From 45203aac55b61a7bd1b44d20e289998368fea9cd Mon Sep 17 00:00:00 2001 From: Ralf Jung Date: Sun, 17 Mar 2024 10:12:25 +0100 Subject: [PATCH] move assert_unsafe_preconditions to its own file These macros and functions are not intrinsics, after all. --- library/core/src/char/convert.rs | 2 +- library/core/src/hint.rs | 5 +- library/core/src/intrinsics.rs | 160 ++-------------------------- library/core/src/lib.rs | 1 + library/core/src/num/nonzero.rs | 5 +- library/core/src/ops/index_range.rs | 5 +- library/core/src/ptr/const_ptr.rs | 2 +- library/core/src/ptr/mod.rs | 33 +++--- library/core/src/ptr/non_null.rs | 2 +- library/core/src/slice/index.rs | 2 +- library/core/src/slice/mod.rs | 2 +- library/core/src/slice/raw.rs | 16 ++- library/core/src/str/traits.rs | 2 +- library/core/src/ub_checks.rs | 145 +++++++++++++++++++++++++ 14 files changed, 194 insertions(+), 188 deletions(-) create mode 100644 library/core/src/ub_checks.rs diff --git a/library/core/src/char/convert.rs b/library/core/src/char/convert.rs index 70b9e89f9ea93..8f61292911030 100644 --- a/library/core/src/char/convert.rs +++ b/library/core/src/char/convert.rs @@ -4,9 +4,9 @@ use crate::char::TryFromCharError; use crate::convert::TryFrom; use crate::error::Error; use crate::fmt; -use crate::intrinsics::assert_unsafe_precondition; use crate::mem::transmute; use crate::str::FromStr; +use crate::ub_checks::assert_unsafe_precondition; /// Converts a `u32` to a `char`. See [`char::from_u32`]. #[must_use] diff --git a/library/core/src/hint.rs b/library/core/src/hint.rs index ffe059bf65cad..b27d0db461917 100644 --- a/library/core/src/hint.rs +++ b/library/core/src/hint.rs @@ -4,6 +4,7 @@ //! Hints may be compile time or runtime. use crate::intrinsics; +use crate::ub_checks; /// Informs the compiler that the site which is calling this function is not /// reachable, possibly enabling further optimizations. @@ -98,7 +99,7 @@ use crate::intrinsics; #[rustc_const_stable(feature = "const_unreachable_unchecked", since = "1.57.0")] #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces pub const unsafe fn unreachable_unchecked() -> ! { - intrinsics::assert_unsafe_precondition!( + ub_checks::assert_unsafe_precondition!( check_language_ub, "hint::unreachable_unchecked must never be reached", () => false @@ -148,7 +149,7 @@ pub const unsafe fn unreachable_unchecked() -> ! { pub const unsafe fn assert_unchecked(cond: bool) { // SAFETY: The caller promised `cond` is true. unsafe { - intrinsics::assert_unsafe_precondition!( + ub_checks::assert_unsafe_precondition!( check_language_ub, "hint::assert_unchecked must never be called when the condition is false", (cond: bool = cond) => cond, diff --git a/library/core/src/intrinsics.rs b/library/core/src/intrinsics.rs index 86b9a39d68a67..0ed26b6ed4ee4 100644 --- a/library/core/src/intrinsics.rs +++ b/library/core/src/intrinsics.rs @@ -66,6 +66,7 @@ use crate::marker::DiscriminantKind; use crate::marker::Tuple; use crate::mem::align_of; +use crate::ub_checks; pub mod mir; pub mod simd; @@ -2727,147 +2728,6 @@ pub unsafe fn retag_box_to_raw(ptr: *mut T) -> *mut T { // (`transmute` also falls into this category, but it cannot be wrapped due to the // check that `T` and `U` have the same size.) -/// Check that the preconditions of an unsafe function are followed. The check is enabled at -/// runtime if debug assertions are enabled when the caller is monomorphized. In const-eval/Miri -/// checks implemented with this macro for language UB are always ignored. -/// -/// This macro should be called as -/// `assert_unsafe_precondition!