core/ub_checks.rs
1//! Provides the [`assert_unsafe_precondition`] macro as well as some utility functions that cover
2//! common preconditions.
3
4use crate::intrinsics::{self, const_eval_select};
5
6/// Checks that the preconditions of an unsafe function are followed.
7///
8/// The check is enabled at runtime if debug assertions are enabled when the
9/// caller is monomorphized. In const-eval/Miri checks implemented with this
10/// macro for language UB are always ignored.
11///
12/// This macro should be called as
13/// `assert_unsafe_precondition!(check_{library,language}_ub, "message", (ident: type = expr, ident: type = expr) => check_expr)`
14/// where each `expr` will be evaluated and passed in as function argument `ident: type`. Then all
15/// those arguments are passed to a function with the body `check_expr`.
16/// Pick `check_language_ub` when this is guarding a violation of language UB, i.e., immediate UB
17/// according to the Rust Abstract Machine. Pick `check_library_ub` when this is guarding a violation
18/// of a documented library precondition that does not *immediately* lead to language UB.
19///
20/// If `check_library_ub` is used but the check is actually guarding language UB, the check will
21/// slow down const-eval/Miri and we'll get the panic message instead of the interpreter's nice
22/// diagnostic, but our ability to detect UB is unchanged.
23/// But if `check_language_ub` is used when the check is actually for library UB, the check is
24/// omitted in const-eval/Miri and thus UB might occur undetected. Even if we eventually execute
25/// language UB which relies on the library UB, the backtrace Miri reports may be far removed from
26/// original cause.
27///
28/// These checks are behind a condition which is evaluated at codegen time, not expansion time like
29/// [`debug_assert`]. This means that a standard library built with optimizations and debug
30/// assertions disabled will have these checks optimized out of its monomorphizations, but if a
31/// caller of the standard library has debug assertions enabled and monomorphizes an expansion of
32/// this macro, that monomorphization will contain the check.
33///
34/// Since these checks cannot be optimized out in MIR, some care must be taken in both call and
35/// implementation to mitigate their compile-time overhead. Calls to this macro always expand to
36/// this structure:
37/// ```ignore (pseudocode)
38/// if ::core::intrinsics::check_language_ub() {
39/// precondition_check(args)
40/// }
41/// ```
42/// where `precondition_check` is monomorphic with the attributes `#[rustc_nounwind]`, `#[inline]` and
43/// `#[rustc_no_mir_inline]`. This combination of attributes ensures that the actual check logic is
44/// compiled only once and generates a minimal amount of IR because the check cannot be inlined in
45/// MIR, but *can* be inlined and fully optimized by a codegen backend.
46///
47/// Callers should avoid introducing any other `let` bindings or any code outside this macro in
48/// order to call it. Since the precompiled standard library is built with full debuginfo and these
49/// variables cannot be optimized out in MIR, an innocent-looking `let` can produce enough
50/// debuginfo to have a measurable compile-time impact on debug builds.
51#[macro_export]
52#[unstable(feature = "ub_checks", issue = "none")]
53macro_rules! assert_unsafe_precondition {
54 ($kind:ident, $message:expr, ($($name:ident:$ty:ty = $arg:expr),*$(,)?) => $e:expr $(,)?) => {
55 {
56 // This check is inlineable, but not by the MIR inliner.
57 // The reason for this is that the MIR inliner is in an exceptionally bad position
58 // to think about whether or not to inline this. In MIR, this call is gated behind `debug_assertions`,
59 // which will codegen to `false` in release builds. Inlining the check would be wasted work in that case and
60 // would be bad for compile times.
61 //
62 // LLVM on the other hand sees the constant branch, so if it's `false`, it can immediately delete it without
63 // inlining the check. If it's `true`, it can inline it and get significantly better performance.
