[RFC,v2,4/6] elf/dl-load.c, elf-dl-open.c: Implement RTLD_SHARED dlmopen proxying

Message ID 20180606181934.2480-5-vivek@collabora.com
State New
Headers show
Series
  • Proof-of-Concept implementation of RTLD_SHARED for dlmopen
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Commit Message

Vivek Das Mohapatra June 6, 2018, 6:19 p.m.
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 | 45 +++++++++++++++++++++++++++++++++++++++++++++
 elf/dl-open.c | 39 +++++++++++++++++++++++++++++++++++++--
 2 files changed, 82 insertions(+), 2 deletions(-)

Patch

diff --git a/elf/dl-load.c b/elf/dl-load.c
index 431236920f..6364030168 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -1848,6 +1848,37 @@  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 *
@@ -1864,6 +1895,20 @@  _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 9dde4acfbc..00a40671fd 100644
--- a/elf/dl-open.c
+++ b/elf/dl-open.c
@@ -237,6 +237,15 @@  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;
+    }
+
   /* It was already open.  */
   if (__glibc_unlikely (new->l_searchlist.r_list != NULL))
     {
@@ -252,6 +261,16 @@  dl_open_worker (void *a)
 
       assert (_dl_debug_initialize (0, args->nsid)->r_state == RT_CONSISTENT);
 
+      if (__glibc_unlikely (mode & RTLD_SHARED))
+        {
+          args->map = new = _dl_new_proxy (new, mode, args->nsid);
+          ++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;
     }
 
@@ -509,6 +528,14 @@  TLS generation counter wrapped!  Please report this."));
       /* It failed.  */
       return;
 
+  if (__glibc_unlikely (mode & RTLD_SHARED))
+    {
+      /* 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, args->nsid);
+      ++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.  */
@@ -517,8 +544,16 @@  TLS generation counter wrapped!  Please report this."));
 
   /* 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);
+    }
 }