-zeroing dynamic drop to be added later.


pnkfelix has a prototype branch of rustc with this change, at drop-flag-hacks.

That url again, for those of you looking at a PDF or screen capture:

pnkfelix believes the prototype might pass make check, but he has not had a chance to confirm that yet.

  • (He only just this evening found a cause of a bug that was causing rustc itself to segfault on one compile-fail test case, and now sees his prototype has gotten through all of run-pass and compile-fail on stage2, and is working on the rest of rpass-full; but he will not have the chance to update this thread for a few days, so the outcome will remain a mystery for you, dear reader)

If you want to help, it would be great if you could try just to clone the drop-flag-hacks repo and try it out on your host platform and your favorite crates.

Or if you want to offer advice on other ways to future-proof the compiler and runtime for nonzeroing-drop, feel free to chime in on this topic thread.

Thanks for reading!


Sounds like a solid plan, thanks for the detailed writeup!

I recommend using 0xBE for DONE_DROP, as it is unaligned for all reasonable alignments. ASAN also uses it for this reason. (I also wonder if it might be reasonable in the future for rustc to emit valgrind client requests. For example, MAKE_MEM_UNDEFINED after a drop call returns…)

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Mystery solved: It passed make check, hooray!

(Now its time to refactor, revise, and do those measurements I promised. And by “now” I mean in five or six days.)

Thanks a lot for this detailed writeup!

Does this mean marking a struct as #[unsafe_no_drop_flag] will skip the filling? I’m picturing cases where Vec's ptr member is set to zero at the end of the destructor, and checked for zero at the beginning.

Agreed, #[unsafe_no_drop_flag] should imply that the destructor uses a custom encoding inside the structure, but does not rely on compiler-provided zero-filling.

That is not part of this first step; just as before we would zero out such structs, after filling-drop they would be filled with the marker byte.

(I believe some such zeroing/filling is necessary in general because such structs can have other "normal" structs with drop flags embedded within them.)

In the current prototype (and as I think is alluded to in my post) all such structs instead look for the new filling-drop bytes, via an unstable binding provided by std::mem.

However, I do think that a slight modification of this idea, where no-drop-flag structs are not entirely zeroed/filled, but rather solely their substructures that do have drop flags, would be an excellent second incremental step towards non-zeroing/non-filling drop in general.

In fact, I will at least attempt to prototype the latter and see if I can incorporate it into the initial filling-drop Pull Request. That would make life a little less tumultuous in the long term for libraries that define non-drop structs, since with that change, I think their code will then match what it should be with non-zeroing/non-filling dynamic drop.

(However, I think it is more important that filling-drop land in some form for the beta, than to delay landing it at all to incorporate enhancements like this.)

Ah, I overlooked a problem that was exposed during my attempt to prototype this handling of #[unsafe_no_drop_flag].

While I do believe one can skip the zeroing/filling after the destructor runs for a struct with #[unsafe_no_drop_flag] (assuming its destructor has been written accordingly to ensure resources are cleaned up only once), there is another case where zeroing/filling is done currently in the generated code: When you move a value out of one location into another, we fill the original location with zeroes today (and with DONE_DROP tomorrow) so that we do not run the destructor at all when we hit the end of the scope for the original location.

I had not really thought carefully about this before; certainly if we are going to support #[unsafe_no_drop_flag] in any proper form, we need to actually provide the means for structs to customize how those moves are handled.

E.g. I am currently thinking maybe a second method on Drop, perhaps named forget, with an inlined empty default implementation, that structs with #[unsafe_no_drop_flag] are expected to implement. And then a move such a let b = a; would also call a.forget().

So for example one might have:

pub struct Vec<T> { ptr: Unique<T>, len: usize, cap: usize }

impl<T> Drop for Vec<T> {
    fn forget(&mut self) {
        // a zero capacity indicates no work to do
        self.cap = 0;
    fn drop(&mut self) {
        // a zero capacity indicates no work to do
        if self.cap != 0 {
            unsafe {
                for x in &*self {
                dealloc(*self.ptr, self.cap)
            self.cap = 0;

(Or maybe we should just remove support for #[unsafe_no_drop_flag] entirely; it becomes harder to justify supporting it with stack-local drop flags.)

I don’t think it’s reasonable to have #[unsafe_no_drop_flag] remove the stack drop flags - it only exists as a layout optimization, after all. It seems to me that it’s almost more work to fiddle around with this filling drop than implementing the full stack flags, unless I’m missing something.

Filling drop itself (the fully-filling kind) was not that much work.

The business with partially-filling drop for #[unsafe_no_drop_flag] does look like a rabbit hole where time would probably be better spent trying to go straight to stack-local drop flags.

(This statement only makes sense if you also assume that we will just get rid of #[unsafe_no_drop_flag] when we shift to stack-local drop flags. But that was the context in which the statement was written, since @eddyb had said he does not think #[unsafe_no_drop_flag] should remove the stack-local drop flags, which I see as synonymous with saying that we should plan to get rid of #[unsafe_no_drop_flag].)

I agree with @eddyb here.

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