64 #[rustc_no_mir_inline]
65 #[inline]
66 #[rustc_nounwind]
67 #[track_caller]
68 const fn precondition_check($($name:$ty),*) {
69 if !$e {
70 let msg = concat!("unsafe precondition(s) violated: ", $message,
71 "\n\nThis indicates a bug in the program. \
72 This Undefined Behavior check is optional, and cannot be relied on for safety.");
73 ::core::panicking::panic_nounwind_fmt(::core::fmt::Arguments::new_const(&[msg]), false);
74 }
75 }
76
77 if ::core::ub_checks::$kind() {
78 precondition_check($($arg,)*);
79 }
80 }
81 };
82}
83#[unstable(feature = "ub_checks", issue = "none")]
84pub use assert_unsafe_precondition;
85/// Checking library UB is always enabled when UB-checking is done
86/// (and we use a reexport so that there is no unnecessary wrapper function).
87#[unstable(feature = "ub_checks", issue = "none")]
88pub use intrinsics::ub_checks as check_library_ub;
89
90/// Determines whether we should check for language UB.
91///
92/// The intention is to not do that when running in the interpreter, as that one has its own
93/// language UB checks which generally produce better errors.
94#[inline]
95#[rustc_allow_const_fn_unstable(const_eval_select)]
96pub(crate) const fn check_language_ub() -> bool {
97 // Only used for UB checks so we may const_eval_select.
98 intrinsics::ub_checks()
99 && const_eval_select!(
100 @capture { } -> bool:
101 if const {
102 // Always disable UB checks.
103 false
104 } else {
105 // Disable UB checks in Miri.
106 !cfg!(miri)
107 }
108 )
109}
110
111/// Checks whether `ptr` is properly aligned with respect to the given alignment, and
112/// if `is_zst == false`, that `ptr` is not null.
113///
114/// In `const` this is approximate and can fail spuriously. It is primarily intended
115/// for `assert_unsafe_precondition!` with `check_language_ub`, in which case the
116/// check is anyway not executed in `const`.
117#[inline]
118#[rustc_allow_const_fn_unstable(const_eval_select)]
119pub(crate) const fn maybe_is_aligned_and_not_null(
120 ptr: *const (),
121 align: usize,
122 is_zst: bool,
123) -> bool {
124 // This is just for safety checks so we can const_eval_select.
125 maybe_is_aligned(ptr, align) && (is_zst || !ptr.is_null())
126}
127
128/// Checks whether `ptr` is properly aligned with respect to the given alignment.
129///
130/// In `const` this is approximate and can fail spuriously. It is primarily intended
131/// for `assert_unsafe_precondition!` with `check_language_ub`, in which case the
132/// check is anyway not executed in `const`.
133#[inline]
134#[rustc_allow_const_fn_unstable(const_eval_select)]
135pub(crate) const fn maybe_is_aligned(ptr: *const (), align: usize) -> bool {
136 // This is just for safety checks so we can const_eval_select.
137 const_eval_select!(
138 @capture { ptr: *const (), align: usize } -> bool:
139 if const {
140 true
141 } else {
142 ptr.is_aligned_to(align)
143 }
144 )
145}
146
147#[inline]
148pub(crate) const fn is_valid_allocation_size(size: usize, len: usize) -> bool {
149 let max_len = if size == 0 { usize::MAX } else { isize::MAX as usize / size };
150 len <= max_len
151}
152
153/// Checks whether the regions of memory starting at `src` and `dst` of size
154/// `count * size` do *not* overlap.
155///
156/// Note that in const-eval this function just returns `true` and therefore must
157/// only be used with `assert_unsafe_precondition!`, similar to `is_aligned_and_not_null`.
158#[inline]
159#[rustc_allow_const_fn_unstable(const_eval_select)]
160pub(crate) const fn maybe_is_nonoverlapping(
161 src: *const (),
162 dst: *const (),
163 size: usize,
164 count: usize,
165) -> bool {
166 // This is just for safety checks so we can const_eval_select.
167 const_eval_select!(
168 @capture { src: *const (), dst: *const (), size: usize, count: usize } -> bool:
169 if const {
170 true
171 } else {
172 let src_usize = src.addr();
173 let dst_usize = dst.addr();
174 let Some(size) = size.checked_mul(count) else {
175 crate::panicking::panic_nounwind(
176 "is_nonoverlapping: `size_of::<T>() * count` overflows a usize",
177 )
178 };
179 let diff = src_usize.abs_diff(dst_usize);
180 // If the absolute distance between the ptrs is at least as big as the size of the buffer,
181 // they do not overlap.
182 diff >= size
183 }
184 )
185}