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Don't try to force_ptr pointers to zsts #68088

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Jan 13, 2020
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Don't try to force_ptr pointers to zsts #68088

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11 changes: 8 additions & 3 deletions src/librustc_mir/interpret/validity.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -608,9 +608,14 @@ impl<'rt, 'mir, 'tcx, M: Machine<'mir, 'tcx>> ValueVisitor<'mir, 'tcx, M>
return Ok(());
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}
// This is the element type size.
let ty_size = self.ecx.layout_of(tys)?.size;
let layout = self.ecx.layout_of(tys)?;
// Empty tuples and fieldless structs (the only ZSTs that allow reaching this code)
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This makes it very odd that we have them in the same match arm as integers, given that we handle them entirely separate... we could have a match arm for these ZST that just doesn't do anything, I think. But that is a pre-existing condition, so we can leave that for later and fix Miri now.

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well.. with the check_mplace_access we additionally get a check ensuring that the zst is correctly aligned (due to the early abort we're not checking this anymore), so if we want to keep doing such checks we're going to have to either duplicate all the logic in a struct/tuple arm or we have to intermingle it here as I did earlier with check_mplace_access.

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IIRC validity checking assumes that the place itself is aligned and dereferencable? For references, we check those before adding them to the TODO list.

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@RalfJung RalfJung Jan 13, 2020 •
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We definitely assume it to be dereferencable:

rust/src/librustc_mir/interpret/validity.rs

Line 668 in 19b9b26

/// This function checks the data at `op`. `op` is assumed to cover valid memory if it

I don't think we check alignment elsewhere... and that's okay, we want to use this to validate packed structs as well!

// have no data to be checked.
if layout.is_zst() {
return Ok(());
}
// This is the size in bytes of the whole array.
let size = ty_size * len;
let size = layout.size * len;
// Size is not 0, get a pointer.
let ptr = self.ecx.force_ptr(mplace.ptr)?;

Expand Down Expand Up @@ -640,7 +645,7 @@ impl<'rt, 'mir, 'tcx, M: Machine<'mir, 'tcx>> ValueVisitor<'mir, 'tcx, M>
// Some byte was undefined, determine which
// element that byte belongs to so we can
// provide an index.
let i = (offset.bytes() / ty_size.bytes()) as usize;
let i = (offset.bytes() / layout.size.bytes()) as usize;
self.path.push(PathElem::ArrayElem(i));

throw_validation_failure!("undefined bytes", self.path)
Expand Down