From 609b2bdc67cca4ee584547966567fc8ee512acea Mon Sep 17 00:00:00 2001
From: Paul Pluzhnikov <ppluzhnikov@google.com>
Date: Fri, 31 Jan 2020 20:36:09 -0800
Subject: [PATCH 6/8] Add dlopen_with_offset to glibc.

This patchset is used in Google internal and is needed for
Loonix, https://crbug.com/1015890.

The patch is a squashed diff of following commits:
https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=0010a400dcb13304c9081b0f9ad156ce88178320
https://sourceware.org/git/?p=glibc.git;a=commit;h=024f0eb409268e704a83cae2c1511d31167c6f49
https://sourceware.org/git/?p=glibc.git;a=commit;h=295883290d7f26f483e574e80a851cea2d639d12
https://sourceware.org/git/?p=glibc.git;a=commit;h=771d3bf7065f6c25219cf206816b695a628a4618
https://sourceware.org/git/?p=glibc.git;a=commit;h=a6241b657547f0f708629bc254ebcaafb6f25074
https://sourceware.org/git/?p=glibc.git;a=commit;h=c0342bb26e0251dc2d7ca00a884ea831705e32b4

Then modified to support LARGEFILE 32 bit builds and to only call lseek
if the offset is larger than 0 (to avoid irritating sandbox policy).

This is requested by Loonix for use in Google internal.

[Adrian: forward ported to glibc 2.32]
---
 dlfcn/Versions             |  6 ++++
 dlfcn/dlfcn.h              | 27 ++++++++++++++
 dlfcn/dlmopen.c            | 56 ++++++++++++++++++++++-------
 dlfcn/dlopen.c             | 66 +++++++++++++++++++++++++++-------
 dlfcn/dlopenold.c          |  2 +-
 elf/dl-deps.c              |  2 +-
 elf/dl-libc.c              |  4 ++-
 elf/dl-load.c              | 72 ++++++++++++++++++++++++++++----------
 elf/dl-open.c              |  7 ++--
 elf/rtld.c                 |  6 ++--
 include/dlfcn.h            | 11 ++++++
 include/link.h             |  6 ++++
 sysdeps/generic/ldsodefs.h | 12 ++++---
 13 files changed, 223 insertions(+), 54 deletions(-)

diff --git a/dlfcn/Versions b/dlfcn/Versions
index 1df6925a92..6032584242 100644
--- a/dlfcn/Versions
+++ b/dlfcn/Versions
@@ -11,6 +11,12 @@ libdl {
   GLIBC_2.3.4 {
     dlmopen;
   }
+  GLIBC_2.15 {
+    __google_dlopen_with_offset; __google_dlmopen_with_offset;
+  }
+  GLIBC_2.27 {
+    __google_dlopen_with_offset64;
+  }
   GLIBC_PRIVATE {
     _dlfcn_hook;
     __libdl_freeres;
diff --git a/dlfcn/dlfcn.h b/dlfcn/dlfcn.h
index c629fc7f42..880c72e614 100644
--- a/dlfcn/dlfcn.h
+++ b/dlfcn/dlfcn.h
@@ -22,6 +22,7 @@
 #include <features.h>
 #define __need_size_t
 #include <stddef.h>
+#include <sys/types.h>
 
 /* Collect various system dependent definitions and declarations.  */
 #include <bits/dlfcn.h>
@@ -55,6 +56,26 @@ __BEGIN_DECLS
    passed to `dlsym' to get symbol values from it.  */
 extern void *dlopen (const char *__file, int __mode) __THROWNL;
 
+/* Same as above, but ELF header is at OFF from the start of file.  */
+#ifndef __USE_FILE_OFFSET64
+extern void *__google_dlopen_with_offset (__const char *__file,
+					  __off_t offset,
+					  int __mode) __THROW;
+#else
+# ifdef __REDIRECT_NTH
+extern void *__REDIRECT_NTH (__google_dlopen_with_offset,
+			(__const char *__file, __off64_t offset, int __mode),
+			__google_dlopen_with_offset64);
+# else
+#define __google_dlopen_with_offset __google_dlopen_with_offset64
+# endif
+#endif  /* not __USE_FILE_OFFSET64 */
+#ifdef __USE_LARGEFILE64
+extern void *__google_dlopen_with_offset64 (__const char *__file,
+					    __off64_t offset,
+					    int __mode) __THROW;
+#endif
+
 /* Unmap and close a shared object opened by `dlopen'.
    The handle cannot be used again after calling `dlclose'.  */
 extern int dlclose (void *__handle) __THROWNL __nonnull ((1));
@@ -68,6 +89,12 @@ extern void *dlsym (void *__restrict __handle,
 /* Like `dlopen', but request object to be allocated in a new namespace.  */
 extern void *dlmopen (Lmid_t __nsid, const char *__file, int __mode) __THROWNL;
 
