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Cygwin: dlfcn: Fix reference counting

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The original dll_init code was living under the wrong assumption that
dll_dllcrt0_1 and in turn dll_list::alloc will be called for each
LoadLibrary call.  The same wrong assumption was made for
cygwin_detach_dll/dll_list::detach called via FreeLibrary.

In reality, dll_dllcrt0_1 gets only called once at first LoadLibrary
and cygwin_detach_dll once at last FreeLibrary.

In effect, reference counting for DLLs was completely broken after fork:

  parent:
    l1 = dlopen ("lib1");  // LoadLibrary, LoadCount = 1
    l2 = dlopen ("lib1");  // LoadLibrary, LoadCount = 2

    fork ();               // LoadLibrary in the child, LoadCount = 1!
      child:
        dlclose (l1);      // FreeLibrary actually frees the lib
        x = dlsym (l2);    // SEGV

* Move reference counting to dlopen/dlclose since only those functions
  have to keep track of loading/unloading DLLs in the application context.

* Remove broken accounting code from dll_list::alloc and dll_list::detach.

* Fix error handling in dlclose.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
This commit is contained in:
Corinna Vinschen
2017-03-21 14:30:24 +01:00
parent 33c7b2b544
commit 33297d810d
3 changed files with 57 additions and 34 deletions
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49
winsup/cygwin/dll_init.cc
View File

@@ -200,9 +200,8 @@ dll_list::alloc (HINSTANCE h, per_process *p, dll_type type)
dll *d = (type == DLL_LINK) ? dlls.find_by_modname (modname) : dlls[name];
if (d)
{
if (!in_forkee)
d->count++; /* Yes. Bump the usage count. */
else if (d->handle != h)
/* We only get here in the forkee. */
if (d->handle != h)
fabort ("%W: Loaded to different address: parent(%p) != child(%p)",
name, d->handle, h);
/* If this DLL has been linked against, and the full path differs, try
@@ -224,15 +223,14 @@ dll_list::alloc (HINSTANCE h, per_process *p, dll_type type)
}
else
{
/* FIXME: Change this to new at some point. */
d = (dll *) cmalloc (HEAP_2_DLL, sizeof (*d) + (namelen * sizeof (*name)));
d = (dll *) cmalloc (HEAP_2_DLL,
sizeof (*d) + (namelen * sizeof (*name)));
/* Now we've allocated a block of information. Fill it in with the
supplied info about this DLL. */
d->count = 1;
wcscpy (d->name, name);
d->modname = d->name + (modname - name);
d->handle = h;
d->count = 0; /* Reference counting performed in dlopen/dlclose. */
d->has_dtors = true;
d->p = p;
d->ndeps = 0;
@@ -438,25 +436,21 @@ dll_list::detach (void *retaddr)
guard (true);
if ((d = find (retaddr)))
{
if (d->count <= 0)
system_printf ("WARNING: trying to detach an already detached dll ...");
if (--d->count == 0)
{
/* Ensure our exception handler is enabled for destructors */
exception protect;
/* Call finalize function if we are not already exiting */
if (!exit_state)
__cxa_finalize (d->handle);
d->run_dtors ();
d->prev->next = d->next;
if (d->next)
d->next->prev = d->prev;
if (d->type == DLL_LOAD)
loaded_dlls--;
if (end == d)
end = d->prev;
cfree (d);
}
system_printf ("HERE %W", d->name);
/* Ensure our exception handler is enabled for destructors */
exception protect;
/* Call finalize function if we are not already exiting */
if (!exit_state)
__cxa_finalize (d->handle);
d->run_dtors ();
d->prev->next = d->next;
if (d->next)
d->next->prev = d->prev;
if (d->type == DLL_LOAD)
loaded_dlls--;
if (end == d)
end = d->prev;
cfree (d);
}
guard (false);
}
@@ -641,6 +635,9 @@ void dll_list::load_after_fork_impl (HANDLE parent, dll* d, int retries)
if (h != d->handle)
fabort ("unable to map %W to same address as parent: %p != %p",
d->modname, d->handle, h);
/* Fix OS reference count. */
for (int cnt = 1; cnt < d->count; ++cnt)
LoadLibraryW (d->name);
}
}