mkWeak# has a very dangerous type
The mkWeak#
primop has a type looking like this:
primop MkWeakOp "mkWeak#" GenPrimOp
o -> b -> c -> State# RealWorld -> (# State# RealWorld, Weak# b #)
But the third argument is allowed to be a null pointer (indicating no finaliser). That is why we see this code in GHC.TopHandler
:
mkWeakThreadId :: ThreadId -> IO (Weak ThreadId)
mkWeakThreadId t@(ThreadId t#) = IO $ \s ->
case mkWeak# t# t (unsafeCoerce# 0#) s of
(# s1, w #) -> (# s1, Weak w #)
But coercing a non-pointer to a pointer is dangerous. Very dangerous. Could crash the garbage collector. This is Not Right.
I found this when tidying up Core Lint. For now I have commented out the extra test, but it should be there!
lintCoercion (UnsafeCo ty1 ty2)
= do { k1 <- lintType ty1
; _k2 <- lintType ty2
-- ; unless (k1 `eqKind` k2) $
-- failWithL (hang (ptext (sLit "Unsafe coercion changes kind"))
-- 2 (ppr co))
; return (k1, ty1, ty2) }
}
Trac metadata
Trac field | Value |
---|---|
Version | 7.4.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |