diff mbox series

[v14,5/7] Implement dlmopen RTLD_SHARED flag (bug 22745)

Message ID 20211010163836.14666-6-vivek@collabora.com
State New
Headers show
Series Implementation of RTLD_SHARED for dlmopen | expand

Checks

Context Check Description
dj/TryBot-apply_patch success Patch applied to master at the time it was sent

Commit Message

Vivek Das Mohapatra Oct. 10, 2021, 4:38 p.m. UTC
This flag will instruct dlmopen to create a shared object present
in the main namespace and accessible from the selected namespace
when supplied in the MODE argument.

include/link.h: Update the link_map struct to allow proxies

We already have an l_real pointer, used for a similar purpose by
the linker for copies of ld.so in secondary namespaces. Update its
documentation and add a bitfield to indicate when link_map entry
is a proxy.

elf/dl-object.c: Implement a helper function to proxy link_map entries

Provides the minimal functionality needed to take an existing
link_map entry and create a proxy for it in the specified namespace.

elf/dl-load.c, elf/dl-open.c: Implement RTLD_SHARED dlmopen proxying

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-fini.c: Handle proxy link_maps in the shutdown path (bug 22745)

When cleaning up before exit we should not call destructors or
otherwise free [most of] the contents of proxied link_map entries
since they share [most of] their contents with the LM_ID_BASE
objects to which they point.

elf/dl-init.c: Skip proxy link_map entries in dl init path

Proxies should not trigger calls to DT_INIT constructors since they're
just shims that point to the real, already loaded and initialised, objects.

elf/dl-open.c: Skip libc init if namespace has no libc map

Secondary namespaces which share their libc mapping with the main
namespace cannot (and should not) have _dl_call_libc_early_init
called for them by dl_open_worker.

elf/dl-open.c: When creating a proxy check NS 0 libc map

The libc_already_loaded check normally considers the libc_map entry
in GL(dl_ns)[args->nsid].libc_map.

This is not correct for proxies, which use the libc_map from
the default namespace (as proxies are dummy entries that point
to the base namespace via their l_real members).

elf/dl-load.c, dl-open.c: Compare DSOs by file ID & check DF_GNU_1_UNIQUE

If _dl_map_object_from_fd finds that a DSO it was asked to
load into a non-base namespace is already loaded (into the
main namespace) and is flagged DF_GNU_1_UNIQUE then it should
return that DSO's link map entry.

In such cases _dl_open_worker must notice that this has
happened and continue down the link map proxy generation
path instead of normal link map entry preparation.

elf/dl-load.c: error if RTLD_SHARED = DF_GNU_1_UNIQUE semantics violated

elf/dl-open.c: Use search helper to find preloaded DT_GNU_UNIQUE DSOs

If a DSO already exists (with the same name) in the base namespace
and it is flagged DT_GNU_UNIQUE then we should behave as if a proxy
had been requested.

elf/dl-load.c: When loading DSOs in other namespaces check DT_GNU_UNIQUE

If a DSO has not already been loaded and the target is not the main
namespace then we must check to see if it's been DT_GNU_UNIQUE tagged
and load it into the main namespace instead.

dl_open_worker has alread been modified to notice the discrepancy
between the request and the result in such cases, and will set up
a proxy in the target namespace.

elf/dl-load.c: Suppress audit calls when a (new) namespace is empty

When preparing an RTLD_SHARED proxy in a new namespace
it is possible for the target namespace to be empty:

This can happen for RTLD_SHARED + LM_ID_NEWLM.

The audit infrastructure should not be invoked at this
point (as there's nothing there to audit yet).

bits/dlfcn.h, elf/dl-load.c, elf/dl-open.c, elf/rtld.c,
sysdeps/mips/bits/dlfcn.h:
Suppress inter-namespace DSO sharing for audit libraries

Audit libraries should not participate in DSO sharing: In
particular libraries tagged with DF_GNU_1_UNIQUE should not
be shared between the audit namespace and any others - they
should get their own copy.

This is signalled to the loader code by passing the RTLD_ISOLATE
flag from the relevant entry point in the dl modes argument.

elf/dl-sym.c: dlsym, dlvsym must be able to resolve symbols via proxies

elf/tst-tls-ie-dlmopen.c: Enforce separate libc for TLS storage test

tst-tls-ie-dlmopen checks to see that new namespaces consume
TLS memory as expected: This does not happen when new namespaces
share the same libc instance (since TLS is allocated only when
a new libc instance insitialises its threading infrastructure).

Adding RTLD_ISOLATE to the dlmopen flags in the test restores
the old behaviour which allows the test to check what it
actually needs to.
---
 bits/dlfcn.h               |  10 ++
 elf/dl-close.c             |  41 +++++---
 elf/dl-deps.c              |  17 ++++
 elf/dl-fini.c              |   6 +-
 elf/dl-init.c              |   4 +-
 elf/dl-load.c              | 194 ++++++++++++++++++++++++++++++-------
 elf/dl-lookup.c            |  23 ++++-
 elf/dl-object.c            |  83 ++++++++++++++++
 elf/dl-open.c              | 162 ++++++++++++++++++++++++++-----
 elf/dl-sym.c               |  14 +++
 elf/rtld.c                 |   2 +-
 elf/tst-tls-ie-dlmopen.c   |   4 +-
 include/link.h             |   6 +-
 sysdeps/generic/ldsodefs.h |   7 ++
 sysdeps/mips/bits/dlfcn.h  |  10 ++
 15 files changed, 498 insertions(+), 85 deletions(-)
diff mbox series

Patch

diff --git a/bits/dlfcn.h b/bits/dlfcn.h
index f3bc63e958..26d715d813 100644
--- a/bits/dlfcn.h
+++ b/bits/dlfcn.h
@@ -32,6 +32,16 @@ 
    visible as if the object were linked directly into the program.  */
 #define RTLD_GLOBAL	0x00100
 
+/* If the following bit is set in the MODE argument to dlmopen
+   then the target object is loaded into the main namespace (if
+   it is not already there) and a shallow copy (proxy) is placed
+   in the target namespace.  This allows multiple namespaces to
+   share a single instance of a DSO.  */
+#define RTLD_SHARED 0x00080
+
+/* Suppress RTLD_SHARED and/or DT_GNU_FLAGS_1/DF_GNU_1_UNIQUE  */
+#define RTLD_ISOLATE 0x00040
+
 /* Unix98 demands the following flag which is the inverse to RTLD_GLOBAL.
    The implementation does this by default and so we can define the
    value to zero.  */
diff --git a/elf/dl-close.c b/elf/dl-close.c
index cfe0f1c0c9..6c6e6aa0d4 100644
--- a/elf/dl-close.c
+++ b/elf/dl-close.c
@@ -286,8 +286,9 @@  _dl_close_worker (struct link_map *map, bool force)
 
