rts: Refactor foreign export tracking

This avoids calling `libc` in the initializers which are responsible for
registering foreign exports. We believe this should avoid the corruption
observed in #18548.

See Note [Tracking foreign exports] in rts/ForeignExports.c for an
overview of the new scheme.

9 files changed, 209 insertions, 45 deletions

diff --git a/compiler/GHC/HsToCore/Foreign/Decl.hs b/compiler/GHC/HsToCore/Foreign/Decl.hs
index 08cfc7aee6..3c46ef5cf9 100644
--- a/compiler/GHC/HsToCore/Foreign/Decl.hs
+++ b/compiler/GHC/HsToCore/Foreign/Decl.hs
@@ -91,15 +91,16 @@ dsForeigns' :: [LForeignDecl GhcTc]
 dsForeigns' []
   = return (NoStubs, nilOL)
 dsForeigns' fos = do
+    mod <- getModule
     fives <- mapM do_ldecl fos
     let
         (hs, cs, idss, bindss) = unzip4 fives
         fe_ids = concat idss
-        fe_init_code = map foreignExportInitialiser fe_ids
+        fe_init_code = foreignExportsInitialiser mod fe_ids
     --
     return (ForeignStubs
              (vcat hs)
-             (vcat cs $$ vcat fe_init_code),
+             (vcat cs $$ fe_init_code),
             foldr (appOL . toOL) nilOL bindss)
   where
    do_ldecl (L loc decl) = putSrcSpanDs loc (do_decl decl)
@@ -700,8 +701,8 @@ mkFExportCBits dflags c_nm maybe_target arg_htys res_hty is_IO_res_ty cc
      ]
 
 
-foreignExportInitialiser :: Id -> SDoc
-foreignExportInitialiser hs_fn =
+foreignExportsInitialiser :: Module -> [Id] -> SDoc
+foreignExportsInitialiser mod hs_fns =
    -- Initialise foreign exports by registering a stable pointer from an
    -- __attribute__((constructor)) function.
    -- The alternative is to do this from stginit functions generated in
@@ -710,14 +711,24 @@ foreignExportInitialiser hs_fn =
    -- all modules that are imported directly or indirectly are actually used by
    -- the program.
    -- (this is bad for big umbrella modules like Graphics.Rendering.OpenGL)
+   --
+   -- See Note [Tracking foreign exports] in rts/ForeignExports.c
    vcat
-    [ text "static void stginit_export_" <> ppr hs_fn
-         <> text "() __attribute__((constructor));"
-    , text "static void stginit_export_" <> ppr hs_fn <> text "()"
-    , braces (text "foreignExportStablePtr"
-       <> parens (text "(StgPtr) &" <> ppr hs_fn <> text "_closure")
-       <> semi)
+    [ text "static struct ForeignExportsList" <+> list_symbol <+> equals
+         <+> braces (text ".exports = " <+> export_list) <> semi
+    , text "static void " <> ctor_symbol <> text "(void)"
+         <+> text " __attribute__((constructor));"
+    , text "static void " <> ctor_symbol <> text "()"
+    , braces (text "registerForeignExports" <> parens (char '&' <> list_symbol) <> semi)
     ]
+  where
+    mod_str = pprModuleName (moduleName mod)
+    ctor_symbol = text "stginit_export_" <> mod_str
+    list_symbol = text "stg_exports_" <> mod_str
+    export_list = braces $ pprWithCommas closure_ptr hs_fns
+
+    closure_ptr :: Id -> SDoc
+    closure_ptr fn = text "(StgPtr) &" <> ppr fn <> text "_closure"
 
 
 mkHObj :: Type -> SDoc
diff --git a/includes/Rts.h b/includes/Rts.h
index 989394b590..589ef8b82c 100644
--- a/includes/Rts.h
+++ b/includes/Rts.h
@@ -212,6 +212,9 @@ void _assertFail(const char *filename, unsigned int linenum)
 #include "rts/storage/GC.h"
 #include "rts/NonMoving.h"
 
