Help! An invalid closure

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A walk-through debugging session of GHC runtime system

This is an attempt at documenting one of my sessions debugging [GHC’s] runtime system. In particular, I’ll try to write down some of the background I’ve accrued bringing up GHC on the ARM architecture. However, most of the below should be largely architecture-agnostic.

The symptom

When debugging the GHC runtime system (RTS), one is often faced with errors of the form,

test: internal error: invalid closure, info=0x122a8
(GHC version for arm_unknown_linux)
Please report this as a GHC bug:

This error is produced by GHC’s garbage collector. In particular, the garbage collector is trying to tell you that the runtime system has encountered an unrecognized closure type while traversing the heap, a likely sign of corruption.

Finding closure(s)

The first thing I do when faced with such a crash is to recompile the code in question with GHC’s -debug flag, linking against the debug build of the runtime system. After running the resulting executable in gdb, you’ll likely find a backtrace containing frames similar to frames 0 through 11 of the following,

(gdb) bt
#0  __libc_do_syscall () at ../ports/sysdeps/unix/sysv/linux/arm/libc-do-syscall.S:44
#1  0xb64035fe in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
#2  0xb6405d9a in __GI_abort () at abort.c:90
#3  0xb65dce48 in rtsFatalInternalErrorFn (s=0xb6615b74 "invalid closure, info=%p", ap=...) at rts/RtsMessages.c:170
#4  0xb65dcc22 in barf (s=0xb6615b74 "invalid closure, info=%p") at rts/RtsMessages.c:42
#5  0xb65ec8ba in evacuate (p=0xb6fd6418 <SgDT_srt+88>) at rts/sm/Evac.c:384
#6  0xb65f2c1a in scavenge_srt (srt=0xb6fd63f4 <SgDT_srt+52>, srt_bitmap=1793) at rts/sm/Scav.c:332
#7  0xb65f2caa in scavenge_fun_srt (info=0xb6e8604c <base_SystemziPosixziInternals_fdFileSizze1_info_itable+8>) at rts/sm/Scav.c:360
#8  0xb65f458a in scavenge_static () at rts/sm/Scav.c:1572
#9  0xb65f4aae in scavenge_loop () at rts/sm/Scav.c:1919
#10 0xb65eea4a in scavenge_until_all_done () at rts/sm/GC.c:995
#11 0xb65edc5c in GarbageCollect (collect_gen=1, do_heap_census=rtsFalse, gc_type=0, cap=0xb661b540 <MainCapability>) at rts/sm/GC.c:393
#12 0xb65df046 in scheduleDoGC (pcap=0xbefff4e8, task=0x1c758, force_major=rtsTrue) at rts/Schedule.c:1652
#13 0xb65df8a0 in exitScheduler (wait_foreign=rtsFalse) at rts/Schedule.c:2466
#14 0xb65dd2f2 in hs_exit_ (wait_foreign=rtsFalse) at rts/RtsStartup.c:336
#15 0xb65dd42a in shutdownHaskellAndExit (n=0, fastExit=0) at rts/RtsStartup.c:466
#16 0xb65dcb9a in real_main () at rts/RtsMain.c:87
#17 0xb65dcbf2 in hs_main (argc=1, argv=0xbefff6e4, main_closure=0x12268 <ZCMain_main_closure>, rts_config=...) at rts/RtsMain.c:114
#18 0x00009e8e in main ()

Looking in rts/sm/Eval.c, we find the assertion in question,

ASSERTM(LOOKS_LIKE_CLOSURE_PTR(q), "invalid closure, info=%p", q->;

Here q is a pointer to an StgClosure, GHC’s representation of an STG closure. We find the definition of LOOKS_LIKE_CLOSURE_PTR in includes/rts/storage/ClosureMacros.h,

static inline StgClosure *
UNTAG_CLOSURE(StgClosure * p)
    return (StgClosure*)((StgWord)p & ~TAG_MASK);


    return LOOKS_LIKE_INFO_PTR((StgWord)(UNTAG_CLOSURE((StgClosure *)(p)))->;

Some background may be in order here. Since accessing unaligned addresses is frowned upon in general, the RTS claims the bottom few bits (2 or 3 depending upon the architecture’s word size) of pointers to encode a tag. In the case of function references the tag encodes the function’s arity whereas in the case of data the tag contains the data constructor. Note that this is merely an optimization; in the event that the arity or constructor doesn’t fit in the tag the compiler automatically falls back to storing this information in the object itself.