+/* Same as above, but ELF header is at OFF from the start of file.  */
+extern void *__google_dlmopen_with_offset (Lmid_t __nsid,
+					   __const char *__file,
+					   off_t offset,
+					   int __mode) __THROW;
+
 /* Find the run-time address in the shared object HANDLE refers to
    of the symbol called NAME with VERSION.  */
 extern void *dlvsym (void *__restrict __handle,
diff --git a/dlfcn/dlmopen.c b/dlfcn/dlmopen.c
index 1396c818ac..141b0b769b 100644
--- a/dlfcn/dlmopen.c
+++ b/dlfcn/dlmopen.c
@@ -38,6 +38,8 @@ struct dlmopen_args
 {
   /* Namespace ID.  */
   Lmid_t nsid;
+  /* ELF header at offset in file.  */
+  off_t offset;
   /* The arguments for dlopen_doit.  */
   const char *file;
   int mode;
@@ -68,13 +70,52 @@ dlmopen_doit (void *a)
 	_dl_signal_error (EINVAL, NULL, NULL, N_("invalid mode"));
     }
 
-  args->new = GLRO(dl_open) (args->file ?: "", args->mode | __RTLD_DLOPEN,
+  args->new = GLRO(dl_open) (args->file ?: "", args->offset, args->mode | __RTLD_DLOPEN,
 			     args->caller,
 			     args->nsid, __dlfcn_argc, __dlfcn_argv,
 			     __environ);
 }
 
 
+static void *
+__dlmopen_common (struct dlmopen_args *args)
+{
+
+# ifdef SHARED
+  return _dlerror_run (dlmopen_doit, args) ? NULL : args->new;
+# else
+  if (_dlerror_run (dlmopen_doit, args))
+    return NULL;
+
+  __libc_register_dl_open_hook ((struct link_map *) args->new);
+  __libc_register_dlfcn_hook ((struct link_map *) args->new);
+
+  return args->new;
+# endif
+}
+
+void *
+__dlmopen_with_offset (Lmid_t nsid, const char *file, off_t offset,
+		       int mode DL_CALLER_DECL)
+{
+# ifdef SHARED
+  if (!rtld_active ())
+    return _dlfcn_hook->dlmopen_with_offset (nsid, file, offset, mode, RETURN_ADDRESS (0));
+# endif
+
+  struct dlmopen_args oargs;
+  oargs.nsid = nsid;
+  oargs.file = file;
+  oargs.offset = offset;
+  oargs.mode = mode;
+  oargs.caller = DL_CALLER;
+
+  return __dlmopen_common (&oargs);
+}
+# ifdef SHARED
+strong_alias (__dlmopen_with_offset, __google_dlmopen_with_offset)
+# endif
+
 void *
 __dlmopen (Lmid_t nsid, const char *file, int mode DL_CALLER_DECL)
 {
@@ -86,20 +127,11 @@ __dlmopen (Lmid_t nsid, const char *file, int mode DL_CALLER_DECL)
   struct dlmopen_args args;
   args.nsid = nsid;
   args.file = file;
+  args.offset = 0;
   args.mode = mode;
   args.caller = DL_CALLER;
 