(check_{library,lang}_ub, "message", (ident: type = expr, ident: type = expr) => check_expr)` -/// where each `expr` will be evaluated and passed in as function argument `ident: type`. Then all -/// those arguments are passed to a function with the body `check_expr`. -/// Pick `check_language_ub` when this is guarding a violation of language UB, i.e., immediate UB -/// according to the Rust Abstract Machine. Pick `check_library_ub` when this is guarding a violation -/// of a documented library precondition that does not *immediately* lead to language UB. -/// -/// If `check_library_ub` is used but the check is actually guarding language UB, the check will -/// slow down const-eval/Miri and we'll get the panic message instead of the interpreter's nice -/// diagnostic, but our ability to detect UB is unchanged. -/// But if `check_language_ub` is used when the check is actually for library UB, the check is -/// omitted in const-eval/Miri and thus if we eventually execute language UB which relies on the -/// library UB, the backtrace Miri reports may be far removed from original cause. -/// -/// These checks are behind a condition which is evaluated at codegen time, not expansion time like -/// [`debug_assert`]. This means that a standard library built with optimizations and debug -/// assertions disabled will have these checks optimized out of its monomorphizations, but if a -/// caller of the standard library has debug assertions enabled and monomorphizes an expansion of -/// this macro, that monomorphization will contain the check. -/// -/// Since these checks cannot be optimized out in MIR, some care must be taken in both call and -/// implementation to mitigate their compile-time overhead. Calls to this macro always expand to -/// this structure: -/// ```ignore (pseudocode) -/// if ::core::intrinsics::check_language_ub() { -/// precondition_check(args) -/// } -/// ``` -/// where `precondition_check` is monomorphic with the attributes `#[rustc_nounwind]`, `#[inline]` and -/// `#[rustc_no_mir_inline]`. This combination of attributes ensures that the actual check logic is -/// compiled only once and generates a minimal amount of IR because the check cannot be inlined in -/// MIR, but *can* be inlined and fully optimized by a codegen backend. -/// -/// Callers should avoid introducing any other `let` bindings or any code outside this macro in -/// order to call it. Since the precompiled standard library is built with full debuginfo and these -/// variables cannot be optimized out in MIR, an innocent-looking `let` can produce enough -/// debuginfo to have a measurable compile-time impact on debug builds. -#[allow_internal_unstable(ub_checks)] // permit this to be called in stably-const fn -macro_rules! assert_unsafe_precondition { - ($kind:ident, $message:expr, ($($name:ident:$ty:ty = $arg:expr),*$(,)?) => $e:expr $(,)?) => { - { - // #[cfg(bootstrap)] (this comment) - // When the standard library is compiled with debug assertions, we want the check to inline for better performance. - // This is important when working on the compiler, which is compiled with debug assertions locally. - // When not compiled with debug assertions (so the precompiled std) we outline the check to minimize the compile - // time impact when debug assertions are disabled. - // The proper solution to this is the `#[rustc_no_mir_inline]` below, but we still want decent performance for cfg(bootstrap). - #[cfg_attr(all(debug_assertions, bootstrap), inline(always))] - #[cfg_attr(all(not(debug_assertions), bootstrap), inline(never))] - - // This check is inlineable, but not by the MIR inliner. - // The reason for this is that the MIR inliner is in an exceptionally bad position - // to think about whether or not to inline this. In MIR, this call is gated behind `debug_assertions`, - // which will codegen to `false` in release builds. Inlining the check would be wasted work in that case and - // would be bad for compile times. - // - // LLVM on the other hand sees the constant branch, so if it's `false`, it can immediately delete it without - // inlining the check. If it's `true`, it can inline it and get significantly better performance. - #[cfg_attr(not(bootstrap), rustc_no_mir_inline)] - #[cfg_attr(not(bootstrap), inline)] - #[rustc_nounwind] - #[rustc_const_unstable(feature = "ub_checks", issue = "none")] - const fn precondition_check($($name:$ty),*) { - if !