 	  /* Call its termination function.  Do not do it for
 	     half-cooked objects.  Temporarily disable exception
-	     handling, so that errors are fatal.  */
-	  if (imap->l_init_called)
+	     handling, so that errors are fatal.
+	     Proxies should never have this flag set, but we double check.  */
+	  if (imap->l_init_called && !imap->l_proxy)
 	    {
 	      /* When debugging print a message first.  */
 	      if (__builtin_expect (GLRO(dl_debug_mask) & DL_DEBUG_IMPCALLS,
@@ -363,7 +364,9 @@  _dl_close_worker (struct link_map *map, bool force)
 	     one for the terminating NULL pointer.  */
 	  size_t remain = (new_list != NULL) + 1;
 	  bool removed_any = false;
-	  for (size_t cnt = 0; imap->l_scope[cnt] != NULL; ++cnt)
+	  for (size_t cnt = 0;
+	       imap->l_scope && imap->l_scope[cnt] != NULL;
+	       ++cnt)
 	    /* This relies on l_scope[] entries being always set either
 	       to its own l_symbolic_searchlist address, or some map's
 	       l_searchlist address.  */
@@ -694,8 +697,10 @@  _dl_close_worker (struct link_map *map, bool force)
 
 	  /* We can unmap all the maps at once.  We determined the
 	     start address and length when we loaded the object and
-	     the `munmap' call does the rest.  */
-	  DL_UNMAP (imap);
+	     the `munmap' call does the rest. Proxies do not have
+             any segments of their own to unmap.  */
+          if (!imap->l_proxy)
+            DL_UNMAP (imap);
 
 	  /* Finally, unlink the data structure and free it.  */
 #if DL_NNS == 1
@@ -735,19 +740,23 @@  _dl_close_worker (struct link_map *map, bool force)
 	    _dl_debug_printf ("\nfile=%s [%lu];  destroying link map\n",
 			      imap->l_name, imap->l_ns);
 
-	  /* This name always is allocated.  */
-	  free (imap->l_name);
-	  /* Remove the list with all the names of the shared object.  */
-
-	  struct libname_list *lnp = imap->l_libname;
-	  do
+	  /* Skip structures borrowed by proxies from the real map.  */
+	  if (!imap->l_proxy)
 	    {
-	      struct libname_list *this = lnp;
-	      lnp = lnp->next;
-	      if (!this->dont_free)
-		free (this);
+	      /* This name is always allocated.  */
+	      free (imap->l_name);
+
+	      /* Remove the list with all the names of the shared object.  */
+	      struct libname_list *lnp = imap->l_libname;
+	      do
+		{
+		  struct libname_list *this = lnp;
+		  lnp = lnp->next;
+		  if (!this->dont_free)
+		    free (this);
+		}
+	      while (lnp != NULL);
 	    }
-	  while (lnp != NULL);
 
 	  /* Remove the searchlists.  */
 	  free (imap->l_initfini);
diff --git a/elf/dl-deps.c b/elf/dl-deps.c
index 087a49b212..303d0f42c0 100644
--- a/elf/dl-deps.c
+++ b/elf/dl-deps.c
@@ -66,6 +66,23 @@  openaux (void *a)
 			       ? lt_library : args->map->l_type),
 			      args->trace_mode, args->open_mode,
 			      args->map->l_ns);
+
+  /* This implies that we need to prepare a proxy in the target namespace.  */
+  if (__glibc_unlikely (args->map->l_ns != LM_ID_BASE &&
+                        args->aux->l_ns == LM_ID_BASE))
+    {
+      if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_FILES))
+	_dl_debug_printf ("need proxy for file=%s [%lu]; initstate=%d\n\n",
+			  args->aux->l_name, args->aux->l_ns,
+			  args->aux->l_real->l_relocated);
+
+      args->aux = _dl_new_proxy (args->aux, args->open_mode, args->map->l_ns);
+
+      if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_FILES))
+	_dl_debug_printf ("proxying dependency=%s [%lu]; direct_opencount=%u\n\n",
+			  args->aux->l_name, args->aux->l_ns,
+			  args->aux->l_direct_opencount);
+    }
 }
 
 static ptrdiff_t
diff --git a/elf/dl-fini.c b/elf/dl-fini.c
index 6dbdfe4b3e..51f45be9fe 100644
--- a/elf/dl-fini.c
+++ b/elf/dl-fini.c
@@ -73,7 +73,7 @@  _dl_fini (void)
 	  assert (nloaded != 0 || GL(dl_ns)[ns]._ns_loaded == NULL);
 	  for (l = GL(dl_ns)[ns]._ns_loaded, i = 0; l != NULL; l = l->l_next)
 	    /* Do not handle ld.so in secondary namespaces.  */
-	    if (l == l->l_real)
+	    if (l == l->l_real || l->l_proxy)
 	      {
 		assert (i < nloaded);
 
@@ -111,7 +111,9 @@  _dl_fini (void)
 	    {
 	      struct link_map *l = maps[i];
 
-	      if (l->l_init_called)
+	      /* Do not call fini functions via proxies, or for
+		 objects which are not marked as initialised.  */
+	      if (l->l_init_called && !l->l_proxy)
 		{
 		  /* Make sure nothing happens if we are called twice.  */
 		  l->l_init_called = 0;
diff --git a/elf/dl-init.c b/elf/dl-init.c
index f924d26642..00ce8bb9d6 100644
--- a/elf/dl-init.c
+++ b/elf/dl-init.c
@@ -30,8 +30,8 @@  call_init (struct link_map *l, int argc, char **argv, char **env)
      need relocation, and neither do proxy objects.)  */
   assert (l->l_real->l_relocated || l->l_real->l_type == lt_executable);
 
-  if (l->l_init_called)
-    /* This object is all done.  */
+  if (l->l_init_called || l->l_proxy)
+    /* This object is all done, or a proxy (and therefore initless).  */
     return;
 
   /* Avoid handling this constructor again in case we have a circular
diff --git a/elf/dl-load.c b/elf/dl-load.c
index ff49c7a831..460e157123 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -854,7 +854,6 @@  _dl_init_paths (const char *llp, const char *source,
     __rtld_env_path_list.dirs = (void *) -1;
 }
 