So, LOOKS_LIKE_CLOSURE_PTR deferences the pointer (after removing the low-order tag bits), and performs a sanity check on its info table pointer. This sanity check is also defined in ClosureMacros.h,

#define IS_FORWARDING_PTR(p) ((((StgWord)p) & 1) != 0)

    StgInfoTable *info = INFO_PTR_TO_STRUCT((StgInfoTable *)p);
    return (info->type != INVALID_OBJECT && info->type < N_CLOSURE_TYPES) ? rtsTrue : rtsFalse;

    return (p && (IS_FORWARDING_PTR(p) || LOOKS_LIKE_INFO_PTR_NOT_NULL(p))) ? rtsTrue : rtsFalse;

This is, a valid info table pointer can’t be NULL and must be either,

  1. A forwarding pointer, having the low order bit set, or
  2. A valid info table pointer, therefore
    1. Is not of type INVALID_OBJECT (0), and
    2. Has a type in the valid range of closure types (defined in includes/rts/storage/ClosureTypes.h; in GHC 7.8 N_CLOSURE_TYPES == 61)

If we look at the definition of INFO_PTR_TO_STRUCT we will notice there is a slight subtly here, however,

EXTERN_INLINE StgInfoTable *INFO_PTR_TO_STRUCT(const StgInfoTable *info) {return (StgInfoTable *)info - 1;}
EXTERN_INLINE StgInfoTable *INFO_PTR_TO_STRUCT(const StgInfoTable *info) {return (StgInfoTable *)info;}

This is where GHC handles tables-next-to-code, a memory-layout optimization GHC performs to keep info tables local to the objects they describe. We should keep this in mind as we traverse the heap.

In this example, the object in question is,

(gdb) print *q
$5 = {header = {info = 0x122a8 <integerzmgmp_GHCziIntegerziType_Szh_static_info>},
      payload = 0xb6fb7ab4 <base_SystemziPosixziInternals_fdFileSizze2_closure+4>}

We see here that the object in question is an Integer, namely the S# constructor from integer-gmp,

data Integer
   = S# Int# -- ^ \"small\" integers fitting into an 'Int#'
   | J# Int# ByteArray# -- ^ \"big\" integers represented as GMP's @mpz_t@ structure.

The fact that the constructor’s field is of type Int# tells us that the payload should be an integer,

(gdb) print *(int *) q->payload
$14 = -1

q-> is not NULL so let’s look at the info table itself (keeping in mind tables-next-to-code),

(gdb) print q->[-1]
$3 = {layout = {payload = {ptrs = 50404, nptrs = 46814},
                bitmap = 3068052708,
                large_bitmap_offset = -1226914588,
                selector_offset = 3068052708},
      type = 0,
      srt_bitmap = 0, 
      code = 0x122a8 <integerzmgmp_GHCziIntegerziType_Szh_static_info> ""}

While type == INVALID_OBJECT is clearly concerning, it is unclear whether the rest of this is reasonable. With a simple test program on my x86-64 box we find that a healthy-looking integerzmgmp_GHCziIntegerziType_Szh_static_info should look something like,

(gdb) print * ( (StgInfoTable *) integerzmgmp_GHCziIntegerziType_Szh_static_info - 1)
$3 = {layout = {payload = {ptrs = 0, nptrs = 1},
                bitmap = 4294967296,
                large_bitmap_offset = 0,
                __pad_large_bitmap_offset = 0,
                selector_offset = 4294967296},
      type = 8,
      srt_bitmap = 0, 
      code = 0x455990 <integerzmgmp_GHCziIntegerziType_Szh_static_info> "H\377\303\377e"}

Let’s now check the shared library to see what it says,

(gdb) file libraries/base/dist-install/build/ 
A program is being debugged already.
Are you sure you want to change the file? (y or n) y
Load new symbol table from "/home/ben/ghc/ghc/libraries/base/dist-install/build/"? (y or n) y
Reading symbols from /home/ben/ghc/ghc/libraries/base/dist-install/build/ debugging symbols found)...done.
(gdb) print * ( (StgInfoTable *) integerzmgmp_GHCziIntegerziType_Szh_static_info - 1)
$8 = {layout = {payload = {ptrs = 0, nptrs = 1},
                bitmap = 65536,
                large_bitmap_offset = 65536,
                selector_offset = 65536},
      type = 8,
      srt_bitmap = 0, 
      code = 0xb66f9dac <integerzmgmp_GHCziIntegerziType_Szh_static_info> ""}

Examining the core

However, I did notice at the outset that the code in question works without -O,

$ inplace/bin/ghc-stage2 testsuite/tests/boxy/T2193.hs -o test -dynamic -fforce-recomp
[1 of 1] Compiling Main             ( testsuite/tests/boxy/T2193.hs, testsuite/tests/boxy/T2193.o )
Linking test ...
$ ./test
$ inplace/bin/ghc-stage2 testsuite/tests/boxy/T2193.hs -o test -dynamic -fforce-recomp -O
[1 of 1] Compiling Main             ( testsuite/tests/boxy/T2193.hs, testsuite/tests/boxy/T2193.o )
Linking test ...
$ ./test
test: internal error: evacuate(static): strange closure type 0
    (GHC version for arm_unknown_linux)
    Please report this as a GHC bug:

Let’s have a look at the core,

$ inplace/bin/ghc-stage2 testsuite/tests/boxy/T2193.hs -o test -dynamic -O -fforce-recomp -ddump-simpl -dsuppress-idinfo -dsuppress-coercions -dsuppress-type-applications >test.o1.core
$ inplace/bin/ghc-stage2 testsuite/tests/boxy/T2193.hs -o test -dynamic -O0 -fforce-recomp -ddump-simpl -dsuppress-idinfo -dsuppress-coercions -dsuppress-type-applications >test.o0.core

Work in progress

This is about as far as I have made it so far.