$e { - ::core::panicking::panic_nounwind( - concat!("unsafe precondition(s) violated: ", $message) - ); - } - } - - if ::core::intrinsics::$kind() { - precondition_check($($arg,)*); - } - } - }; -} -pub(crate) use assert_unsafe_precondition; - -/// Checks whether `ptr` is properly aligned with respect to -/// `align_of::()`. -/// -/// In `const` this is approximate and can fail spuriously. It is primarily intended -/// for `assert_unsafe_precondition!` with `check_language_ub`, in which case the -/// check is anyway not executed in `const`. -#[inline] -pub(crate) const fn is_aligned_and_not_null(ptr: *const (), align: usize) -> bool { - !ptr.is_null() && ptr.is_aligned_to(align) -} - -#[inline] -pub(crate) const fn is_valid_allocation_size(size: usize, len: usize) -> bool { - let max_len = if size == 0 { usize::MAX } else { isize::MAX as usize / size }; - len <= max_len -} - -/// Checks whether the regions of memory starting at `src` and `dst` of size -/// `count * size` do *not* overlap. -/// -/// Note that in const-eval this function just returns `true` and therefore must -/// only be used with `assert_unsafe_precondition!`, similar to `is_aligned_and_not_null`. -#[inline] -pub(crate) const fn is_nonoverlapping( - src: *const (), - dst: *const (), - size: usize, - count: usize, -) -> bool { - #[inline] - fn runtime(src: *const (), dst: *const (), size: usize, count: usize) -> bool { - let src_usize = src.addr(); - let dst_usize = dst.addr(); - let Some(size) = size.checked_mul(count) else { - crate::panicking::panic_nounwind( - "is_nonoverlapping: `size_of::() * count` overflows a usize", - ) - }; - let diff = src_usize.abs_diff(dst_usize); - // If the absolute distance between the ptrs is at least as big as the size of the buffer, - // they do not overlap. - diff >= size - } - - #[inline] - const fn comptime(_: *const (), _: *const (), _: usize, _: usize) -> bool { - true - } - - #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block - // SAFETY: This function's precondition is equivalent to that of `const_eval_select`. - // Programs which do not execute UB will only see this function return `true`, which makes the - // const and runtime implementation indistinguishable. - unsafe { - const_eval_select((src, dst, size, count), comptime, runtime) - } -} - /// Copies `count * size_of::()` bytes from `src` to `dst`. The source /// and destination must *not* overlap. /// @@ -2966,7 +2826,7 @@ pub const unsafe fn copy_nonoverlapping(src: *const T, dst: *mut T, count: us pub fn copy_nonoverlapping(src: *const T, dst: *mut T, count: usize); } - assert_unsafe_precondition!( + ub_checks::assert_unsafe_precondition!( check_language_ub, "ptr::copy_nonoverlapping requires that both pointer arguments are aligned and non-null \ and the specified memory ranges do not overlap", @@ -2977,9 +2837,9 @@ pub const unsafe fn copy_nonoverlapping(src: *const T, dst: *mut T, count: us align: usize = align_of::(), count: usize = count, ) => - is_aligned_and_not_null(src, align) - && is_aligned_and_not_null(dst, align) - && is_nonoverlapping(src, dst, size, count) + ub_checks::is_aligned_and_not_null(src, align) + && ub_checks::is_aligned_and_not_null(dst, align) + && ub_checks::is_nonoverlapping(src, dst, size, count) ); // SAFETY: the safety contract for `copy_nonoverlapping` must be @@ -3070,7 +2930,7 @@ pub const unsafe fn copy(src: *const T, dst: *mut T, count: usize) { // SAFETY: the safety contract for `copy` must be upheld by the caller. unsafe { - assert_unsafe_precondition!( + ub_checks::assert_unsafe_precondition!