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Rollup merge of #122537 - RalfJung:interpret-allocation, r=oli-obk

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interpret/allocation: fix aliasing issue in interpreter and refactor getters a bit

That new raw getter will be needed to let Miri pass pointers to natively executed FFI code ("extern-so" mode).

While doing that I realized our get_bytes_mut are named less scary than get_bytes_unchecked so I rectified that. Also I realized `mem_copy_repeatedly` would break if we called it for multiple overlapping copies so I made sure this does not happen.

And I realized that we are actually [violating Stacked Borrows in the interpreter](https://rust-lang.zulipchat.com/#narrow/stream/136281-t-opsem/topic/I.20think.20Miri.20violates.20Stacked.20Borrows.20.F0.9F.99.88).^^ That was introduced in https://github.com/rust-lang/rust/pull/87777.

r? ```@oli-obk```
This commit is contained in:
Matthias Krüger 2024-03-22 11:36:59 +01:00 committed by GitHub
commit 84e55be8da
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2 changed files with 45 additions and 14 deletions
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17
compiler/rustc_const_eval/src/interpret/memory.rs
View File

@ -1159,11 +1159,11 @@ pub fn write_bytes_ptr(
};
// Side-step AllocRef and directly access the underlying bytes more efficiently.
// (We are staying inside the bounds here so all is good.)
// (We are staying inside the bounds here and all bytes do get overwritten so all is good.)
let alloc_id = alloc_ref.alloc_id;
let bytes = alloc_ref
.alloc
.get_bytes_mut(&alloc_ref.tcx, alloc_ref.range)
.get_bytes_unchecked_for_overwrite(&alloc_ref.tcx, alloc_ref.range)
.map_err(move |e| e.to_interp_error(alloc_id))?;
// `zip` would stop when the first iterator ends; we want to definitely
// cover all of `bytes`.
@ -1184,6 +1184,11 @@ pub fn mem_copy(
self.mem_copy_repeatedly(src, dest, size, 1, nonoverlapping)
}
/// Performs `num_copies` many copies of `size` many bytes from `src` to `dest + i*size` (where
/// `i` is the index of the copy).
///
/// Either `nonoverlapping` must be true or `num_copies` must be 1; doing repeated copies that
/// may overlap is not supported.
pub fn mem_copy_repeatedly(
&mut self,
src: Pointer<Option<M::Provenance>>,
@ -1245,8 +1250,9 @@ pub fn mem_copy_repeatedly(
(dest_alloc_id, dest_prov),
dest_range,
)?;
// Yes we do overwrite all bytes in `dest_bytes`.
let dest_bytes = dest_alloc
.get_bytes_mut_ptr(&tcx, dest_range)
.get_bytes_unchecked_for_overwrite_ptr(&tcx, dest_range)
.map_err(|e| e.to_interp_error(dest_alloc_id))?
.as_mut_ptr();
@ -1280,6 +1286,9 @@ pub fn mem_copy_repeatedly(
}
}
}
if num_copies > 1 {
assert!(nonoverlapping, "multi-copy only supported in non-overlapping mode");
}
let size_in_bytes = size.bytes_usize();
// For particularly large arrays (where this is perf-sensitive) it's common that
@ -1292,6 +1301,8 @@ pub fn mem_copy_repeatedly(
} else if src_alloc_id == dest_alloc_id {
let mut dest_ptr = dest_bytes;
for _ in 0..num_copies {
// Here we rely on `src` and `dest` being non-overlapping if there is more than
// one copy.
ptr::copy(src_bytes, dest_ptr, size_in_bytes);
dest_ptr = dest_ptr.add(size_in_bytes);
}

