#5879: mkWeak# has a very dangerous type
---------------------------------+------------------------------------------
Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 7.6.1
Component: Compiler | Version: 7.4.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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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) }
}}}
}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5879>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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