( check_language_ub, "ptr::copy_nonoverlapping requires that both pointer arguments are aligned and non-null \ and the specified memory ranges do not overlap", @@ -3079,8 +2939,8 @@ pub const unsafe fn copy(src: *const T, dst: *mut T, count: usize) { dst: *mut () = dst as *mut (), align: usize = align_of::(), ) => - is_aligned_and_not_null(src, align) - && is_aligned_and_not_null(dst, align) + ub_checks::is_aligned_and_not_null(src, align) + && ub_checks::is_aligned_and_not_null(dst, align) ); copy(src, dst, count) } @@ -3151,13 +3011,13 @@ pub const unsafe fn write_bytes(dst: *mut T, val: u8, count: usize) { // SAFETY: the safety contract for `write_bytes` must be upheld by the caller. unsafe { - assert_unsafe_precondition!( + ub_checks::assert_unsafe_precondition!( check_language_ub, "ptr::write_bytes requires that the destination pointer is aligned and non-null", ( addr: *const () = dst as *const (), align: usize = align_of::(), - ) => is_aligned_and_not_null(addr, align) + ) => ub_checks::is_aligned_and_not_null(addr, align) ); write_bytes(dst, val, count) } diff --git a/library/core/src/lib.rs b/library/core/src/lib.rs index 9b786feba8988..626e85fc0484b 100644 --- a/library/core/src/lib.rs +++ b/library/core/src/lib.rs @@ -368,6 +368,7 @@ pub mod hint; pub mod intrinsics; pub mod mem; pub mod ptr; +mod ub_checks; /* Core language traits */ diff --git a/library/core/src/num/nonzero.rs b/library/core/src/num/nonzero.rs index 1f7a4e276f553..12e22ad14a361 100644 --- a/library/core/src/num/nonzero.rs +++ b/library/core/src/num/nonzero.rs @@ -9,6 +9,7 @@ use crate::ops::{BitOr, BitOrAssign, Div, Neg, Rem}; use crate::panic::{RefUnwindSafe, UnwindSafe}; use crate::ptr; use crate::str::FromStr; +use crate::ub_checks; use super::from_str_radix; use super::{IntErrorKind, ParseIntError}; @@ -369,7 +370,7 @@ where None => { // SAFETY: The caller guarantees that `n` is non-zero, so this is unreachable. unsafe { - intrinsics::assert_unsafe_precondition!( + ub_checks::assert_unsafe_precondition!( check_language_ub, "NonZero::new_unchecked requires the argument to be non-zero", () => false, @@ -409,7 +410,7 @@ where None => { // SAFETY: The caller guarantees that `n` references a value that is non-zero, so this is unreachable. unsafe { - intrinsics::assert_unsafe_precondition!( + ub_checks::assert_unsafe_precondition!( check_library_ub, "NonZero::from_mut_unchecked requires the argument to dereference as non-zero", () => false, diff --git a/library/core/src/ops/index_range.rs b/library/core/src/ops/index_range.rs index b28d88fa5bfc2..65bda9177c7be 100644 --- a/library/core/src/ops/index_range.rs +++ b/library/core/src/ops/index_range.rs @@ -1,6 +1,7 @@ -use crate::intrinsics::{assert_unsafe_precondition, unchecked_add, unchecked_sub}; +use crate::intrinsics::{unchecked_add, unchecked_sub}; use crate::iter::{FusedIterator, TrustedLen}; use crate::num::NonZero; +use crate::ub_checks; /// Like a `Range`, but with a safety invariant that `start <= end`. /// @@ -19,7 +20,7 @@ impl IndexRange { /// - `start <= end` #[inline] pub const unsafe fn new_unchecked(start: usize, end: usize) -> Self { - assert_unsafe_precondition!( + ub_checks::assert_unsafe_precondition!( check_library_ub, "IndexRange::new_unchecked requires `start <= end`", (start: usize = start, end: usize = end) => start <= end, diff --git a/library/core/src/ptr/const_ptr.rs b/library/core/src/ptr/const_ptr.rs index a0c04d3f65dfd..02fc920a0fac1 100644 --- a/library/core/src/ptr/const_ptr.rs +++ b/library/core/src/ptr/const_ptr.rs @@ -827,7 +827,7 @@ impl *const T { } } - assert_unsafe_precondition!( + ub_checks::assert_unsafe_precondition!( check_language_ub, "ptr::sub_ptr requires `self >= origin`", ( diff --git a/library/core/src/ptr/mod.rs b/library/core/src/ptr/mod.rs index 1f0204daf7238..55f0c7db82db4 100644 --- a/library/core/src/ptr/mod.rs +++ b/library/core/src/ptr/mod.rs @@ -388,10 +388,9 @@ use crate::cmp::Ordering; use crate::fmt; use crate::hash; -use crate::intrinsics::{ - self, assert_unsafe_precondition, is_aligned_and_not_null, is_nonoverlapping, -}; +use crate::intrinsics; use crate::marker::FnPtr; +use crate::ub_checks; use crate::mem::{self, align_of, size_of, MaybeUninit}; @@ -1019,7 +1018,7 @@ pub const unsafe fn swap_nonoverlapping(x: *mut T, y: *mut T, count: usize) { }; } - assert_unsafe_precondition!