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Rollup merge of #123411 - saethlin:ub-checks, r=Urgau,RalfJung
Put checks that detect UB under their own flag below debug_assertions Implementation of rust-lang/compiler-team#725
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Original file line number | Diff line number | Diff line change |
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# `ub-checks` | ||
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The tracking issue for this feature is: [#123499](https://github.com/rust-lang/rust/issues/123499). | ||
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-------------------- | ||
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The `-Zub-checks` compiler flag enables additional runtime checks that detect some causes of Undefined Behavior at runtime. | ||
By default, `-Zub-checks` flag inherits the value of `-Cdebug-assertions`. | ||
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All checks are generated on a best-effort basis; even if we have a check implemented for some cause of Undefined Behavior, it may be possible for the check to not fire. | ||
If a dependency is compiled with `-Zub-checks=no` but the final binary or library is compiled with `-Zub-checks=yes`, UB checks reached by the dependency are likely to be optimized out. | ||
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When `-Zub-checks` detects UB, a non-unwinding panic is produced. | ||
That means that we will not unwind the stack and will not call any `Drop` impls, but we will execute the configured panic hook. | ||
We expect that unsafe code has been written which relies on code not unwinding which may have UB checks inserted. | ||
Ergo, an unwinding panic could easily turn works-as-intended UB into a much bigger problem. | ||
Calling the panic hook theoretically has the same implications, but we expect that the standard library panic hook will be stateless enough to be always called, and that if a user has configured a panic hook that the hook may be very helpful to debugging the detected UB. |
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Original file line number | Diff line number | Diff line change |
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// With -Zub-checks=yes (enabled by default by -Cdebug-assertions=yes) we will produce a runtime | ||
// check that the index to slice::get_unchecked is in-bounds of the slice. That is tested for by | ||
// tests/ui/precondition-checks/out-of-bounds-get-unchecked.rs | ||
// | ||
// This test ensures that such a runtime check is *not* emitted when debug-assertions are enabled, | ||
// but ub-checks are explicitly disabled. | ||
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//@ revisions: DEBUG NOCHECKS | ||
//@ [DEBUG] compile-flags: | ||
//@ [NOCHECKS] compile-flags: -Zub-checks=no | ||
//@ compile-flags: -O -Cdebug-assertions=yes | ||
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#![crate_type = "lib"] | ||
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use std::ops::Range; | ||
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// CHECK-LABEL: @slice_get_unchecked( | ||
#[no_mangle] | ||
pub unsafe fn slice_get_unchecked(x: &[i32], i: usize) -> &i32 { | ||
// CHECK: icmp ult | ||
// NOCHECKS: tail call void @llvm.assume | ||
// DEBUG: br i1 | ||
// DEBUG: call core::panicking::panic_nounwind | ||
// DEBUG: unreachable | ||
// CHECK: getelementptr inbounds | ||
// CHECK: ret ptr | ||
x.get_unchecked(i) | ||
} |
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