-
 /* Process PT_GNU_PROPERTY program header PH in module L after
    PT_LOAD segments are mapped.  Only one NT_GNU_PROPERTY_TYPE_0
    note is handled which contains processor specific properties.
@@ -931,14 +930,11 @@  _dl_process_pt_gnu_property (struct link_map *l, int fd, const ElfW(Phdr) *ph)
 /* Map in the shared object NAME, actually located in REALNAME, and already
    opened on FD.  */
 
-#ifndef EXTERNAL_MAP_FROM_FD
-static
-#endif
-struct link_map *
+static struct link_map *
 _dl_map_object_from_fd (const char *name, const char *origname, int fd,
 			struct filebuf *fbp, char *realname,
 			struct link_map *loader, int l_type, int mode,
-			void **stack_endp, Lmid_t nsid)
+			void **stack_endp, bool has_gnu_unique, Lmid_t nsid)
 {
   struct link_map *l = NULL;
   const ElfW(Ehdr) *header;
@@ -1041,6 +1037,32 @@  _dl_map_object_from_fd (const char *name, const char *origname, int fd,
     }
 #endif
 
+  /* DSOs in the main namespace which are flagged DF_GNU_1_UNIQUE should only
+     be opened into the main namespace.  Other namespaces should only get
+     proxies.  */
+  if (__glibc_unlikely ((nsid != LM_ID_BASE) && !(mode & RTLD_ISOLATE)))
+    {
+      /* Check base ns to see if the name matched another already loaded.  */
+      for (l = GL(dl_ns)[LM_ID_BASE]._ns_loaded; l != NULL; l = l->l_next)
+	if (!l->l_removed && _dl_file_id_match_p (&l->l_file_id, &id))
+	  {
+	    if (!(l->l_gnu_flags_1 & DF_GNU_1_UNIQUE))
+	      continue;
+
+	    /* Already loaded. Bump its reference count and return it.  */
+	    __close_nocancel (fd);
+
+	    /* If the name is not listed for this object add it.  */
+	    free (realname);
+	    add_name_to_object (l, name);
+
+	    /* NOTE: It is important that our caller picks up on the fact
+	       that we have NOT returned an object in the requested namespace
+	       and handles the proxying correctly.  */
+	    return l;
+	  }
+    }
+
   if (mode & RTLD_NOLOAD)
     {
       /* We are not supposed to load the object unless it is already
@@ -1066,8 +1088,11 @@  _dl_map_object_from_fd (const char *name, const char *origname, int fd,
 	  && __glibc_unlikely (GLRO(dl_naudit) > 0))
 	{
 	  struct link_map *head = GL(dl_ns)[nsid]._ns_loaded;
-	  /* Do not call the functions for any auditing object.  */
-	  if (head->l_auditing == 0)
+	  /* Do not call the functions for any auditing object or try to
+	     call auditing functions if the namespace is currently empty.
+	     This can happen when opening the first DSO in a new namespace.
+	   */
+	  if ((head != NULL) && !head->l_auditing)
 	    {
 	      struct audit_ifaces *afct = GLRO(dl_audit);
 	      for (unsigned int cnt = 0; cnt < GLRO(dl_naudit); ++cnt)
@@ -1093,6 +1118,32 @@  _dl_map_object_from_fd (const char *name, const char *origname, int fd,
   else
     assert (r->r_state == RT_ADD);
 
+  /* Load the ELF header using preallocated struct space if it's big enough.  */
+  maplength = header->e_phnum * sizeof (ElfW(Phdr));
+  if (header->e_phoff + maplength <= (size_t) fbp->len)
+    phdr = (void *) (fbp->buf + header->e_phoff);
+  else
+    {
+      phdr = alloca (maplength);
+      if ((size_t) __pread64_nocancel (fd, (void *) phdr, maplength,
+				       header->e_phoff) != maplength)
+	{
+	  errstring = N_("cannot read file data");
+	  goto lose_errno;
+	}
+    }
+
+  /* We need to check for DT_GNU_FLAGS_1/DF_GNU_1_UNIQUE before we start
+     initialising any namespace dependent metatada.  */
+  if (__glibc_unlikely ((nsid != LM_ID_BASE) && !(mode & RTLD_ISOLATE)))
+    {
+      /* Target DSO is flagged as unique: Make sure it gets loaded into
+         the base namespace.  It is up to our caller to generate a proxy
+         in the target nsid.  */
+      if (has_gnu_unique)
+        nsid = LM_ID_BASE;
+    }
+
   /* Enter the new object in the list of loaded objects.  */
   l = _dl_new_object (realname, name, l_type, loader, mode, nsid);
   if (__glibc_unlikely (l == NULL))
@@ -1110,20 +1161,6 @@  _dl_map_object_from_fd (const char *name, const char *origname, int fd,
   type = header->e_type;
   l->l_phnum = header->e_phnum;
 
-  maplength = header->e_phnum * sizeof (ElfW(Phdr));
-  if (header->e_phoff + maplength <= (size_t) fbp->len)
-    phdr = (void *) (fbp->buf + header->e_phoff);
-  else
-    {
-      phdr = alloca (maplength);
-      if ((size_t) __pread64_nocancel (fd, (void *) phdr, maplength,
-				       header->e_phoff) != maplength)
-	{
-	  errstring = N_("cannot read file data");
-	  goto lose_errno;
-	}
-    }
-
    /* On most platforms presume that PT_GNU_STACK is absent and the stack is
     * executable.  Other platforms default to a nonexecutable stack and don't
     * need PT_GNU_STACK to do so.  */
@@ -1588,7 +1625,8 @@  print_search_path (struct r_search_path_elem **list,
 static int
 open_verify (const char *name, int fd,
              struct filebuf *fbp, struct link_map *loader,
-	     int whatcode, int mode, bool *found_other_class, bool free_name)
+	     int whatcode, int mode, bool *found_other_class,
+	     bool *has_gnu_unique, bool free_name)
 {
   /* This is the expected ELF header.  */
 #define ELF32_CLASS ELFCLASS32
@@ -1867,6 +1905,30 @@  open_verify (const char *name, int fd,
 
 	    break;
 	  }
+	else if (ph->p_type == PT_DYNAMIC)
+	  {
+	    off64_t pos = ph->p_offset;
+	    off64_t end = pos + ph->p_filesz;
+
+	    while (pos < end)
+	      {
+		ElfW (Dyn) entry;
+		ssize_t bytes = __pread64_nocancel (fd, &entry,
+						    sizeof (ElfW (Dyn)), pos);
+		if (bytes != sizeof (entry))
+		  goto read_error;
+		pos += bytes;
+
+		switch (entry.d_tag)
+		  {
+		  case DT_GNU_FLAGS_1:
+		    *has_gnu_unique = entry.d_un.d_val & DF_GNU_1_UNIQUE;
+		    /* fallthrough  */
+		  case DT_NULL:
+		    break;
+		  }
+	      }
+	  }
       free (abi_note_malloced);
     }
 