+/* Foreign exports */
+#include "rts/ForeignExports.h"
+
 /* Other RTS external APIs */
 #include "rts/Parallel.h"
 #include "rts/Signals.h"
diff --git a/includes/rts/ForeignExports.h b/includes/rts/ForeignExports.h
new file mode 100644
index 0000000000..f8828e59d4
--- /dev/null
+++ b/includes/rts/ForeignExports.h
@@ -0,0 +1,36 @@
+/* -----------------------------------------------------------------------------
+ *
+ * (c) The GHC Team, 1995-2009
+ *
+ * Interface to the RTS's foreign export tracking code.
+ *
+ * Do not #include this file directly: #include "Rts.h" instead.
+ *
+ * To understand the structure of the RTS headers, see the wiki:
+ *   https://gitlab.haskell.org/ghc/ghc/wikis/commentary/source-tree/includes
+ *
+ * ---------------------------------------------------------------------------*/
+
+#pragma once
+
+struct _ObjectCode;
+
+/* N.B. See Note [Tracking foreign exports] in
+ * rts/ForeignExports.c. */
+struct ForeignExportsList {
+      /* a link field for linking these together into lists.
+       */
+    struct ForeignExportsList *next;
+      /* the length of ->exports */
+    int n_entries;
+      /* if the RTS linker loaded the module,
+       * to which ObjectCode these exports belong. */
+    struct _ObjectCode *oc;
+      /* if the RTS linker loaded the module,
+       * this points to an array of length ->n_entries
+       * recording the StablePtr for each export. */
+    StgPtr exports[];
+};
+
+void registerForeignExports(struct ForeignExportsList *exports);
+
diff --git a/rts/ForeignExports.c b/rts/ForeignExports.c
new file mode 100644
index 0000000000..3195b6e779
--- /dev/null
+++ b/rts/ForeignExports.c
@@ -0,0 +1,113 @@
+/* -----------------------------------------------------------------------------
+ *
+ * (c) The GHC Team 2020
+ *
+ * Management of foreign exports.
+ *
+ * ---------------------------------------------------------------------------*/
+
+#include "Rts.h"
+#include "RtsUtils.h"
+#include "ForeignExports.h"
+
+/* protected by linker_mutex after start-up */
+static struct ForeignExportsList *pending = NULL;
+static ObjectCode *loading_obj = NULL;
+
+/*
+ * Note [Tracking foreign exports]
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * Foreign exports are garbage collection roots. That is, things (e.g. CAFs)
+ * depended upon by a module's `foreign export`s need to be kept alive for as
+ * long an module is loaded. To ensure this we create a stable pointer to each
+ * `foreign export`'d closure. This works as follows:
+ *
+ * 1. The compiler  (namely GHC.HsToCore.Foreign.Decl.foreignExports)
+ *    inserts a C-stub into each module containing a `foreign export`. This
+ *    stub contains two things:
+ *
+ *    - A `ForeignExportsList` listing all of the exported closures, and
+ *
+ *    - An initializer which calls `registerForeignExports` with a reference to
+ *      the `ForeignExportsList`.
+ *
+ * 2. When the module's object code is loaded, its initializer is called by the
+ *    linker (this might be the system's dynamic linker or GHC's own static
+ *    linker). `registerForeignExports` then places the module's
+ *    `ForeignExportsList` on `pending` list.
+ *
+ * 3. When loading has finished (e.g. during RTS initialization or at the end
+ *    of `Linker.c:ocTryLoad`) `processForeignExports` is called. Here we
+ *    traverse the `pending` list and create a `StablePtr` for each export
+ *    therein.
+ *
+ * The reason for this two-step process is that we are very restricted in what
+ * we can do in an initializer function. For instance, we cannot necessarily
+ * call `malloc`  since the `libc`'s own initializer may not have run yet.
+ * For instance, doing exactly this resulted in #18548.
+ *
+ * Another consideration here is that the linker needs to know which
+ * `StablePtr`s belong to each `ObjectCode` it loads for the sake of unloading.
+ * For this reason, the linker informs us when it is loading an object by calling
+ * `foreignExportsLoadingObject` and `foreignExportsFinishedLoadingObject`. We
+ * take note of the `ObjectCode*` we are loading in `loading_obj` such that we
+ * can associate the `StablePtr` with the `ObjectCode` in
+ * `processForeignExports`.
+ *
+ */
+
+void registerForeignExports(struct ForeignExportsList *exports)
+{
+    ASSERT(exports->next == NULL);
+    ASSERT(exports->oc == NULL);
+    exports->next = pending;
+    exports->oc = loading_obj;
+    pending = exports;
+}
+
+/* -----------------------------------------------------------------------------
+   Create a StablePtr for a foreign export.  This is normally called by
+   a C function with __attribute__((constructor)), which is generated
+   by GHC and linked into the module.
+
+   If the object code is being loaded dynamically, then we remember
+   which StablePtrs were allocated by the constructors and free them
+   again in unloadObj().
+   -------------------------------------------------------------------------- */
+
+void foreignExportsLoadingObject(ObjectCode *oc)
+{
+    ASSERT(loading_obj == NULL);
+    loading_obj = oc;
+}
+
+void foreignExportsFinishedLoadingObject()
+{
+    ASSERT(loading_obj != NULL);
+    loading_obj = NULL;
+    processForeignExports();
+}
+
+/* Caller must own linker_mutex so that we can safely modify
+ * oc->stable_ptrs. */
+void processForeignExports()
+{
+    while (pending) {
+        for (int i=0; i < pending->n_entries; i++) {
+            StgPtr p = pending->exports[i];
+            StgStablePtr *sptr = getStablePtr(p);
+
+            if (loading_obj != NULL) {
+                ForeignExportStablePtr *fe_sptr = (ForeignExportStablePtr *)
+                  stgMallocBytes(sizeof(ForeignExportStablePtr),
+                                 "foreignExportStablePtr");
+                fe_sptr->stable_ptr = sptr;
+                fe_sptr->next = loading_obj->stable_ptrs;
+                pending->oc->stable_ptrs = fe_sptr;
+            }
+        }
+
+        pending = pending->next;
+    }
+}
diff --git a/rts/ForeignExports.h b/rts/ForeignExports.h
new file mode 100644
index 0000000000..84bf3bc484
--- /dev/null
+++ b/rts/ForeignExports.h
@@ -0,0 +1,21 @@
+/* -----------------------------------------------------------------------------
+ *
+ * (c) The GHC Team 2020
+ *
+ * Management of foreign exports.
+ *
+ * ---------------------------------------------------------------------------*/
+
+#pragma once
+
+#include "Rts.h"
+#include "LinkerInternals.h"
+
+#include "BeginPrivate.h"
+
+void foreignExportsLoadingObject(ObjectCode *oc);
+void foreignExportsFinishedLoadingObject(void);
+void processForeignExports(void);
+
+#include "EndPrivate.h"
+
diff --git a/rts/Linker.c b/rts/Linker.c
index a8ab511719..fa38480fb6 100644
--- a/rts/Linker.c
+++ b/rts/Linker.c
@@ -26,6 +26,7 @@
 #include "RtsSymbols.h"
 #include "RtsSymbolInfo.h"
 #include "Profiling.h"
+#include "ForeignExports.h"
 #include "sm/OSMem.h"
 #include "linker/M32Alloc.h"
 #include "linker/CacheFlush.h"
@@ -969,37 +970,6 @@ SymbolAddr* lookupSymbol( SymbolName* lbl )
 }
 
