memory-leak detector in +RTS -DS fails to track allocations in constructors

$ cat Hello.hs
module Main where

main = print "Hello world"

$ ghc -debug -dynamic  --make Hello.hs && ./Hello +RTS -DS
Linking Hello ...
"Hello world"
Warning: Freeing non-allocated memory at 0x1ff87850
Warning: Freeing non-allocated memory at 0x1ff85820
Warning: Freeing non-allocated memory at 0x1ff85010
Warning: Freeing non-allocated memory at 0x1ff89860
Warning: Freeing non-allocated memory at 0x1ff8b860
Warning: 192 bytes at 0x1ff9b920 still allocated at shutdown
Warning: 2048 bytes at 0x1ff8ef50 still allocated at shutdown
Warning: 48 bytes at 0x1ff8eef0 still allocated at shutdown
Warning: 2048 bytes at 0x1ff8e6c0 still allocated at shutdown
Warning: 48 bytes at 0x1ff8e660 still allocated at shutdown
Warning: 2048 bytes at 0x1ff8de30 still allocated at shutdown
Warning: 48 bytes at 0x1ff8ddd0 still allocated at shutdown
Warning: 4100 bytes at 0x1ff8cca0 still allocated at shutdown
Warning: 16 bytes at 0x1ff8c980 still allocated at shutdown
Warning: 16 bytes at 0x1ff8c940 still allocated at shutdown
Warning: 4 bytes at 0x1ff8c880 still allocated at shutdown
Warning: 5 bytes at 0x1ff8c840 still allocated at shutdown
Warning: 8 bytes at 0x1ff8c800 still allocated at shutdown
Warning: 32 bytes at 0x1ff8c7b0 still allocated at shutdown

Note the freeing of non-allocated memory. This is because "base" contains a foreign export of forkOS_entry. Foreign exports are desugared to uses of attribute((constructor)) which call getStablePtr (in this case initializing stginit_export_base_ControlziConcurrent_zdfforkOSzuentryzua1bd). On our system this initializer is being run when libHSbase-4.2.0.0-ghc6.12.0.20091010.so gets loaded, which is before the runtime is actually initialized.

This leads to a call to allocHashTable (from initStablePtrTable) which allocates memory. This allocation is recorded in the allocs linked list but when the runtime is initialized initAllocator gets called which overwrites that entry. Hence when we come to free that hash table we free a pointer which is not in the allocs list and get the warning.

I don't get this message with static linking on 6.10 - I can't check 6.12 because there is no static debug runtime for 6.12 RC1 by default. This makes me suspect that this is a peculiarity of shared object initialization, and the call to getStablePtr will happen more lazily with static linking (probably on the first reference to a function in base), such that it comes after RTS initialization is complete.

I observed this behaviour when trying to use -DS to isolate a segfault in Haskell code in a very large program using -dynamic and Haskell shared libraries. I'm not sure if this problem is actually causing my segfault, but it certainly feels like it has the potential to do so, especially as I tend to get my segfaults from Evac.c, which could be affected by the stable ptr table.

Trac metadata

Trac field	Value
Version	6.12.1 RC1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
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CC	batterseapower@hotmail.com, ndmitchell@gmail.com
Operating system
Architecture