@@ -1884,7 +1946,7 @@  static int
 open_path (const char *name, size_t namelen, int mode,
 	   struct r_search_path_struct *sps, char **realname,
 	   struct filebuf *fbp, struct link_map *loader, int whatcode,
-	   bool *found_other_class)
+	   bool *found_other_class, bool *has_gnu_unique)
 {
   struct r_search_path_elem **dirs = sps->dirs;
   char *buf;
@@ -1938,7 +2000,7 @@  open_path (const char *name, size_t namelen, int mode,
 	    _dl_debug_printf ("  trying file=%s\n", buf);
 
 	  fd = open_verify (buf, -1, fbp, loader, whatcode, mode,
-			    found_other_class, false);
+			    found_other_class, has_gnu_unique, false);
 	  if (this_dir->status[cnt] == unknown)
 	    {
 	      if (fd != -1)
@@ -2032,6 +2094,37 @@  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 != NULL; 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 != NULL)
+    {
+      if (l->l_proxy)
+	return l;
+
+      _dl_signal_error (EEXIST, name, NULL,
+			N_("object cannot be demoted to a proxy"));
+    }
+
+  return NULL;
+}
+
 /* Search for a shared object in a given namespace.  */
 struct link_map *
 _dl_find_dso (const char *name, Lmid_t nsid)
@@ -2085,12 +2178,42 @@  _dl_map_object (struct link_map *loader, const char *name,
 
   assert (nsid >= 0);
   assert (nsid < GL(dl_nns));
+  assert (!((mode & RTLD_ISOLATE) && (mode & RTLD_SHARED)));
+
+#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.  */
+      l = _dl_find_proxy (nsid, name);
+      if (l != 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.  */
   l = _dl_find_dso (name, nsid);
 
   if (l != NULL)
     {
+#ifdef SHARED
+      /* If we are trying to load a DF_GNU_1_UNIQUE flagged DSO which WAS
+	 already opened in the target NS but with RTLD_ISOLATE so it WAS NOT
+	 created as a proxy we need to error out since we cannot satisfy the
+	 DF_GNU_1_UNIQUE is-equivalent-to RTLD_SHARED semantics.  */
+      if (!(mode & RTLD_ISOLATE) &&
+	  (l->l_ns != LM_ID_BASE) &&
+	  (l->l_gnu_flags_1 & DF_GNU_1_UNIQUE) &&
+	  !l->l_proxy)
+      {
+        _dl_signal_error (EEXIST, name, NULL,
+			  N_("object cannot be demoted to a proxy"));
+      }
+#endif
       return l;
     }
 
@@ -2140,6 +2263,8 @@  _dl_map_object (struct link_map *loader, const char *name,
 
   /* Will be true if we found a DSO which is of the other ELF class.  */
   bool found_other_class = false;
+  /* Will be true if the DSO has a DT_GNU_FLAGS_1/DF_GNU_1_UNIQUE flag.  */
+  bool has_gnu_unique = false;
 
   if (strchr (name, '/') == NULL)
     {
@@ -2169,7 +2294,7 @@  _dl_map_object (struct link_map *loader, const char *name,
 		fd = open_path (name, namelen, mode,
 				&l->l_rpath_dirs,
 				&realname, &fb, loader, LA_SER_RUNPATH,
-				&found_other_class);
+				&found_other_class, &has_gnu_unique);
 		if (fd != -1)
 		  break;
 
@@ -2185,7 +2310,7 @@  _dl_map_object (struct link_map *loader, const char *name,
 	    fd = open_path (name, namelen, mode,
 			    &main_map->l_rpath_dirs,
 			    &realname, &fb, loader ?: main_map, LA_SER_RUNPATH,
-			    &found_other_class);
+			    &found_other_class, &has_gnu_unique);
 	}
 
       /* Try the LD_LIBRARY_PATH environment variable.  */
@@ -2193,7 +2318,7 @@  _dl_map_object (struct link_map *loader, const char *name,
 	fd = open_path (name, namelen, mode, &__rtld_env_path_list,
 			&realname, &fb,
 			loader ?: GL(dl_ns)[LM_ID_BASE]._ns_loaded,
-			LA_SER_LIBPATH, &found_other_class);
+			LA_SER_LIBPATH, &found_other_class, &has_gnu_unique);
 
       /* Look at the RUNPATH information for this binary.  */
       if (fd == -1 && loader != NULL
@@ -2201,7 +2326,7 @@  _dl_map_object (struct link_map *loader, const char *name,
 			  DT_RUNPATH, "RUNPATH"))
 	fd = open_path (name, namelen, mode,
 			&loader->l_runpath_dirs, &realname, &fb, loader,
-			LA_SER_RUNPATH, &found_other_class);
+			LA_SER_RUNPATH, &found_other_class, &has_gnu_unique);
 
       if (fd == -1)
         {
@@ -2211,7 +2336,7 @@  _dl_map_object (struct link_map *loader, const char *name,
               fd = open_verify (realname, fd,
                                 &fb, loader ?: GL(dl_ns)[nsid]._ns_loaded,
                                 LA_SER_CONFIG, mode, &found_other_class,
-                                false);
+                                &has_gnu_unique, false);
               if (fd == -1)
                 free (realname);
             }
@@ -2265,7 +2390,7 @@  _dl_map_object (struct link_map *loader, const char *name,
 		  fd = open_verify (cached, -1,
 				    &fb, loader ?: GL(dl_ns)[nsid]._ns_loaded,
 				    LA_SER_CONFIG, mode, &found_other_class,
-				    false);
+				    &has_gnu_unique, false);
 		  if (__glibc_likely (fd != -1))
 		    realname = cached;
 		  else
@@ -2281,7 +2406,8 @@  _dl_map_object (struct link_map *loader, const char *name,
 	      || __glibc_likely (!(l->l_flags_1 & DF_1_NODEFLIB)))
 	  && __rtld_search_dirs.dirs != (void *) -1)
 	fd = open_path (name, namelen, mode, &__rtld_search_dirs,
-			&realname, &fb, l, LA_SER_DEFAULT, &found_other_class);
+			&realname, &fb, l, LA_SER_DEFAULT, &found_other_class,
+			&has_gnu_unique);
 
