From e6e78c0bc9f45cbc37cbce939b95229411f0425b Mon Sep 17 00:00:00 2001
From: Petros Angelatos <petrosagg@gmail.com>
Date: Thu, 15 May 2025 15:46:53 +0300
Subject: [PATCH] avoid violating `slice::from_raw_parts` safety contract in
 `Vec::extract_if`

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:

```rust
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`.

Signed-off-by: Petros Angelatos <petrosagg@gmail.com>
---
 library/alloc/src/vec/extract_if.rs | 56 ++++++++++++++++-------------
 1 file changed, 31 insertions(+), 25 deletions(-)

diff --git a/library/alloc/src/vec/extract_if.rs b/library/alloc/src/vec/extract_if.rs
index a456d3d9e602d..68c4ee5f81e10 100644
--- a/library/alloc/src/vec/extract_if.rs
+++ b/library/alloc/src/vec/extract_if.rs
@@ -64,27 +64,29 @@ where
     type Item = T;
 
     fn next(&mut self) -> Option<T> {
-        unsafe {
-            while self.idx < self.end {
-                let i = self.idx;
-                let v = slice::from_raw_parts_mut(self.vec.as_mut_ptr(), self.old_len);
-                let drained = (self.pred)(&mut v[i]);
-                // Update the index *after* the predicate is called. If the index
-                // is updated prior and the predicate panics, the element at this
-                // index would be leaked.
-                self.idx += 1;
-                if drained {
-                    self.del += 1;
-                    return Some(ptr::read(&v[i]));
-                } else if self.del > 0 {
-                    let del = self.del;
-                    let src: *const T = &v[i];
-                    let dst: *mut T = &mut v[i - del];
-                    ptr::copy_nonoverlapping(src, dst, 1);
+        while self.idx < self.end {
+            let i = self.idx;
+            // SAFETY: Unchecked element must be valid.
+            let cur = unsafe { &mut *self.vec.as_mut_ptr().add(i) };
+            let drained = (self.pred)(cur);
+            // Update the index *after* the predicate is called. If the index
+            // is updated prior and the predicate panics, the element at this
+            // index would be leaked.
+            self.idx += 1;
+            if drained {
+                self.del += 1;
+                // SAFETY: We never touch this element again after returning it.
+                return Some(unsafe { ptr::read(cur) });
+            } else if self.del > 0 {
+                // SAFETY: `self.del` > 0, so the hole slot must not overlap with current element.
+                // We use copy for move, and never touch this element again.
+                unsafe {
+                    let hole_slot = self.vec.as_mut_ptr().add(i - self.del);
+                    ptr::copy_nonoverlapping(cur, hole_slot, 1);
                 }
             }
-            None
         }
+        None
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
@@ -95,14 +97,18 @@ where
 #[stable(feature = "extract_if", since = "1.87.0")]
 impl<T, F, A: Allocator> Drop for ExtractIf<'_, T, F, A> {
     fn drop(&mut self) {
-        unsafe {
-            if self.idx < self.old_len && self.del > 0 {
-                let ptr = self.vec.as_mut_ptr();
-                let src = ptr.add(self.idx);
-                let dst = src.sub(self.del);
-                let tail_len = self.old_len - self.idx;
-                src.copy_to(dst, tail_len);
+        if self.del > 0 {
+            // SAFETY: Trailing unchecked items must be valid since we never touch them.
+            unsafe {
+                ptr::copy(
+                    self.vec.as_ptr().add(self.idx),
+                    self.vec.as_mut_ptr().add(self.idx - self.del),
+                    self.old_len - self.idx,
+                );
             }
+        }
+        // SAFETY: After filling holes, all items are in contiguous memory.
+        unsafe {
             self.vec.set_len(self.old_len - self.del);
         }
     }