( + ub_checks::assert_unsafe_precondition!( check_language_ub, "ptr::swap_nonoverlapping requires that both pointer arguments are aligned and non-null \ and the specified memory ranges do not overlap", @@ -1030,9 +1029,9 @@ pub const unsafe fn swap_nonoverlapping(x: *mut T, y: *mut T, count: usize) { align: usize = align_of::(), count: usize = count, ) => - is_aligned_and_not_null(x, align) - && is_aligned_and_not_null(y, align) - && is_nonoverlapping(x, y, size, count) + ub_checks::is_aligned_and_not_null(x, align) + && ub_checks::is_aligned_and_not_null(y, align) + && ub_checks::is_nonoverlapping(x, y, size, count) ); // Split up the slice into small power-of-two-sized chunks that LLVM is able @@ -1120,13 +1119,13 @@ pub const unsafe fn replace(dst: *mut T, src: T) -> T { // and cannot overlap `src` since `dst` must point to a distinct // allocated object. unsafe { - assert_unsafe_precondition!( + ub_checks::assert_unsafe_precondition!( check_language_ub, "ptr::replace requires that the pointer argument is aligned and non-null", ( addr: *const () = dst as *const (), align: usize = align_of::(), - ) => is_aligned_and_not_null(addr, align) + ) => ub_checks::is_aligned_and_not_null(addr, align) ); mem::replace(&mut *dst, src) } @@ -1272,13 +1271,13 @@ pub const unsafe fn read(src: *const T) -> T { // SAFETY: the caller must guarantee that `src` is valid for reads. unsafe { #[cfg(debug_assertions)] // Too expensive to always enable (for now?) - assert_unsafe_precondition!( + ub_checks::assert_unsafe_precondition!( check_language_ub, "ptr::read requires that the pointer argument is aligned and non-null", ( addr: *const () = src as *const (), align: usize = align_of::(), - ) => is_aligned_and_not_null(addr, align) + ) => ub_checks::is_aligned_and_not_null(addr, align) ); crate::intrinsics::read_via_copy(src) } @@ -1481,13 +1480,13 @@ pub const unsafe fn write(dst: *mut T, src: T) { // to `dst` while `src` is owned by this function. unsafe { #[cfg(debug_assertions)] // Too expensive to always enable (for now?) - assert_unsafe_precondition!( + ub_checks::assert_unsafe_precondition!( check_language_ub, "ptr::write requires that the pointer argument is aligned and non-null", ( addr: *mut () = dst as *mut (), align: usize = align_of::(), - ) => is_aligned_and_not_null(addr, align) + ) => ub_checks::is_aligned_and_not_null(addr, align) ); intrinsics::write_via_move(dst, src) } @@ -1653,13 +1652,13 @@ pub const unsafe fn write_unaligned(dst: *mut T, src: T) { pub unsafe fn read_volatile(src: *const T) -> T { // SAFETY: the caller must uphold the safety contract for `volatile_load`. unsafe { - assert_unsafe_precondition!( + ub_checks::assert_unsafe_precondition!( check_language_ub, "ptr::read_volatile requires that the pointer argument is aligned and non-null", ( addr: *const () = src as *const (), align: usize = align_of::(), - ) => is_aligned_and_not_null(addr, align) + ) => ub_checks::is_aligned_and_not_null(addr, align) ); intrinsics::volatile_load(src) } @@ -1732,13 +1731,13 @@ pub unsafe fn read_volatile(src: *const T) -> T { pub unsafe fn write_volatile(dst: *mut T, src: T) { // SAFETY: the caller must uphold the safety contract for `volatile_store`. unsafe { - assert_unsafe_precondition!( + ub_checks::assert_unsafe_precondition!