       /* Add another newline when we are tracing the library loading.  */
       if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_LIBS))
@@ -2299,7 +2425,7 @@  _dl_map_object (struct link_map *loader, const char *name,
 	{
 	  fd = open_verify (realname, -1, &fb,
 			    loader ?: GL(dl_ns)[nsid]._ns_loaded, 0, mode,
-			    &found_other_class, true);
+			    &found_other_class, &has_gnu_unique, true);
 	  if (__glibc_unlikely (fd == -1))
 	    free (realname);
 	}
@@ -2360,7 +2486,7 @@  _dl_map_object (struct link_map *loader, const char *name,
 
   void *stack_end = __libc_stack_end;
   return _dl_map_object_from_fd (name, origname, fd, &fb, realname, loader,
-				 type, mode, &stack_end, nsid);
+				 type, mode, &stack_end, has_gnu_unique, nsid);
 }
 
 struct add_path_state
diff --git a/elf/dl-lookup.c b/elf/dl-lookup.c
index 430359af39..14e76c8790 100644
--- a/elf/dl-lookup.c
+++ b/elf/dl-lookup.c
@@ -25,6 +25,7 @@ 
 #include <dl-hash.h>
 #include <dl-machine.h>
 #include <dl-protected.h>
+#include <dl-dst.h>
 #include <sysdep-cancel.h>
 #include <libc-lock.h>
 #include <tls.h>
@@ -922,11 +923,25 @@  _dl_lookup_symbol_x (const char *undef_name, struct link_map *undef_map,
 	}
     }
 
+  /* Found a candidate symbol but it resides in the base namespace BUT we are
+     in another namespace.  Therefore we want a proxy (which should already
+     have been created by this point.  */
+  if (current_value.m != NULL && !(IS_RTLD (current_value.m))
+      && undef_map->l_ns != LM_ID_BASE && current_value.m->l_ns == LM_ID_BASE)
+    {
+      struct link_map *proxy = _dl_find_proxy (undef_map->l_ns,
+					       current_value.m->l_name);
+      if (proxy != NULL)
+	current_value.m = proxy;
+      else if (GLRO(dl_debug_mask) & DL_DEBUG_SYMBOLS)
+	_dl_debug_printf ("Failed to find proxy for %s\n", current_value.m->l_name);
+    }
+
   /* We have to check whether this would bind UNDEF_MAP to an object
      in the global scope which was dynamically loaded.  In this case
      we have to prevent the latter from being unloaded unless the
      UNDEF_MAP object is also unloaded.  */
-  if (__glibc_unlikely (current_value.m->l_type == lt_loaded)
+  if (__glibc_unlikely (current_value.m->l_real->l_type == lt_loaded)
       /* Don't do this for explicit lookups as opposed to implicit
 	 runtime lookups.  */
       && (flags & DL_LOOKUP_ADD_DEPENDENCY) != 0
@@ -940,8 +955,8 @@  _dl_lookup_symbol_x (const char *undef_name, struct link_map *undef_map,
 				  version, type_class, flags, skip_map);
 
   /* The object is used.  */
-  if (__glibc_unlikely (current_value.m->l_used == 0))
-    current_value.m->l_used = 1;
+  if (__glibc_unlikely (current_value.m->l_real->l_used == 0))
+    current_value.m->l_real->l_used = 1;
 
   if (__glibc_unlikely (GLRO(dl_debug_mask)
 			& (DL_DEBUG_BINDINGS|DL_DEBUG_PRELINK)))
@@ -949,7 +964,7 @@  _dl_lookup_symbol_x (const char *undef_name, struct link_map *undef_map,
 			&current_value, version, type_class, protected);
 
   *ref = current_value.s;
-  return LOOKUP_VALUE (current_value.m);
+  return LOOKUP_VALUE (current_value.m->l_real);
 }
 
 
diff --git a/elf/dl-object.c b/elf/dl-object.c
index 1875599eb2..31bdf96879 100644
--- a/elf/dl-object.c
+++ b/elf/dl-object.c
@@ -21,6 +21,7 @@ 
 #include <stdlib.h>
 #include <unistd.h>
 #include <ldsodefs.h>
+#include <libintl.h>
 
 #include <assert.h>
 
@@ -50,6 +51,88 @@  _dl_add_to_namespace_list (struct link_map *new, Lmid_t nsid)
   __rtld_lock_unlock_recursive (GL(dl_load_write_lock));
 }
 
+/* Proxy an existing link map entry into a new link map:
+   This is based on _dl_new_object, skipping the steps we know we won't need
+   because this is mostly just a shell for the l_real pointer holding the real
+   link map entry (normally l == l->l_real, but not for ld.so in non-main
+   link maps or RTLD_SHARED proxies).
+   It also flags the proxy by setting l_proxy, and sets the the no-delete
+   flag in the original if it is an lt_loaded.  */
+struct link_map *
+_dl_new_proxy (struct link_map *old, int mode, Lmid_t nsid)
+{
+  const char *name;
+  struct link_map *new;
+  struct libname_list *newname;
+#ifdef SHARED
+  unsigned int na = GLRO(dl_naudit);
+
+  if ((mode & __RTLD_OPENEXEC) != 0)
+    na = DL_NNS;
+
+  size_t audit_space = na * sizeof (struct auditstate);
+#else
+# define audit_space 0
+#endif
+
+  name = old->l_name;
+
+  /* Find the original link map entry if 'old' is itself a proxy. */
+  while (old != NULL && old->l_proxy)
+    old = old->l_real;
+
+  if (old == NULL)
+    _dl_signal_error (EINVAL, name, NULL, N_("cannot proxy NULL link_map"));
+
+  /* Object already exists in the target namespace.  This should get handled
+     by dl_open_worker but just in case we get this far, handle it:  */
+  if (__glibc_unlikely (old->l_ns == nsid))
+    _dl_signal_error (EEXIST, name, NULL,
+                      N_("existing object cannot be demoted to a proxy"));
+
+  /* Now duplicate as little of _dl_new_object as possible to get a
+     working proxied object in the target link map.  */
+  /* NOTE: For a "real" link map entry _dl_new_object allocates space
+     for extra items at the end of the link_map struct but since proxies
+     point at the contents of their parent struct they don't need their
+     own space for these.  */
+  new = (struct link_map *) calloc (sizeof (*new) + audit_space
+                                    // These two allocations may also be unnecessary
+                                    + sizeof (struct link_map *)
+                                    + sizeof (*newname), 1);
+
+  if (new == NULL)
+    _dl_signal_error (ENOMEM, name, NULL,
+                      N_("cannot create shared object descriptor"));
+
+  /* Specific to the proxy.  */
+  new->l_real = old;
+  new->l_proxy = 1;
+  new->l_ns = nsid;
+
+  /* Copied from the origin.  */
+  new->l_libname = old->l_libname;
+  new->l_name = old->l_name;
+  /* Proxies are considered lt_loaded if the real entry type is lt_library.  */
+  new->l_type = (old->l_type == lt_library) ? lt_loaded : old->l_type;
+
+  if (__glibc_unlikely (mode & RTLD_NODELETE))
+    new->l_flags_1 |= DF_1_NODELETE;
+
+  /* Specific to the origin.  Ideally we'd do some accounting here but
+     for now it's easier to pin the original so the proxy remains valid.  */
+  if (old->l_type == lt_loaded)
+    old->l_flags_1 |= DF_1_NODELETE;
+
+  /* Fix up the searchlist so that relocations work.  */
+  _dl_map_object_deps (new, NULL, 0, 0,
+		       mode & (__RTLD_DLOPEN | RTLD_DEEPBIND | __RTLD_AUDIT));
+
+  /* And finally put the proxy in the target namespace.  */
+  _dl_add_to_namespace_list (new, nsid);
+
+  return new;
+}
 
