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Avoid using the copy_nonoverlapping wrapper through mem::replace. #87827

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Aug 8, 2021
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Avoid using the copy_nonoverlapping wrapper through mem::replace. #87827

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7 changes: 7 additions & 0 deletions library/core/src/ptr/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -685,6 +685,13 @@ pub const unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_ptr_read", issue = "80377")]
pub const unsafe fn read<T>(src: *const T) -> T {
// We are calling the intrinsics directly to avoid function calls in the generated code
// as `intrinsics::copy_nonoverlapping` is a wrapper function.
extern "rust-intrinsic" {
#[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
}

let mut tmp = MaybeUninit::<T>::uninit();
// SAFETY: the caller must guarantee that `src` is valid for reads.
// `src` cannot overlap `tmp` because `tmp` was just allocated on
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25 changes: 25 additions & 0 deletions src/test/codegen/mem-replace-direct-memcpy.rs
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@@ -0,0 +1,25 @@
// This test ensures that `mem::replace::<T>` only ever calls `@llvm.memcpy`
// with `size_of::<T>()` as the size, and never goes through any wrapper that
// may e.g. multiply `size_of::<T>()` with a variable "count" (which is only
// known to be `1` after inlining).

// compile-flags: -C no-prepopulate-passes

#![crate_type = "lib"]

pub fn replace_byte(dst: &mut u8, src: u8) -> u8 {
std::mem::replace(dst, src)
}

// NOTE(eddyb) the `CHECK-NOT`s ensure that the only calls of `@llvm.memcpy` in
// the entire output, are the two direct calls we want, from `ptr::{read,write}`.

// CHECK-NOT: call void @llvm.memcpy
// CHECK: ; core::ptr::read
// CHECK-NOT: call void @llvm.memcpy
// CHECK: call void @llvm.memcpy.p0i8.p0i8.i{{.*}}(i8* align 1 %{{.*}}, i8* align 1 %src, i{{.*}} 1, i1 false)
// CHECK-NOT: call void @llvm.memcpy
// CHECK: ; core::ptr::write
// CHECK-NOT: call void @llvm.memcpy
// CHECK: call void @llvm.memcpy.p0i8.p0i8.i{{.*}}(i8* align 1 %dst, i8* align 1 %src, i{{.*}} 1, i1 false)
// CHECK-NOT: call void @llvm.memcpy
45 changes: 15 additions & 30 deletions src/test/ui/const-ptr/out_of_bounds_read.stderr
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@@ -1,35 +1,25 @@
error[E0080]: evaluation of constant value failed
--> $SRC_DIR/core/src/intrinsics.rs:LL:COL
|
LL | unsafe { copy_nonoverlapping(src, dst, count) }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| |
| memory access failed: alloc7 has size 4, so pointer to 4 bytes starting at offset 4 is out-of-bounds
| inside `copy_nonoverlapping::<u32>` at $SRC_DIR/core/src/intrinsics.rs:LL:COL
|
::: $SRC_DIR/core/src/ptr/mod.rs:LL:COL
--> $SRC_DIR/core/src/ptr/mod.rs:LL:COL
|
LL | copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
| --------------------------------------------- inside `std::ptr::read::<u32>` at $SRC_DIR/core/src/ptr/mod.rs:LL:COL
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| |
| memory access failed: alloc7 has size 4, so pointer to 4 bytes starting at offset 4 is out-of-bounds
| inside `std::ptr::read::<u32>` at $SRC_DIR/core/src/ptr/mod.rs:LL:COL
|
::: $DIR/out_of_bounds_read.rs:13:33
|
LL | const _READ: u32 = unsafe { ptr::read(PAST_END_PTR) };
| ----------------------- inside `_READ` at $DIR/out_of_bounds_read.rs:13:33

error[E0080]: evaluation of constant value failed
--> $SRC_DIR/core/src/intrinsics.rs:LL:COL
|
LL | unsafe { copy_nonoverlapping(src, dst, count) }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| |
| memory access failed: alloc7 has size 4, so pointer to 4 bytes starting at offset 4 is out-of-bounds
| inside `copy_nonoverlapping::<u32>` at $SRC_DIR/core/src/intrinsics.rs:LL:COL
|
::: $SRC_DIR/core/src/ptr/mod.rs:LL:COL
--> $SRC_DIR/core/src/ptr/mod.rs:LL:COL
|
LL | copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
| --------------------------------------------- inside `std::ptr::read::<u32>` at $SRC_DIR/core/src/ptr/mod.rs:LL:COL
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| |
| memory access failed: alloc7 has size 4, so pointer to 4 bytes starting at offset 4 is out-of-bounds
| inside `std::ptr::read::<u32>` at $SRC_DIR/core/src/ptr/mod.rs:LL:COL
|
::: $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
|
Expand All @@ -42,18 +32,13 @@ LL | const _CONST_READ: u32 = unsafe { PAST_END_PTR.read() };
| ------------------- inside `_CONST_READ` at $DIR/out_of_bounds_read.rs:14:39

error[E0080]: evaluation of constant value failed
--> $SRC_DIR/core/src/intrinsics.rs:LL:COL
|
LL | unsafe { copy_nonoverlapping(src, dst, count) }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| |
| memory access failed: alloc7 has size 4, so pointer to 4 bytes starting at offset 4 is out-of-bounds
| inside `copy_nonoverlapping::<u32>` at $SRC_DIR/core/src/intrinsics.rs:LL:COL
|
::: $SRC_DIR/core/src/ptr/mod.rs:LL:COL
--> $SRC_DIR/core/src/ptr/mod.rs:LL:COL
|
LL | copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
| --------------------------------------------- inside `std::ptr::read::<u32>` at $SRC_DIR/core/src/ptr/mod.rs:LL:COL
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| |
| memory access failed: alloc7 has size 4, so pointer to 4 bytes starting at offset 4 is out-of-bounds
| inside `std::ptr::read::<u32>` at $SRC_DIR/core/src/ptr/mod.rs:LL:COL
|
::: $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
|
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