( check_language_ub, "ptr::write_volatile requires that the pointer argument is aligned and non-null", ( addr: *mut () = dst as *mut (), align: usize = align_of::(), - ) => is_aligned_and_not_null(addr, align) + ) => ub_checks::is_aligned_and_not_null(addr, align) ); intrinsics::volatile_store(dst, src); } diff --git a/library/core/src/ptr/non_null.rs b/library/core/src/ptr/non_null.rs index 9c0236c172a93..7097b6fbf2f85 100644 --- a/library/core/src/ptr/non_null.rs +++ b/library/core/src/ptr/non_null.rs @@ -2,7 +2,6 @@ use crate::cmp::Ordering; use crate::fmt; use crate::hash; use crate::intrinsics; -use crate::intrinsics::assert_unsafe_precondition; use crate::marker::Unsize; use crate::mem::{MaybeUninit, SizedTypeProperties}; use crate::num::NonZero; @@ -10,6 +9,7 @@ use crate::ops::{CoerceUnsized, DispatchFromDyn}; use crate::ptr; use crate::ptr::Unique; use crate::slice::{self, SliceIndex}; +use crate::ub_checks::assert_unsafe_precondition; /// `*mut T` but non-zero and [covariant]. /// diff --git a/library/core/src/slice/index.rs b/library/core/src/slice/index.rs index 64f1f360821da..18f63e4a461eb 100644 --- a/library/core/src/slice/index.rs +++ b/library/core/src/slice/index.rs @@ -1,10 +1,10 @@ //! Indexing implementations for `[T]`. -use crate::intrinsics::assert_unsafe_precondition; use crate::intrinsics::const_eval_select; use crate::intrinsics::unchecked_sub; use crate::ops; use crate::ptr; +use crate::ub_checks::assert_unsafe_precondition; #[stable(feature = "rust1", since = "1.0.0")] impl ops::Index for [T] diff --git a/library/core/src/slice/mod.rs b/library/core/src/slice/mod.rs index 4a574bf034745..92080c5020ad8 100644 --- a/library/core/src/slice/mod.rs +++ b/library/core/src/slice/mod.rs @@ -9,7 +9,6 @@ use crate::cmp::Ordering::{self, Equal, Greater, Less}; use crate::fmt; use crate::hint; -use crate::intrinsics::assert_unsafe_precondition; use crate::intrinsics::exact_div; use crate::mem::{self, SizedTypeProperties}; use crate::num::NonZero; @@ -17,6 +16,7 @@ use crate::ops::{Bound, OneSidedRange, Range, RangeBounds}; use crate::ptr; use crate::simd::{self, Simd}; use crate::slice; +use crate::ub_checks::assert_unsafe_precondition; #[unstable( feature = "slice_internals", diff --git a/library/core/src/slice/raw.rs b/library/core/src/slice/raw.rs index 2199614ce27e4..29a12f106c5ed 100644 --- a/library/core/src/slice/raw.rs +++ b/library/core/src/slice/raw.rs @@ -1,12 +1,10 @@ //! Free functions to create `&[T]` and `&mut [T]`. use crate::array; -use crate::intrinsics::{ - assert_unsafe_precondition, is_aligned_and_not_null, is_valid_allocation_size, -}; use crate::mem::{align_of, size_of}; use crate::ops::Range; use crate::ptr; +use crate::ub_checks; /// Forms a slice from a pointer and a length. /// @@ -95,7 +93,7 @@ use crate::ptr; pub const unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T] { // SAFETY: the caller must uphold the safety contract for `from_raw_parts`. unsafe { - assert_unsafe_precondition!( + ub_checks::assert_unsafe_precondition!( check_language_ub, "slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX`", ( @@ -104,8 +102,8 @@ pub const unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T] align: usize = align_of::(), len: usize = len, ) => - is_aligned_and_not_null(data, align) - && is_valid_allocation_size(size, len) + ub_checks::is_aligned_and_not_null(data, align) + && ub_checks::is_valid_allocation_size(size, len) ); &*ptr::slice_from_raw_parts(data, len) } @@ -149,7 +147,7 @@ pub const unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T] pub const unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a mut [T] { // SAFETY: the caller must uphold the safety contract for `from_raw_parts_mut`. unsafe { - assert_unsafe_precondition!( + ub_checks::assert_unsafe_precondition!