 /* Allocate a `struct link_map' for a new object being loaded,
    and enter it into the _dl_loaded list.  */
diff --git a/elf/dl-open.c b/elf/dl-open.c
index 6ea5dd2457..2cf4599fa8 100644
--- a/elf/dl-open.c
+++ b/elf/dl-open.c
@@ -73,6 +73,12 @@  struct dl_open_args
   int argc;
   char **argv;
   char **env;
+
+  /* Some state needs to be maintained between dl_open_worker_begin and
+     dl_open_worker now that they have been split across 2 functions.  */
+  bool want_proxy;
+  Lmid_t proxy_ns;
+  int dl_mode;
 };
 
 /* Called in case the global scope cannot be extended.  */
@@ -489,8 +495,19 @@  dl_open_worker_begin (void *a)
 {
   struct dl_open_args *args = a;
   const char *file = args->file;
-  int mode = args->mode;
   struct link_map *call_map = NULL;
+  struct link_map *preloaded = NULL;
+  bool dl_isolate = args->mode & RTLD_ISOLATE;
+
+  args->dl_mode = args->mode;
+  args->proxy_ns = LM_ID_BASE;
+  args->want_proxy = false;
+
+  /* Isolation means we should suppress all inter-namespace sharing.  */
+  if (dl_isolate)
+    args->dl_mode &= ~RTLD_SHARED;
+  else
+    args->want_proxy = args->dl_mode & RTLD_SHARED;
 
   /* 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
@@ -515,6 +532,15 @@  dl_open_worker_begin (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 (args->dl_mode & RTLD_SHARED))
+    {
+      args->mode &= ~RTLD_SHARED;
+      args->dl_mode &= ~RTLD_SHARED;
+      args->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).  */
@@ -528,26 +554,73 @@  dl_open_worker_begin (void *a)
      may not be true if this is a recursive call to dlopen.  */
   _dl_debug_initialize (0, args->nsid);
 
+  /* Target Lmid is not the base and we haven't explicitly asked for a proxy:
+     We need to check for a matching DSO in the base Lmid in case it is flagged
+     DT_GNU_FLAGS_1/DF_GNU_1_UNIQUE in which case we add RTLD_SHARED to the
+     mode and set want_proxy.
+     NOTE: RTLD_ISOLATE in the mode suppresses this behaviour.  */
+  if (__glibc_unlikely (args->nsid != LM_ID_BASE) &&
+      __glibc_likely (!dl_isolate) &&
+      __glibc_likely (!args->want_proxy))
+    {
+      preloaded = _dl_find_dso (file, LM_ID_BASE);
+
+      if ((preloaded != NULL) && (preloaded->l_gnu_flags_1 & DF_GNU_1_UNIQUE))
+	{
+	  args->want_proxy = true;
+	  args->dl_mode |= RTLD_SHARED;
+	}
+    }
+
   /* Load the named object.  */
   struct link_map *new;
   args->map = new = _dl_map_object (call_map, file, lt_loaded, 0,
-				    mode | __RTLD_CALLMAP, args->nsid);
+				    args->dl_mode | __RTLD_CALLMAP, args->nsid);
 
   /* If the pointer returned is NULL this means the RTLD_NOLOAD flag is
      set and the object is not already loaded.  */
   if (new == NULL)
     {
-      assert (mode & RTLD_NOLOAD);
+      assert (args->dl_mode & RTLD_NOLOAD);
       return;
     }
 
-  if (__glibc_unlikely (mode & __RTLD_SPROF))
+  if (__glibc_unlikely (args->dl_mode & __RTLD_SPROF))
     /* This happens only if we load a DSO for 'sprof'.  */
     return;
 
   /* 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 are trying to load a DF_GNU_1_UNIQUE flagged DSO which was
+     NOT ALREADY LOADED (or not loaded with the name we are using) then
+     _dl_map_object will have returned an instance from the main namespace.
+     We need to detect this and set up the RTLD_SHARED flags.  */
+  if (__glibc_unlikely (args->nsid != LM_ID_BASE && new->l_ns == LM_ID_BASE))
+    {
+      args->want_proxy = RTLD_SHARED;
+      args->dl_mode |= RTLD_SHARED;
+    }
+
+  /* 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 (args->want_proxy))
+    {
+      args->proxy_ns = args->nsid;
+      args->nsid = LM_ID_BASE;
+      args->libc_already_loaded = GL(dl_ns)[LM_ID_BASE].libc_map != NULL;
+    }
+
   /* It was already open.  */
   if (__glibc_unlikely (new->l_searchlist.r_list != NULL))
     {
@@ -559,12 +632,12 @@  dl_open_worker_begin (void *a)
       /* If the user requested the object to be in the global
 	 namespace but it is not so far, prepare to add it now.  This
 	 can raise an exception to do a malloc failure.  */
-      if ((mode & RTLD_GLOBAL) && new->l_global == 0)
+      if ((args->dl_mode & RTLD_GLOBAL) && new->l_global == 0)
 	add_to_global_resize (new);
 
