Protect Box.unbox from dereferencing null pointer#16514
Merged
straight-shoota merged 2 commits intocrystal-lang:masterfrom Dec 20, 2025
Merged
Protect Box.unbox from dereferencing null pointer#16514straight-shoota merged 2 commits intocrystal-lang:masterfrom
Box.unbox from dereferencing null pointer#16514straight-shoota merged 2 commits intocrystal-lang:masterfrom
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Unboxing a null pointer leads to invalid memory access when the target type is not nilable. We can avoid this by raising an explicit exception. This makes `Box.unbox` a little bit safer. Of course we can still not protect against any other invalid pointer. But null pointers are easily identifiable as invalid, so this is low hanging fruit.
Box.unbox from dereferncing null pointerBox.unbox from dereferencing null pointer
ysbaddaden
reviewed
Dec 18, 2025
spec/std/box_spec.cr
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| Box(Int32?).unbox(Pointer(Void).null).should be_nil | ||
| Box(Int32 | String?).unbox(Pointer(Void).null).should be_nil |
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Thought: I'm wondering if these two cases shouldn't raise; they're mixed unions so Box will allocate them in the HEAP, so a NULL pointer when unboxing should be unexpected.
Box(Nil).box(nil) # => Pointer(Void).null
Box(String?).box(nil) # => Pointer(Void).null
Box(Int32?).box(nil) # => Pointer(Void)@0x7ef22cce1900
Box(Int32 | String | Nil).box(nil) # => Pointer(Void)@0x7ef229049140
```
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Yes, that's another option. I actually wanted to mention that, but apparently I forgot 🙈
The intention of a null pointer is clear, so we could interpret as nil even though it's not a correctly typed value. But I suppose raising might be better because it is really an exceptional condition.
ysbaddaden
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Dec 18, 2025
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Unboxing a null pointer leads to invalid memory access when the target type is not nilable.
We can avoid this by raising an explicit exception. This makes
Box.unboxa little bit safer.Of course we can still not protect against any other invalid pointer. But null pointers are easily identifiable as invalid.
This is low hanging fruit for a bit more implicit memory safety.