 /* -----------------------------------------------------------------------------
-   Create a StablePtr for a foreign export.  This is normally called by
-   a C function with __attribute__((constructor)), which is generated
-   by GHC and linked into the module.
-
-   If the object code is being loaded dynamically, then we remember
-   which StablePtrs were allocated by the constructors and free them
-   again in unloadObj().
-   -------------------------------------------------------------------------- */
-
-static ObjectCode *loading_obj = NULL;
-
-StgStablePtr foreignExportStablePtr (StgPtr p)
-{
-    ForeignExportStablePtr *fe_sptr;
-    StgStablePtr *sptr;
-
-    sptr = getStablePtr(p);
-
-    if (loading_obj != NULL) {
-        fe_sptr = stgMallocBytes(sizeof(ForeignExportStablePtr),
-                                 "foreignExportStablePtr");
-        fe_sptr->stable_ptr = sptr;
-        fe_sptr->next = loading_obj->stable_ptrs;
-        loading_obj->stable_ptrs = fe_sptr;
-    }
-
-    return sptr;
-}
-
-
-/* -----------------------------------------------------------------------------
  * Debugging aid: look in GHCi's object symbol tables for symbols
  * within DELTA bytes of the specified address, and show their names.
  */
@@ -1793,7 +1763,8 @@ int ocTryLoad (ObjectCode* oc) {
 
