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Fix unintentional UB in ui tests #107972

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Feb 16, 2023
+15 −12
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Fix unintentional UB in ui tests #107972

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4 changes: 2 additions & 2 deletions tests/ui/closures/2229_closure_analysis/migrations/auto_traits.fixed
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Original file line number Diff line number Diff line change
Expand Up @@ -26,7 +26,7 @@ fn test_send_trait() {
//~| HELP: add a dummy let to cause `fptr` to be fully captured
*fptr.0 = 20;
//~^ NOTE: in Rust 2018, this closure captures all of `fptr`, but in Rust 2021, it will only capture `fptr.0`
} });
} }).join().unwrap();
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Why is this needed? Doesn't look like a UB fix. Is this about fixing memory leaks? Is that even a goal or should Miri be run with -Zmiri-ignore-leaks when running ui tests?

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Without this join, the thread can execute after the function returns, which is a use-after-free.

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Oh good point!

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No need to give me too much credit here, I just looked at Miri's output :p

}

/* Test Sync Trait Migration */
Expand All @@ -47,7 +47,7 @@ fn test_sync_trait() {
//~| HELP: add a dummy let to cause `fptr` to be fully captured
*fptr.0.0 = 20;
//~^ NOTE: in Rust 2018, this closure captures all of `fptr`, but in Rust 2021, it will only capture `fptr.0.0`
} });
} }).join().unwrap();
}

/* Test Clone Trait Migration */
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4 changes: 2 additions & 2 deletions tests/ui/closures/2229_closure_analysis/migrations/auto_traits.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -26,7 +26,7 @@ fn test_send_trait() {
//~| HELP: add a dummy let to cause `fptr` to be fully captured
*fptr.0 = 20;
//~^ NOTE: in Rust 2018, this closure captures all of `fptr`, but in Rust 2021, it will only capture `fptr.0`
});
}).join().unwrap();
}

/* Test Sync Trait Migration */
Expand All @@ -47,7 +47,7 @@ fn test_sync_trait() {
//~| HELP: add a dummy let to cause `fptr` to be fully captured
*fptr.0.0 = 20;
//~^ NOTE: in Rust 2018, this closure captures all of `fptr`, but in Rust 2021, it will only capture `fptr.0.0`
});
}).join().unwrap();
}

/* Test Clone Trait Migration */
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4 changes: 2 additions & 2 deletions tests/ui/closures/2229_closure_analysis/migrations/auto_traits.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -19,7 +19,7 @@ LL ~ thread::spawn(move || { let _ = &fptr; unsafe {
LL |
...
LL |
LL ~ } });
LL ~ } }).join().unwrap();
|

error: changes to closure capture in Rust 2021 will affect which traits the closure implements
Expand All @@ -41,7 +41,7 @@ LL ~ thread::spawn(move || { let _ = &fptr; unsafe {
LL |
...
LL |
LL ~ } });
LL ~ } }).join().unwrap();
|

error: changes to closure capture in Rust 2021 will affect drop order and which traits the closure implements
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2 changes: 1 addition & 1 deletion tests/ui/closures/2229_closure_analysis/migrations/multi_diagnostics.fixed
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Original file line number Diff line number Diff line change
Expand Up @@ -145,7 +145,7 @@ fn test_multi_traits_issues() {
//~^ NOTE: in Rust 2018, this closure captures all of `fptr1`, but in Rust 2021, it will only capture `fptr1.0.0`
*fptr2.0 = 20;
//~^ NOTE: in Rust 2018, this closure captures all of `fptr2`, but in Rust 2021, it will only capture `fptr2.0`
} });
} }).join().unwrap();
}

fn main() {
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2 changes: 1 addition & 1 deletion tests/ui/closures/2229_closure_analysis/migrations/multi_diagnostics.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -141,7 +141,7 @@ fn test_multi_traits_issues() {
//~^ NOTE: in Rust 2018, this closure captures all of `fptr1`, but in Rust 2021, it will only capture `fptr1.0.0`
*fptr2.0 = 20;
//~^ NOTE: in Rust 2018, this closure captures all of `fptr2`, but in Rust 2021, it will only capture `fptr2.0`
});
}).join().unwrap();
}

fn main() {
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2 changes: 1 addition & 1 deletion tests/ui/closures/2229_closure_analysis/migrations/multi_diagnostics.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -111,7 +111,7 @@ LL ~ thread::spawn(move || { let _ = (&fptr1, &fptr2); unsafe {
LL |
...
LL |
LL ~ } });
LL ~ } }).join().unwrap();
|

error: aborting due to 5 previous errors
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5 changes: 4 additions & 1 deletion tests/ui/consts/const-eval/issue-91827-extern-types.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -28,7 +28,10 @@ pub struct ListImpl<T, const N: usize> {

impl<T> List<T> {
const fn as_slice(&self) -> &[T] {
unsafe { std::slice::from_raw_parts(self.data.as_ptr(), self.len) }
unsafe {
let ptr = addr_of!(self.tail) as *const T;
std::slice::from_raw_parts(ptr, self.len)
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What is this even doing? It looks like the original code would run into the &Header issue, but that issue doesn't have a work-around in Stacked Borrows...

