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Prevent FixedSizeBinaryArray::value offset truncation #9850

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Prevent FixedSizeBinaryArray::value offset truncation #9850

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15 changes: 8 additions & 7 deletions arrow-array/src/array/fixed_size_binary_array.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -55,7 +55,7 @@ pub struct FixedSizeBinaryArray {
value_data: Buffer,
nulls: Option<NullBuffer>,
len: usize,
value_length: i32,
value_length: i32, // validated to be a valid usize by `try_new`
}

impl FixedSizeBinaryArray {
Expand Down Expand Up @@ -169,12 +169,11 @@ impl FixedSizeBinaryArray {
self.len()
);
let offset = i + self.offset();
let value_length = self.value_length as usize; // validated in Self::try_new
let value_offset = offset.checked_mul(value_length).expect("offset overflow");
// SAFETY: value offset/length computed correctly using checked arithmetic
unsafe {
let pos = self.value_offset_at(offset);
std::slice::from_raw_parts(
self.value_data.as_ptr().offset(pos as isize),
(self.value_offset_at(offset + 1) - pos) as usize,
)
std::slice::from_raw_parts(self.value_data.as_ptr().add(value_offset), value_length)
}
}

Expand All @@ -186,7 +185,7 @@ impl FixedSizeBinaryArray {
/// # Safety
///
/// Caller is responsible for ensuring that the index is within the bounds
/// of the array
/// of the array and the resulting byte offset fits in `i32`
pub unsafe fn value_unchecked(&self, i: usize) -> &[u8] {
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@adamreeve adamreeve Apr 30, 2026

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Would it make sense to add new methods that are alternatives to value_offset and value_offset_at that return usize, so we don't need this limitation? Or at least update this method so it doesn't use them and doesn't suffer from i32 overflow. Because with this change, value now handles when value_offset is greater than max i32, but value_unchecked still doesn't.

I would expect value_unchecked to work correctly for all cases where value doesn't panic.

And existing code that uses value_unchecked might validate the index but not be aware of this hidden safety requirement.

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@alamb alamb Apr 30, 2026

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Short answer is yes. I also spent some more time reviewing the code in FixedSizeBinaryArray and I am now convinced there are several other miuses of i32 <-> usize . I am working on an improvement, though I worry it will be a larger PR

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@alamb alamb May 1, 2026

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let offset = i + self.offset();
let pos = self.value_offset_at(offset);
Expand Down Expand Up @@ -486,6 +485,8 @@ impl FixedSizeBinaryArray {
})
}

/// Returns the byte offset for the element at index `i` without
/// checking for overflow.
#[inline]
fn value_offset_at(&self, i: usize) -> i32 {
self.value_length * i as i32
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@alamb alamb Apr 29, 2026

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this arithmetic can overflow if the result is larger than i32::MAX

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