[RFC][PATCH v7 04/20] elf/dl-load.c, elf-dl-open.c: Implement RTLD_SHARED dlmopen proxying

["Vivek Das Mohapatra" <vivek@collabora.com>]

This uses the new infrastructure to implement RTLD_SHARED object
proxying via dlmopen: Instead of opening the specified object in
the requested namespace we open it in the main namespace (if it
is not already present there) and proxy it to the destination.

The following rules apply:

If a proxy of the object is already present in the requested namespace,
we simply return it (with an incremented direct-open count).

If the object is already present in the requested namespace, a dl
error is signalled, since we cannot satisfy the user's request.

Proxies are never created in the main namespace: RTLD_SHARED has no
effect when the requested namespace is LM_ID_BASE.
---
 elf/dl-load.c | 46 +++++++++++++++++++++++++++++++++++++++
 elf/dl-open.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 103 insertions(+), 2 deletions(-)

diff --git a/elf/dl-load.c b/elf/dl-load.c
index 21c2920396..0c8cb763c1 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -2006,6 +2006,38 @@ open_path (const char *name, size_t namelen, int mode,
   return -1;
 }
 
+/* Search for a link map proxy in the given namespace by name.
+   Consider it to be an error if the found object is not a proxy.  */
+
+struct link_map *
+_dl_find_proxy (Lmid_t nsid, const char *name)
+{
+  struct link_map *l;
+
+  for (l = GL(dl_ns)[nsid]._ns_loaded; l; l = l->l_next)
+    {
+      if (__glibc_unlikely ((l->l_faked | l->l_removed) != 0))
+        continue;
+
+      if (!_dl_name_match_p (name, l))
+        continue;
+
+      /* We have a match - stop searching.  */
+      break;
+    }
+
+  if (l)
+    {
+      if (l->l_proxy)
+        return l;
+
+      _dl_signal_error (EEXIST, name, NULL,
+                        N_("object cannot be demoted to a proxy"));
+    }
+
+  return NULL;
+}
+
 /* Map in the shared object file NAME.  */
 
 struct link_map *
@@ -2022,6 +2054,20 @@ _dl_map_object (struct link_map *loader, const char *name,
   assert (nsid >= 0);
   assert (nsid < GL(dl_nns));
 
+#ifdef SHARED
+  /* Only need to do proxy checks if `nsid' is not LM_ID_BASE.  */
+  if (__glibc_unlikely ((mode & RTLD_SHARED) && (nsid != LM_ID_BASE)))
+    {
+      /* Search the namespace in case the object is already proxied.  */
+      if((l = _dl_find_proxy (nsid, name)) != NULL)
+        return l;
+
+      /* Further searches should be in the base ns: We will proxy the
+         resulting object in dl_open_worker *after* it is initialised.  */
+      nsid = LM_ID_BASE;
+    }
+#endif
+
   /* Look for this name among those already loaded.  */
   for (l = GL(dl_ns)[nsid]._ns_loaded; l; l = l->l_next)
     {
diff --git a/elf/dl-open.c b/elf/dl-open.c
index 8769e47051..809845e5e9 100644
--- a/elf/dl-open.c
+++ b/elf/dl-open.c
@@ -484,6 +484,8 @@ dl_open_worker (void *a)
   const char *file = args->file;
   int mode = args->mode;
   struct link_map *call_map = NULL;
+  int want_proxy = mode & RTLD_SHARED;
+  Lmid_t proxy_ns = LM_ID_BASE;
 
   /* Determine the caller's map if necessary.  This is needed in case
      we have a DST, when we don't know the namespace ID we have to put
@@ -508,6 +510,15 @@ dl_open_worker (void *a)
 	args->nsid = call_map->l_ns;
     }
 
+  /* Now that we know the NS for sure, sanity check the mode.  */
+  if (__glibc_likely(args->nsid == LM_ID_BASE) &&
+      __glibc_unlikely(mode & RTLD_SHARED))
+    {
+      args->mode &= ~RTLD_SHARED;
+      mode &= ~RTLD_SHARED;
+      want_proxy = 0;
+    }
+
   /* The namespace ID is now known.  Keep track of whether libc.so was
      already loaded, to determine whether it is necessary to call the
      early initialization routine (or clear libc_map on error).  */
@@ -541,6 +552,24 @@ dl_open_worker (void *a)
   /* This object is directly loaded.  */
   ++new->l_direct_opencount;
 
+  /* Proxy already existed in the target ns, nothing left to do.  */
+  if (__glibc_unlikely (new->l_proxy))
+    {
+      if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_FILES))
+	_dl_debug_printf ("proxied file=%s [%lu]; direct_opencount=%u\n\n",
+			  new->l_name, new->l_ns, new->l_direct_opencount);
+      return;
+    }
+
+  /* If we want proxy and we get this far then the entry in ‘new’ will
+     be in the main namespace: we should revert to the main namespace code
+     path(s), but remember the namespace we want the proxy to be in.  */
+  if (__glibc_unlikely (want_proxy))
+    {
+      proxy_ns = args->nsid;
+      args->nsid = LM_ID_BASE;
+    }
+
   /* It was already open.  */
   if (__glibc_unlikely (new->l_searchlist.r_list != NULL))
     {
@@ -572,6 +601,16 @@ dl_open_worker (void *a)
 
       assert (_dl_debug_initialize (0, args->nsid)->r_state == RT_CONSISTENT);
 
+      if (__glibc_unlikely (want_proxy))
+        {
+          args->map = new = _dl_new_proxy (new, mode, proxy_ns);
+          ++new->l_direct_opencount;
+
+          if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_FILES))
+            _dl_debug_printf ("proxying file=%s [%lu]; direct_opencount=%u\n\n",
+                              new->l_name, new->l_ns, new->l_direct_opencount);
+        }
+
       return;
     }
 
@@ -787,6 +826,14 @@ dl_open_worker (void *a)
   if (mode & RTLD_GLOBAL)
     add_to_global_update (new);
 
+  if (__glibc_unlikely (want_proxy))
+    {
+      /* args->map is the return slot which the caller sees, but keep
+         the original value of new hanging around so we can see the
+         real link map entry (for logging etc).  */
+      args->map = _dl_new_proxy (new, mode, proxy_ns);
+      ++args->map->l_direct_opencount;
+    }
 #ifndef SHARED
   /* We must be the static _dl_open in libc.a.  A static program that
      has loaded a dynamic object now has competition.  */
@@ -795,8 +842,16 @@ dl_open_worker (void *a)
 
   /* Let the user know about the opencount.  */
   if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_FILES))
-    _dl_debug_printf ("opening file=%s [%lu]; direct_opencount=%u\n\n",
-		      new->l_name, new->l_ns, new->l_direct_opencount);
+    {
+      _dl_debug_printf ("opening file=%s [%lu]; direct_opencount=%u\n\n",
+                        new->l_name, new->l_ns, new->l_direct_opencount);
+
+      if (args->map->l_proxy)
+        _dl_debug_printf ("proxying file=%s [%lu]; direct_opencount=%u\n\n",
+                          args->map->l_name,
+                          args->map->l_ns,
+                          args->map->l_direct_opencount);
+    }
 }
 
 void *
-- 
2.20.1