Preserve order of dlopen'd modules in dll_list::topsortnewlib-snapshot-20170228

This patch alters the behaviour of dll_list::topsort to preserve the
order of dlopen'd units.

The load order of unrelated DLLs is reversed every time fork is called,
since dll_list::topsort finds the tail of the list and then unwinds to
reinsert items. My change takes advantage of what should be undefined
behaviour in dll_list::populate_deps (ndeps non-zero and ndeps and deps
not initialised) to allow the deps field to be initialised prior to the
call and appended to, rather than overwritten.

All DLLs which have been dlopen'd have their deps list initialised with
the list of all previously dlopen'd units. These extra dependencies mean
that the unwind preserves the order of dlopen'd units.

The motivation for this is the FlexDLL linker used in OCaml. The FlexDLL
linker allows a dlopen'd unit to refer to symbols in previously dlopen'd
units and it resolves these symbols in DllMain before anything else has
initialised (including the Cygwin DLL). This means that dependencies may
exist between dlopen'd units (which the OCaml runtime system
understands) but which Windows is unaware of. During fork, the
process-level table which FlexDLL uses to get the symbol table of each
DLL is copied over but because the load order of dlopen'd DLLs is
reversed, it is possible for FlexDLL to attempt to access memory in the
DLL before it has been loaded and hence it fails with an access
violation. Because the list is reversed on each call to fork, it means
that a subsequent call to fork puts the DLLs back into the correct
order, hence "even" invocations of fork work!

An interesting side-effect is that this only occurs if the DLLs load at
their preferred base address - if they have to be rebased, then FlexDLL
works because at the time that the dependent unit is loaded out of
order, there is still in memory the "dummy" DONT_RESOLVE_DLL_REFERENCES
version of the dependency which, as it happens, will contain the correct
symbol table in the data section. For my tests, this initially appeared
to be an x86-only problem, but that was only because the two DLLs on x64
should have been rebased.

Signed-off-by: David Allsopp <david.allsopp@metastack.com>

1 files changed, 48 insertions, 5 deletions

diff --git a/winsup/cygwin/dll_init.cc b/winsup/cygwin/dll_init.cc
index 0fe5714..86776f2 100644
--- a/winsup/cygwin/dll_init.cc
+++ b/winsup/cygwin/dll_init.cc
@@ -271,9 +271,16 @@ void dll_list::populate_deps (dll* d)
   PIMAGE_DATA_DIRECTORY dd = pef->idata_dir (IMAGE_DIRECTORY_ENTRY_IMPORT);
   /* Annoyance: calling crealloc with a NULL pointer will use the
      wrong heap and crash, so we have to replicate some code */
-  long maxdeps = 4;
-  d->deps = (dll**) cmalloc (HEAP_2_DLL, maxdeps*sizeof (dll*));
-  d->ndeps = 0;
+  long maxdeps;
+  if (!d->ndeps)
+    {
+      maxdeps = 4;
+      d->deps = (dll**) cmalloc (HEAP_2_DLL, maxdeps*sizeof (dll*));
+    }
+  else
+    {
+      maxdeps = d->ndeps;
+    }
   for (PIMAGE_IMPORT_DESCRIPTOR id=
 	(PIMAGE_IMPORT_DESCRIPTOR) pef->rva (dd->VirtualAddress);
       dd->Size && id->Name;
@@ -306,9 +313,45 @@ dll_list::topsort ()
 
   /* make sure we have all the deps available */
   dll* d = &start;
+  dll** dlopen_deps = NULL;
+  long maxdeps = 4;
+  long dlopen_ndeps = 0;
+
+  if (loaded_dlls > 0)
+    dlopen_deps = (dll**) cmalloc (HEAP_2_DLL, maxdeps*sizeof (dll*));
+
   while ((d = d->next))
-    if (!d->ndeps)
-      populate_deps (d);
+    {
+      if (!d->ndeps)
+        {
+          /* Ensure that all dlopen'd DLLs depend on previously dlopen'd DLLs.
+             This prevents topsort from reversing the order of dlopen'd DLLs on
+             calls to fork. */
+          if (d->type == DLL_LOAD)
+            {
+              /* Initialise d->deps with all previously dlopen'd DLLs. */
+              if (dlopen_ndeps)
+                {
+                  d->ndeps = dlopen_ndeps;
+                  d->deps = (dll**) cmalloc (HEAP_2_DLL,
+                                             dlopen_ndeps*sizeof (dll*));
+                  memcpy (d->deps, dlopen_deps, dlopen_ndeps*sizeof (dll*));
+                }
+              /* Add this DLL to the list of previously dlopen'd DLLs. */
+              if (dlopen_ndeps >= maxdeps)
+                {
+                  maxdeps = 2*(1+maxdeps);
+                  dlopen_deps = (dll**) crealloc (dlopen_deps,
+						  maxdeps*sizeof (dll*));
+                }
+              dlopen_deps[dlopen_ndeps++] = d;
+            }
+          populate_deps (d);
+        }
+    }
+
+  if (loaded_dlls > 0)
+    cfree (dlopen_deps);
 
   /* unlink head and tail pointers so the sort can rebuild the list */
   d = start.next;