42
compiler/rustc_middle/src/mir/interpret/allocation.rs
View File

@ -37,9 +37,16 @@ pub trait AllocBytes:
/// Create a zeroed `AllocBytes` of the specified size and alignment.
/// Returns `None` if we ran out of memory on the host.
fn zeroed(size: Size, _align: Align) -> Option<Self>;
/// Gives direct access to the raw underlying storage.
///
/// Crucially this pointer is compatible with:
/// - other pointers retunred by this method, and
/// - references returned from `deref()`, as long as there was no write.
fn as_mut_ptr(&mut self) -> *mut u8;
}
// Default `bytes` for `Allocation` is a `Box<[u8]>`.
/// Default `bytes` for `Allocation` is a `Box<u8>`.
impl AllocBytes for Box<[u8]> {
fn from_bytes<'a>(slice: impl Into<Cow<'a, [u8]>>, _align: Align) -> Self {
Box::<[u8]>::from(slice.into())
@ -51,6 +58,11 @@ fn zeroed(size: Size, _align: Align) -> Option<Self> {
let bytes = unsafe { bytes.assume_init() };
Some(bytes)
}
fn as_mut_ptr(&mut self) -> *mut u8 {
// Carefully avoiding any intermediate references.
ptr::addr_of_mut!(**self).cast()
}
}
/// This type represents an Allocation in the Miri/CTFE core engine.
@ -399,10 +411,6 @@ pub fn provenance(&self) -> &ProvenanceMap<Prov> {
/// Byte accessors.
impl<Prov: Provenance, Extra, Bytes: AllocBytes> Allocation<Prov, Extra, Bytes> {
pub fn base_addr(&self) -> *const u8 {
self.bytes.as_ptr()
}
/// This is the entirely abstraction-violating way to just grab the raw bytes without
/// caring about provenance or initialization.
///
@ -452,13 +460,14 @@ pub fn get_bytes_strip_provenance(
Ok(self.get_bytes_unchecked(range))
}
/// Just calling this already marks everything as defined and removes provenance,
/// so be sure to actually put data there!
/// This is the entirely abstraction-violating way to just get mutable access to the raw bytes.
/// Just calling this already marks everything as defined and removes provenance, so be sure to
/// actually overwrite all the data there!
///
/// It is the caller's responsibility to check bounds and alignment beforehand.
/// Most likely, you want to use the `PlaceTy` and `OperandTy`-based methods
/// on `InterpCx` instead.
pub fn get_bytes_mut(
pub fn get_bytes_unchecked_for_overwrite(
&mut self,
cx: &impl HasDataLayout,
range: AllocRange,
@ -469,8 +478,9 @@ pub fn get_bytes_mut(
Ok(&mut self.bytes[range.start.bytes_usize()..range.end().bytes_usize()])
}
/// A raw pointer variant of `get_bytes_mut` that avoids invalidating existing aliases into this memory.
pub fn get_bytes_mut_ptr(
/// A raw pointer variant of `get_bytes_unchecked_for_overwrite` that avoids invalidating existing immutable aliases
/// into this memory.
pub fn get_bytes_unchecked_for_overwrite_ptr(
&mut self,
cx: &impl HasDataLayout,
range: AllocRange,
@ -479,10 +489,19 @@ pub fn get_bytes_mut_ptr(
self.provenance.clear(range, cx)?;
assert!(range.end().bytes_usize() <= self.bytes.len()); // need to do our own bounds-check
// Cruciall, we go via `AllocBytes::as_mut_ptr`, not `AllocBytes::deref_mut`.
let begin_ptr = self.bytes.as_mut_ptr().wrapping_add(range.start.bytes_usize());
let len = range.end().bytes_usize() - range.start.bytes_usize();
Ok(ptr::slice_from_raw_parts_mut(begin_ptr, len))
}
/// This gives direct mutable access to the entire buffer, just exposing their internal state
/// without reseting anything. Directly exposes `AllocBytes::as_mut_ptr`. Only works if
/// `OFFSET_IS_ADDR` is true.
pub fn get_bytes_unchecked_raw_mut(&mut self) -> *mut u8 {
assert!(Prov::OFFSET_IS_ADDR);
self.bytes.as_mut_ptr()
}
}
/// Reading and writing.
@ -589,7 +608,8 @@ pub fn write_scalar(
};
let endian = cx.data_layout().endian;
let dst = self.get_bytes_mut(cx, range)?;
// Yes we do overwrite all the bytes in `dst`.
let dst = self.get_bytes_unchecked_for_overwrite(cx, range)?;
write_target_uint(endian, dst, bytes).unwrap();
// See if we have to also store some provenance.