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@saethlin saethlin Feb 15, 2023 •
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CI is failing because of padding that I failed to account for in my poorly-written, manual, offset_of!.

I don't entirely follow your reasoning, but I think the code is UB before we even get to where you are seeing the &Header pattern. The slice can't be constructed because the array is length 0:

error: Undefined Behavior: trying to retag from <3086> for SharedReadOnly permission at alloc1[0x8], but that tag does not exist in the borrow stack for this location
   --> /home/ben/rust/library/core/src/slice/raw.rs:100:9
    |
100 |         &*ptr::slice_from_raw_parts(data, len)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |         |
    |         trying to retag from <3086> for SharedReadOnly permission at alloc1[0x8], but that tag does not exist in the borrow stack for this location
    |         this error occurs as part of retag at alloc1[0x8..0x38]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <3086> would have been created here, but this is a zero-size retag ([0x8..0x8]) so the tag in question does not exist anywhere
   --> tests/ui/consts/const-eval/issue-91827-extern-types.rs:31:45
    |
31  |         unsafe { std::slice::from_raw_parts(self.data.as_ptr(), self.len) }
    |                                             ^^^^^^^^^^^^^^^^^^

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I added an even more horrifying implementation that passes

x run miri --args tests/ui/consts/const-eval/issue-91827-extern-types.rs --target=wasm32-unknown-unknown

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@RalfJung RalfJung Feb 15, 2023 •
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The empty array is the header, isn't it?

pub struct List<T> {
    len: usize,
    data: [T; 0],
    tail: Opaque,
}

is basically a variable-sized array, serving as the header for ListImpl.

Also doesn't tail_offset further down already compute the offset you need?

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Ah I entirely forgot that I made &Header work with extern types, so what is what is happening here.

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I don't quite understand why this is done in the way it is. Is there any value for the test to not just express this as let ptr = (&self.data) as *const T;?

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@RalfJung RalfJung Feb 15, 2023 •
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& restricts provenance to that field, which has size 0, so the resulting reference is only valid for 0 bytes, which is not very useful.

But addr_of!(self.data) should work.

}
}
}

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4 changes: 2 additions & 2 deletions tests/ui/unsized/unsized3-rpass.rs
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Expand Up @@ -59,7 +59,7 @@ pub fn main() {
}

let data: Box<Foo_<i32>> = Box::new(Foo_ { f: [1, 2, 3] });
let x: &Foo<i32> = mem::transmute(slice::from_raw_parts(&*data, 3));
let x: &Foo<i32> = mem::transmute(ptr::slice_from_raw_parts(&*data, 3));
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I don't think I understand why this helps, given that both are transmuted to the same target type?

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The &Foo<i32> is fine. The problem is the slice creation itself, because that call tries to build a slice of 3 elements from a pointer to a Box of 1 element.

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Oh so the temporary slice actually pretends to have 3 Foo_<i32>. Wtf, why? At that point one might just transmute a (ptr, usize) directly without going through from_raw_parts...

Anyway, patch looks good.

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@saethlin saethlin Feb 13, 2023 •
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transmute a (ptr, usize)

That would require relying on the still-unstable slice layout 😉

But yeah I have no idea why anyone would want to write the code in this test. Considering the amount of code in the SIMD tests that were using a pointer to the first element as a pointer to the whole array it might not hurt for people to skim over the tests for some feature or another from time to time...

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@asquared31415 asquared31415 Feb 13, 2023 •
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It's possible to make a reference to a DST without relying on transmute or assuming that any two slice-like fat pointers have the same layout by using the fact that as preserves metadata. playground example

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@saethlin saethlin Feb 13, 2023 •
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Yeah that's the sort of thing I would advise someone write, but this is a test... so I am not really sure what it is supposed to test. So I'm preserving as much of it as I can in the hope that works.

assert_eq!(x.f.len(), 3);
assert_eq!(x.f[0], 1);

Expand All @@ -70,7 +70,7 @@ pub fn main() {

let data: Box<_> =
Box::new(Baz_ { f1: 42, f2: ['a' as u8, 'b' as u8, 'c' as u8, 'd' as u8, 'e' as u8] });
let x: &Baz = mem::transmute(slice::from_raw_parts(&*data, 5));
let x: &Baz = mem::transmute(ptr::slice_from_raw_parts(&*data, 5));
assert_eq!(x.f1, 42);
let chs: Vec<char> = x.f2.chars().collect();
assert_eq!(chs.len(), 5);
Expand Down