       /* Mark the object as not deletable if the RTLD_NODELETE flags
 	 was passed.  */
-      if (__glibc_unlikely (mode & RTLD_NODELETE))
+      if (__glibc_unlikely (args->dl_mode & RTLD_NODELETE))
 	{
 	  if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_FILES)
 	      && !new->l_nodelete_active)
@@ -574,22 +647,33 @@  dl_open_worker_begin (void *a)
 	}
 
       /* Finalize the addition to the global scope.  */
-      if ((mode & RTLD_GLOBAL) && new->l_global == 0)
+      if ((args->dl_mode & RTLD_GLOBAL) && new->l_global == 0)
 	add_to_global_update (new);
 
       assert (_dl_debug_update (args->nsid)->r_state == RT_CONSISTENT);
 
+      if (__glibc_unlikely (args->want_proxy))
+	{
+	  args->map = new = _dl_new_proxy (new, args->dl_mode, args->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;
     }
 
   /* Schedule NODELETE marking for the directly loaded object if
      requested.  */
-  if (__glibc_unlikely (mode & RTLD_NODELETE))
+  if (__glibc_unlikely (args->dl_mode & RTLD_NODELETE))
     new->l_nodelete_pending = true;
 
   /* Load that object's dependencies.  */
   _dl_map_object_deps (new, NULL, 0, 0,
-		       mode & (__RTLD_DLOPEN | RTLD_DEEPBIND | __RTLD_AUDIT));
+		       args->dl_mode & (__RTLD_DLOPEN | RTLD_DEEPBIND |
+					__RTLD_AUDIT | RTLD_ISOLATE));
 
   /* So far, so good.  Now check the versions.  */
   for (unsigned int i = 0; i < new->l_searchlist.r_nlist; ++i)
@@ -645,9 +729,9 @@  dl_open_worker_begin (void *a)
     _dl_show_scope (new, 0);
 
   /* Only do lazy relocation if `LD_BIND_NOW' is not set.  */
-  int reloc_mode = mode & __RTLD_AUDIT;
+  int reloc_mode = args->dl_mode & __RTLD_AUDIT;
   if (GLRO(dl_lazy))
-    reloc_mode |= mode & RTLD_LAZY;
+    reloc_mode |= args->dl_mode & RTLD_LAZY;
 
   /* Objects must be sorted by dependency for the relocation process.
      This allows IFUNC relocations to work and it also means copy
@@ -683,7 +767,10 @@  dl_open_worker_begin (void *a)
     {
       l = new->l_initfini[i];
 
-      if (l->l_real->l_relocated)
+      if (l->l_proxy)
+	l = l->l_real;
+
+      if (l->l_relocated)
 	continue;
 
       if (! relocation_in_progress)
@@ -729,7 +816,7 @@  dl_open_worker_begin (void *a)
 
   /* Perform the necessary allocations for adding new global objects
      to the global scope below.  */
-  if (mode & RTLD_GLOBAL)
+  if (args->dl_mode & RTLD_GLOBAL)
     add_to_global_resize (new);
 
   /* Demarcation point: After this, no recoverable errors are allowed.
@@ -772,9 +859,21 @@  dl_open_worker_begin (void *a)
      namespace.  */
   if (!args->libc_already_loaded)
     {
-      /* dlopen cannot be used to load an initial libc by design.  */
+      /* If this is a secondary (nsid != LM_ID_BASE) namespace then
+	 it is POSSIBLE there's no libc_map at all - We use the one
+	 shared with LM_ID_BASE instead (which MUST already be
+	 initialised for us to even reach here).  */
       struct link_map *libc_map = GL(dl_ns)[args->nsid].libc_map;
-      _dl_call_libc_early_init (libc_map, false);
+#ifdef SHARED
+      bool initial = (libc_map != NULL) && (libc_map->l_real->l_ns == LM_ID_BASE);
+#else
+      /* In the static case, there is only one namespace, but it
+	 contains a secondary libc (the primary libc is statically
+	 linked).  */
+      bool initial = false;
+#endif
+      if (libc_map != NULL)
+	_dl_call_libc_early_init (libc_map, initial);
     }
 
   args->worker_continue = true;
@@ -804,7 +903,6 @@  dl_open_worker (void *a)
   if (!args->worker_continue)
     return;
 
-  int mode = args->mode;
   struct link_map *new = args->map;
 
   /* Run the initializer functions of new objects.  Temporarily
@@ -822,13 +920,30 @@  dl_open_worker (void *a)
   }
 
   /* Now we can make the new map available in the global scope.  */
-  if (mode & RTLD_GLOBAL)
+  if (args->dl_mode & RTLD_GLOBAL)
     add_to_global_update (new);
 
+  if (__glibc_unlikely (args->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, args->dl_mode, args->proxy_ns);
+      ++args->map->l_direct_opencount;
+    }
+
   /* 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 *
@@ -917,9 +1032,12 @@  no more namespaces available for dlmopen()"));
   if (__glibc_unlikely (exception.errstring != NULL))
     {
       /* Avoid keeping around a dangling reference to the libc.so link
-	 map in case it has been cached in libc_map.  */
-      if (!args.libc_already_loaded)
-	GL(dl_ns)[args.nsid].libc_map = NULL;
+	 map in case it has been cached in libc_map.
+	 If this is a secondary namespace opened with LM_ID_NEWLM and
+	 WITHOUT RTLD_ISOLATE then nsid can still be -1 (LM_ID_NEWLM)
+	 at this point.	 */
+      if (!args.libc_already_loaded && nsid >= LM_ID_BASE)
+	GL(dl_ns)[nsid].libc_map = NULL;
 
       /* Remove the object from memory.  It may be in an inconsistent
 	 state if relocation failed, for example.  */
diff --git a/elf/dl-sym.c b/elf/dl-sym.c
index de5769f926..e3955e7a51 100644
--- a/elf/dl-sym.c
+++ b/elf/dl-sym.c
@@ -96,6 +96,10 @@  do_sym (void *handle, const char *name, void *who,
     {
       match = _dl_sym_find_caller_link_map (caller);
 
