Undefined Behavior in some decomposition methods on SMatrix #1520
Description
I have some code that uses this library and was hitting undefined behavior as reported by Miri, and I was able to simplify it to this simple example that produces UB without any use of unsafe:
fn main() {
let mut mat = nalgebra::SMatrix::<f32, 2, 2>::zeros_generic(nalgebra::Const, nalgebra::Const);
mat[(0, 0)] = 1.0;
mat[(1, 1)] = 1.0;
println!("{:?}", mat.cholesky());
}When run under cargo +nightly miri run, it reports this output:
error: Undefined Behavior: trying to retag from <2370> for Unique permission at alloc642[0xc], but that tag does not exist in the borrow stack for this location
--> /home/jarred/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/slice/raw.rs:192:9
|
192 | &mut *ptr::slice_from_raw_parts_mut(data, len)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| |
| trying to retag from <2370> for Unique permission at alloc642[0xc], but that tag does not exist in the borrow stack for this location
| this error occurs as part of retag at alloc642[0xc..0x10]
|
= 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: <2370> was created by a SharedReadWrite retag at offsets [0x0..0x10]
--> src/main.rs:5:22
|
5 | println!("{:?}", mat.cholesky());
| ^^^^^^^^^^^^^^
help: <2370> was later invalidated at offsets [0x0..0x10] by a Unique retag
--> src/main.rs:5:22
|
5 | println!("{:?}", mat.cholesky());
| ^^^^^^^^^^^^^^
= note: BACKTRACE (of the first span):
= note: inside `std::slice::from_raw_parts_mut::<'_, f32>` at /home/jarred/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/slice/raw.rs:192:9: 192:55
= note: inside `<nalgebra::ViewStorageMut<'_, f32, nalgebra::Dyn, nalgebra::Const<1>, nalgebra::Const<1>, nalgebra::Const<2>> as nalgebra::RawStorageMut<f32, nalgebra::Dyn>>::as_mut_slice_unchecked` at /home/jarred/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/nalgebra-0.33.2/src/base/matrix_view.rs:256:13: 256:56
= note: inside `nalgebra::base::blas_uninit::axcpy_uninit::<nalgebra::uninit::Init, f32, nalgebra::Dyn, nalgebra::Dyn, nalgebra::ViewStorageMut<'_, f32, nalgebra::Dyn, nalgebra::Const<1>, nalgebra::Const<1>, nalgebra::Const<2>>, nalgebra::ViewStorage<'_, f32, nalgebra::Dyn, nalgebra::Const<1>, nalgebra::Const<1>, nalgebra::Const<2>>>` at /home/jarred/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/nalgebra-0.33.2/src/base/blas_uninit.rs:105:13: 105:44
= note: inside `nalgebra::base::blas::<impl nalgebra::Matrix<f32, nalgebra::Dyn, nalgebra::Const<1>, nalgebra::ViewStorageMut<'_, f32, nalgebra::Dyn, nalgebra::Const<1>, nalgebra::Const<1>, nalgebra::Const<2>>>>::axcpy::<nalgebra::Dyn, nalgebra::ViewStorage<'_, f32, nalgebra::Dyn, nalgebra::Const<1>, nalgebra::Const<1>, nalgebra::Const<2>>>` at /home/jarred/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/nalgebra-0.33.2/src/base/blas.rs:300:18: 300:54
= note: inside `nalgebra::base::blas::<impl nalgebra::Matrix<f32, nalgebra::Dyn, nalgebra::Const<1>, nalgebra::ViewStorageMut<'_, f32, nalgebra::Dyn, nalgebra::Const<1>, nalgebra::Const<1>, nalgebra::Const<2>>>>::axpy::<nalgebra::Dyn, nalgebra::ViewStorage<'_, f32, nalgebra::Dyn, nalgebra::Const<1>, nalgebra::Const<1>, nalgebra::Const<2>>>` at /home/jarred/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/nalgebra-0.33.2/src/base/blas.rs:323:9: 323:38
= note: inside `nalgebra::Cholesky::<f32, nalgebra::Const<2>>::new_internal` at /home/jarred/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/nalgebra-0.33.2/src/linalg/cholesky.rs:233:17: 233:65
= note: inside `nalgebra::Cholesky::<f32, nalgebra::Const<2>>::new` at /home/jarred/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/nalgebra-0.33.2/src/linalg/cholesky.rs:196:9: 196:41
= note: inside `nalgebra::linalg::decomposition::<impl nalgebra::Matrix<f32, nalgebra::Const<2>, nalgebra::Const<2>, nalgebra::ArrayStorage<f32, 2, 2>>>::cholesky` at /home/jarred/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/nalgebra-0.33.2/src/linalg/decomposition.rs:260:9: 260:41
note: inside `main`
--> src/main.rs:5:22
|
5 | println!("{:?}", mat.cholesky());
| ^^^^^^^^^^^^^^
note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
error: aborting due to 1 previous error
This seems to happen on the latest release to crates.io (0.33.2) as well as if I use the main branch of this repo. And cholesky() isn't the only method, hessenburg() also encounters UB according to Miri.