avoid violating slice::from_raw_parts safety contract in Vec::extract_if

[petrosagg]

The implementation of the Vec::extract_if iterator violates the safety contract adverized by slice::from_raw_parts by always constructing a mutable slice for the entire length of the vector even though that span of memory can contain holes from items already drained. The safety contract of slice::from_raw_parts requires that all elements must be properly
initialized.

As an example we can look at the following code:

let mut v = vec![Box::new(0u64), Box::new(1u64)];
for item in v.extract_if(.., |x| **x == 0) {
    drop(item);
}

In the second iteration a &mut [Box<u64>] slice of length 2 will be constructed. The first slot of the slice contains the bitpattern of an already deallocated box, which is invalid.

This fixes the issue by only creating references to valid items and using pointer manipulation for the rest. I have also taken the liberty to remove the big unsafe blocks in place of targetted ones with a SAFETY comment. The approach closely mirrors the implementation of Vec::retain_mut.

Note to reviewers: The diff is easier to follow with whitespace hidden.

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[petrosagg]

I was unable to trigger a miri failure with this. I seems to have something to do with how deeply value validity is checked because miri correctly flags this code:

let ptr = std::ptr::null_mut();
let b = unsafe { std::mem::transmute::<*mut u64, Box<u64>>(ptr) };

but doesn't detect UB in this code:

let mut ptr = std::ptr::null_mut();
let b = unsafe { std::mem::transmute::<&mut *mut u64, &mut Box<u64>>(&mut ptr) };

[petrosagg]

@RalfJung if you have any pointers for how to write a miri test for this PR I'm happy to have another go at it.

[pitust]

@petrosagg miri reports the error if you run with MIRIFLAGS=-Zmiri-recursive-validation:

error: Undefined Behavior: constructing invalid value at .<deref>[0]: encountered a dangling box (use-after-free)
   --> /home/pitust/.rustup/toolchains/nightly-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/slice/raw.rs:192:9
    |
192 |         &mut *ptr::slice_from_raw_parts_mut(data, len)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value at .<deref>[0]: encountered a dangling box (use-after-free)
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
    = note: BACKTRACE:
    = note: inside `std::slice::from_raw_parts_mut::<'_, std::boxed::Box<u64>>` at /home/pitust/.rustup/toolchains/nightly-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/slice/raw.rs:192:9: 192:55
    = note: inside `<std::vec::ExtractIf<'_, std::boxed::Box<u64>, {closure@src/main.rs:100:34: 100:37}> as std::iter::Iterator>::next` at /home/pitust/.rustup/toolchains/nightly-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/vec/extract_if.rs:71:25: 71:87
note: inside `main`
   --> src/main.rs:100:17
    |
100 |     for item in v.extract_if(.., |x| **x == 0) {
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[petrosagg]

oh, amazing! I will add a testcase

[RalfJung]

Note that that flag enables UB checks that are very strict, and goes beyond what we are sure will be UB.

[petrosagg]

I see. This particular test exercises very little code but I don't have a strong opinion either. If we want to avoid using this experimental feature until the validity UB rules become more established I can leave the PR as-is.

Any opinions as to whether a testcase for this would be useful to have?

[tgross35]

Cc @the8472 who I believe authored this.

Any opinions as to whether a testcase for this would be useful to have?

Something that exercises the UB with the current implementation would be great, even if it doesn't currently get flagged by default miri.

[rustbot]

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

The Miri subtree was changed

cc @rust-lang/miri

[petrosagg]

@tgross35 great! Just added a miri testcase for this. I did pass the experimental flag, meaning that the test fails without the fix.

[the8472]

I only did some later modifications, this was originally introduced as drain_filter in #43245

In the second iteration a &mut [Box] slice of length 2 will be constructed. The first slot of the slice contains the bitpattern of an already deallocated box, causing UB.

The bytes are still initialized, we don't de-init the bytes when moving the value out. It happens to be the bytes of a pointer that points to a deallocated object.

AIUI validity requirements are still under discussion, but leaning no in this case: rust-lang/unsafe-code-guidelines#412

That said, the slice construction is superfluous and if we can replace it with something that's easier to reason about that's fine.

[pitust]

Note that that flag enables UB checks that are very strict, and goes beyond what we are sure will be UB.

This is true. However, I would argue that the standard to which the standard library should be held to is "definitely not UB" and not "maybe UB or maybe not", especially if the fix is not particularly difficult and doesn't have a meaningful runtime impact.

[saethlin]

I do not think it makes sense to hold this one part of the library to a standard that the rest of it, let alone the compiler's output, does not come close to satisfying: rust-lang/unsafe-code-guidelines#412 (comment)

[petrosagg]

In light of the discussion from the unsafe code guidelines repo it's clear that this PR is in a gray area as to whether it fixes UB or not. The current safety contract for slice::from_raw_parts clearly says it's UB. At the same time many people in the project think it shouldn't be. Let's not decide this in this PR.

I'd argue this PR is valuable on its own even if this behavior is not deemed UB in the future for two reasons:

1. It avoids violating Rust's own documented rules, leading to less confusion to people reading the code. This is how I ended up here in the first place.
2. It makes targeted and justified use of unsafe instead of big unsafe blocks.

I have removed the miri testcase that uses the experimental flag and have rephrased the commit and description of the PR to reflect this new framing.