+      /* Proxies don't contain any symbols: Need to look at the real DSO.  */
+      if (__glibc_unlikely (match->l_proxy))
+	match = match->l_real;
+
       /* Search the global scope.  We have the simple case where
 	 we look up in the scope of an object which was part of
 	 the initial binary.  And then the more complex part
@@ -140,6 +144,11 @@  RTLD_NEXT used in code not dynamically loaded"));
 	}
 
       struct link_map *l = match;
+
+      /* Proxies don't contain any symbols: Need to look at the real DSO.  */
+      if (__glibc_unlikely (l->l_proxy))
+	l = l->l_real;
+
       while (l->l_loader != NULL)
 	l = l->l_loader;
 
@@ -150,6 +159,11 @@  RTLD_NEXT used in code not dynamically loaded"));
     {
       /* Search the scope of the given object.  */
       struct link_map *map = handle;
+
+      /* Proxies don't contain any symbols: Need to look at the real DSO.  */
+      if (__glibc_unlikely (map->l_proxy))
+	map = map->l_real;
+
       result = GLRO(dl_lookup_symbol_x) (name, map, &ref, map->l_local_scope,
 					 vers, 0, flags, NULL);
     }
diff --git a/elf/rtld.c b/elf/rtld.c
index 5eee9e1091..969821ae4d 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -661,7 +661,7 @@  dlmopen_doit (void *a)
   struct dlmopen_args *args = (struct dlmopen_args *) a;
   args->map = _dl_open (args->fname,
 			(RTLD_LAZY | __RTLD_DLOPEN | __RTLD_AUDIT
-			 | __RTLD_SECURE),
+			 | __RTLD_SECURE | RTLD_ISOLATE),
 			dl_main, LM_ID_NEWLM, _dl_argc, _dl_argv,
 			__environ);
 }
diff --git a/elf/tst-tls-ie-dlmopen.c b/elf/tst-tls-ie-dlmopen.c
index a579d72d2d..e67017db2d 100644
--- a/elf/tst-tls-ie-dlmopen.c
+++ b/elf/tst-tls-ie-dlmopen.c
@@ -53,7 +53,7 @@  static void *
 load_and_access (Lmid_t lmid, const char *mod, const char *func)
 {
   /* Load module with TLS.  */
-  void *p = xdlmopen (lmid, mod, RTLD_NOW);
+  void *p = xdlmopen (lmid, mod, RTLD_NOW|RTLD_ISOLATE);
   /* Access the TLS variable to ensure it is allocated.  */
   void (*f) (void) = (void (*) (void))xdlsym (p, func);
   f ();
@@ -95,7 +95,7 @@  do_test (void)
      than 1024 bytes are available (exact number depends on TLS optimizations
      and the libc TLS use).  */
   printf ("The next dlmopen should fail...\n");
-  void *p = dlmopen (LM_ID_BASE, "tst-tls-ie-mod4.so", RTLD_NOW);
+  void *p = dlmopen (LM_ID_BASE, "tst-tls-ie-mod4.so", RTLD_NOW|RTLD_ISOLATE);
   if (p != NULL)
     FAIL_EXIT1 ("error: expected dlmopen to fail because there is "
 		"not enough surplus static TLS.\n");
diff --git a/include/link.h b/include/link.h
index fd39c2207c..befc73a0ed 100644
--- a/include/link.h
+++ b/include/link.h
@@ -107,8 +107,9 @@  struct link_map
        They may change without notice.  */
 
     /* This is an element which is only ever different from a pointer to
-       the very same copy of this type for ld.so when it is used in more
-       than one namespace.  */
+       the very same copy of this type when:
+       - A shallow copy of ld.so is placed in namespaces other than LM_ID_BASE.
+       - An object is proxied into a namespace by dlmopen with RTLD_SHARED.  */
     struct link_map *l_real;
 
     /* Number of the namespace this link map belongs to.  */
@@ -180,6 +181,7 @@  struct link_map
     unsigned int l_relocated:1;	/* Nonzero if object's relocations done.  */
     unsigned int l_init_called:1; /* Nonzero if DT_INIT function called.  */
     unsigned int l_global:1;	/* Nonzero if object in _dl_global_scope.  */
+    unsigned int l_proxy:1;    /* Nonzero if object is a shallow copy.  */
     unsigned int l_reserved:2;	/* Reserved for internal use.  */
     unsigned int l_phdr_allocated:1; /* Nonzero if the data structure pointed
 					to by `l_phdr' is allocated.  */
diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index 017c47cc8e..5c4c42ef77 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -1025,6 +1025,11 @@  extern lookup_t _dl_lookup_symbol_x (const char *undef,
 				     struct link_map *skip_map)
      attribute_hidden;
 
+/* Proxy an existing link map entry into a new link map */
+extern struct link_map *_dl_new_proxy (struct link_map *old,
+				       int mode,
+				       Lmid_t nsid)
+     attribute_hidden;
 
 /* Restricted version of _dl_lookup_symbol_x.  Searches MAP (and only
    MAP) for the symbol UNDEF_NAME, with GNU hash NEW_HASH (computed
@@ -1304,6 +1309,8 @@  rtld_hidden_proto (_dl_find_dso_for_object)
 extern struct link_map *_dl_find_dso (const char *name, Lmid_t nsid);
 rtld_hidden_proto (_dl_find_dso)
 
+extern struct link_map *_dl_find_proxy (Lmid_t nsid, const char *name);
+rtld_hidden_proto (_dl_find_proxy)
 
 /* Initialization which is normally done by the dynamic linker.  */
 extern void _dl_non_dynamic_init (void)
diff --git a/sysdeps/mips/bits/dlfcn.h b/sysdeps/mips/bits/dlfcn.h
index 5cec898de3..898797f863 100644
--- a/sysdeps/mips/bits/dlfcn.h
+++ b/sysdeps/mips/bits/dlfcn.h
@@ -32,6 +32,16 @@ 
    visible as if the object were linked directly into the program.  */
 #define RTLD_GLOBAL	0x0004
 
+/* If the following bit is set in the MODE argument to dlmopen
+   then the target object is loaded into the main namespace (if
+   it is not already there) and a shallow copy (proxy) is placed
+   in the target namespace: This allows multiple namespaces to
+   share a single instance of a DSO.  */
+#define RTLD_SHARED 0x00020
+
+/* Suppress RTLD_SHARED and/or DT_GNU_FLAGS_1/DF_GNU_1_UNIQUE  */
+#define RTLD_ISOLATE 0x00040
+
 /* Unix98 demands the following flag which is the inverse to RTLD_GLOBAL.
    The implementation does this by default and so we can define the
    value to zero.  */