( check_language_ub, "slice::from_raw_parts_mut requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX`", ( @@ -158,8 +156,8 @@ pub const unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a m align: usize = align_of::(), len: usize = len, ) => - is_aligned_and_not_null(data, align) - && is_valid_allocation_size(size, len) + ub_checks::is_aligned_and_not_null(data, align) + && ub_checks::is_valid_allocation_size(size, len) ); &mut *ptr::slice_from_raw_parts_mut(data, len) } diff --git a/library/core/src/str/traits.rs b/library/core/src/str/traits.rs index ec81fd095d530..672af75214936 100644 --- a/library/core/src/str/traits.rs +++ b/library/core/src/str/traits.rs @@ -1,10 +1,10 @@ //! Trait implementations for `str`. use crate::cmp::Ordering; -use crate::intrinsics::assert_unsafe_precondition; use crate::ops; use crate::ptr; use crate::slice::SliceIndex; +use crate::ub_checks::assert_unsafe_precondition; use super::ParseBoolError; diff --git a/library/core/src/ub_checks.rs b/library/core/src/ub_checks.rs new file mode 100644 index 0000000000000..4ef56392fad33 --- /dev/null +++ b/library/core/src/ub_checks.rs @@ -0,0 +1,145 @@ +//! Provides the [`assert_unsafe_precondition`] macro as well as some utility functions that cover +//! common preconditions. + +use crate::intrinsics::const_eval_select; + +/// Check that the preconditions of an unsafe function are followed. The check is enabled at +/// runtime if debug assertions are enabled when the caller is monomorphized. In const-eval/Miri +/// checks implemented with this macro for language UB are always ignored. +/// +/// This macro should be called as +/// `assert_unsafe_precondition!(check_{library,lang}_ub, "message", (ident: type = expr, ident: type = expr) => check_expr)` +/// where each `expr` will be evaluated and passed in as function argument `ident: type`. Then all +/// those arguments are passed to a function with the body `check_expr`. +/// Pick `check_language_ub` when this is guarding a violation of language UB, i.e., immediate UB +/// according to the Rust Abstract Machine. Pick `check_library_ub` when this is guarding a violation +/// of a documented library precondition that does not *immediately* lead to language UB. +/// +/// If `check_library_ub` is used but the check is actually guarding language UB, the check will +/// slow down const-eval/Miri and we'll get the panic message instead of the interpreter's nice +/// diagnostic, but our ability to detect UB is unchanged. +/// But if `check_language_ub` is used when the check is actually for library UB, the check is +/// omitted in const-eval/Miri and thus if we eventually execute language UB which relies on the +/// library UB, the backtrace Miri reports may be far removed from original cause. +/// +/// These checks are behind a condition which is evaluated at codegen time, not expansion time like +/// [`debug_assert`]. This means that a standard library built with optimizations and debug +/// assertions disabled will have these checks optimized out of its monomorphizations, but if a +/// caller of the standard library has debug assertions enabled and monomorphizes an expansion of +/// this macro, that monomorphization will contain the check. +/// +/// Since these checks cannot be optimized out in MIR, some care must be taken in both call and +/// implementation to mitigate their compile-time overhead. Calls to this macro always expand to +/// this structure: +/// ```ignore (pseudocode) +/// if ::core::intrinsics::check_language_ub() { +/// precondition_check(args) +/// } +/// ``` +/// where `precondition_check` is monomorphic with the attributes `#[rustc_nounwind]`, `#[inline]` and +/// `#[rustc_no_mir_inline]`. This combination of attributes ensures that the actual check logic is +/// compiled only once and generates a minimal amount of IR because the check cannot be inlined in +/// MIR, but *can* be inlined and fully optimized by a codegen backend. +/// +/// Callers should avoid introducing any other `let` bindings or any code outside this macro in +/// order to call it. Since the precompiled standard library is built with full debuginfo and these +/// variables cannot be optimized out in MIR, an innocent-looking `let` can produce enough +/// debuginfo to have a measurable compile-time impact on debug builds. +#[allow_internal_unstable(ub_checks)] // permit this to be called in stably-const fn +macro_rules! assert_unsafe_precondition { + ($kind:ident, $message:expr, ($($name:ident:$ty:ty = $arg:expr),*$(,)?) => $e:expr $(,)?) => { + { + // #[cfg(bootstrap)] (this comment) + // When the standard library is compiled with debug assertions, we want the check to inline for better performance. + // This is important when working on the compiler, which is compiled with debug assertions locally. + // When not compiled with debug assertions (so the precompiled std) we outline the check to minimize the compile + // time impact when debug assertions are disabled. + // The proper solution to this is the `#[rustc_no_mir_inline]` below, but we still want decent performance for cfg(bootstrap). + #[cfg_attr(all(debug_assertions, bootstrap), inline(always))] + #[cfg_attr(all(not(debug_assertions), bootstrap), inline(never))] + + // This check is inlineable, but not by the MIR inliner. + // The reason for this is that the MIR inliner is in an exceptionally bad position + // to think about whether or not to inline this. In MIR, this call is gated behind `debug_assertions`, + // which will codegen to `false` in release builds. Inlining the check would be wasted work in that case and + // would be bad for compile times. + // + // LLVM on the other hand sees the constant branch, so if it's `false`, it can immediately delete it without + // inlining the check. If it's `true`, it can inline it and get significantly better performance. + #[cfg_attr(not(bootstrap), rustc_no_mir_inline)] + #[cfg_attr(not(bootstrap), inline)] + #[rustc_nounwind] + #[rustc_const_unstable(feature = "ub_checks", issue = "none")] + const fn precondition_check($($name:$ty),*) { + if !$e { + ::core::panicking::panic_nounwind( + concat!("unsafe precondition(s) violated: ", $message) + ); + } + } + + if ::core::intrinsics::$kind() { + precondition_check($($arg,)*); + } + } + }; +} +pub(crate) use assert_unsafe_precondition; + +/// Checks whether `ptr` is properly aligned with respect to +/// `align_of::()`. +/// +/// In `const` this is approximate and can fail spuriously. It is primarily intended +/// for `assert_unsafe_precondition!` with `check_language_ub`, in which case the +/// check is anyway not executed in `const`. +#[inline] +pub(crate) const fn is_aligned_and_not_null(ptr: *const (), align: usize) -> bool { + !ptr.is_null() && ptr.is_aligned_to(align) +} + +#[inline] +pub(crate) const fn is_valid_allocation_size(size: usize, len: usize) -> bool { + let max_len = if size == 0 { usize::MAX } else { isize::MAX as usize / size }; + len <= max_len +} + +/// Checks whether the regions of memory starting at `src` and `dst` of size +/// `count * size` do *not* overlap. +/// +/// Note that in const-eval this function just returns `true` and therefore must +/// only be used with `assert_unsafe_precondition!`, similar to `is_aligned_and_not_null`. +#[inline] +pub(crate) const fn is_nonoverlapping( + src: *const (), + dst: *const (), + size: usize, + count: usize, +) -> bool { + #[inline] + fn runtime(src: *const (), dst: *const (), size: usize, count: usize) -> bool { + let src_usize = src.addr(); + let dst_usize = dst.addr(); + let Some(size) = size.checked_mul(count) else { + crate::panicking::panic_nounwind( + "is_nonoverlapping: `size_of::() * count` overflows a usize", + ) + }; + let diff = src_usize.abs_diff(dst_usize); + // If the absolute distance between the ptrs is at least as big as the size of the buffer, + // they do not overlap. + diff >= size + } + + #[inline] + const fn comptime(_: *const (), _: *const (), _: usize, _: usize) -> bool { + true + } + + #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block + // SAFETY: This function's precondition is equivalent to that of `const_eval_select`. + // Programs which do not execute UB will only see this function return `true`, which makes the + // const and runtime implementation indistinguishable. + unsafe { + const_eval_select((src, dst, size, count), comptime, runtime) + } +}