     IF_DEBUG(linker, debugBelch("ocTryLoad: ocRunInit start\n"));
 
-    loading_obj = oc; // tells foreignExportStablePtr what to do
+    // See Note [Tracking foreign exports] in ForeignExports.c
+    foreignExportsLoadingObject(oc);
 #if defined(OBJFORMAT_ELF)
     r = ocRunInit_ELF ( oc );
 #elif defined(OBJFORMAT_PEi386)
@@ -1803,7 +1774,7 @@ int ocTryLoad (ObjectCode* oc) {
 #else
     barf("ocTryLoad: initializers not implemented on this platform");
 #endif
-    loading_obj = NULL;
+    foreignExportsFinishedLoadingObject();
 
     if (!r) { return r; }
 
diff --git a/rts/RtsStartup.c b/rts/RtsStartup.c
index 497add5e92..b916010d34 100644
--- a/rts/RtsStartup.c
+++ b/rts/RtsStartup.c
@@ -20,6 +20,7 @@
 #include "STM.h"        /* initSTM */
 #include "RtsSignals.h"
 #include "Weak.h"
+#include "ForeignExports.h"     /* processForeignExports */
 #include "Ticky.h"
 #include "StgRun.h"
 #include "Prelude.h"            /* fixupRTStoPreludeRefs */
@@ -339,7 +340,13 @@ hs_init_ghc(int *argc, char **argv[], RtsConfig rts_config)
     getStablePtr((StgPtr)processRemoteCompletion_closure);
 #endif
 
-    // Initialize the top-level handler system
+    /*
+     * process any foreign exports which were registered while loading the
+     * image
+     * */
+    processForeignExports();
+
+    /* initialize the top-level handler system */
     initTopHandler();
 
     /* initialise the shared Typeable store */
diff --git a/rts/RtsSymbols.c b/rts/RtsSymbols.c
index 2895711089..d14bdbc662 100644
--- a/rts/RtsSymbols.c
+++ b/rts/RtsSymbols.c
@@ -652,7 +652,7 @@
       SymI_HasProto(freeFullProgArgv)                                   \
       SymI_HasProto(getProcessElapsedTime)                              \
       SymI_HasProto(getStablePtr)                                       \
-      SymI_HasProto(foreignExportStablePtr)                             \
+      SymI_HasProto(registerForeignExports)                             \
       SymI_HasProto(hs_init)                                            \
       SymI_HasProto(hs_init_with_rtsopts)                               \
       SymI_HasProto(hs_init_ghc)                                        \
diff --git a/rts/rts.cabal.in b/rts/rts.cabal.in
index 1a1eb30611..89d160c9eb 100644
--- a/rts/rts.cabal.in
+++ b/rts/rts.cabal.in
@@ -140,6 +140,7 @@ library
                       rts/EventLogWriter.h
                       rts/FileLock.h
                       rts/Flags.h
+                      rts/ForeignExports.h
                       rts/GetTime.h
                       rts/Globals.h
                       rts/Hpc.h
@@ -412,6 +413,7 @@ library
                ClosureFlags.c
                Disassembler.c
                FileLock.c
+               ForeignExports.c
                Globals.c
                Hash.c
                Heap.c