-# ifdef SHARED
-  return _dlerror_run (dlmopen_doit, &args) ? NULL : args.new;
-# else
-  if (_dlerror_run (dlmopen_doit, &args))
-    return NULL;
-
-  __libc_register_dl_open_hook ((struct link_map *) args.new);
-  __libc_register_dlfcn_hook ((struct link_map *) args.new);
-
-  return args.new;
-# endif
+  return __dlmopen_common (&args);
 }
 # ifdef SHARED
 strong_alias (__dlmopen, dlmopen)
diff --git a/dlfcn/dlopen.c b/dlfcn/dlopen.c
index 5b4ed6798a..bb43674b9c 100644
--- a/dlfcn/dlopen.c
+++ b/dlfcn/dlopen.c
@@ -21,6 +21,7 @@
 #include <stddef.h>
 #include <unistd.h>
 #include <ldsodefs.h>
+#include <errno.h>
 
 #if !defined SHARED && IS_IN (libdl)
 
@@ -37,6 +38,8 @@ struct dlopen_args
 {
   /* The arguments for dlopen_doit.  */
   const char *file;
+  /* ELF header at offset in file.  */
+  off_t offset;
   int mode;
   /* The return value of dlopen_doit.  */
   void *new;
@@ -63,13 +66,61 @@ dlopen_doit (void *a)
 		     | __RTLD_SPROF))
     _dl_signal_error (0, NULL, NULL, _("invalid mode parameter"));
 
-  args->new = GLRO(dl_open) (args->file ?: "", args->mode | __RTLD_DLOPEN,
+  args->new = GLRO(dl_open) (args->file ?: "", args->offset, args->mode | __RTLD_DLOPEN,
 			     args->caller,
 			     args->file == NULL ? LM_ID_BASE : NS,
 			     __dlfcn_argc, __dlfcn_argv, __environ);
 }
 
 
+static void *
+__dlopen_common (struct dlopen_args *args)
+{
+# ifdef SHARED
+  return _dlerror_run (dlopen_doit, args) ? NULL : args->new;
+# else
+  if (_dlerror_run (dlopen_doit, args))
+    return NULL;
+
+  __libc_register_dl_open_hook ((struct link_map *) args->new);
+  __libc_register_dlfcn_hook ((struct link_map *) args->new);
+
+  return args->new;
+# endif
+}
+
+# ifdef SHARED
+void *
+__dlopen_with_offset (const char *file, off_t offset, int mode DL_CALLER_DECL)
+{
+  if (!rtld_active ())
+    return _dlfcn_hook->dlopen_with_offset (file, offset, mode, DL_CALLER);
+
+  struct dlopen_args oargs;
+  oargs.file = file;
+  oargs.offset = offset;
+  oargs.mode = mode;
+  oargs.caller = DL_CALLER;
+
+  return __dlopen_common (&oargs);
+}
+strong_alias (__dlopen_with_offset, __google_dlopen_with_offset)
+
+void *
+__dlopen_with_offset64 (const char *file, off64_t offset, int mode DL_CALLER_DECL)
+{
+#ifndef __OFF_T_MATCHES_OFF64_T
+  if (offset > 0xFFFFFFFF) {
+    _dl_signal_error(EFBIG, "__dlopen_with_offset64", NULL,
+		     N_("File offset too large. Only 32 bit ELF supported."));
+    return NULL;
+  }
+#endif
+  return __dlopen_with_offset(file, offset, mode);
+}
+strong_alias (__dlopen_with_offset64, __google_dlopen_with_offset64)
+# endif
+
 void *
 __dlopen (const char *file, int mode DL_CALLER_DECL)
 {
@@ -80,20 +131,11 @@ __dlopen (const char *file, int mode DL_CALLER_DECL)
 
   struct dlopen_args args;
   args.file = file;
+  args.offset = 0;
   args.mode = mode;
   args.caller = DL_CALLER;
 
-# ifdef SHARED
-  return _dlerror_run (dlopen_doit, &args) ? NULL : args.new;
-# else
-  if (_dlerror_run (dlopen_doit, &args))
-    return NULL;
-
-  __libc_register_dl_open_hook ((struct link_map *) args.new);
-  __libc_register_dlfcn_hook ((struct link_map *) args.new);
-
-  return args.new;
-# endif
+  return __dlopen_common (&args);
 }
 # ifdef SHARED
 #  include <shlib-compat.h>
diff --git a/dlfcn/dlopenold.c b/dlfcn/dlopenold.c
index f06deec639..1082d087b9 100644
--- a/dlfcn/dlopenold.c
+++ b/dlfcn/dlopenold.c
@@ -51,7 +51,7 @@ dlopen_doit (void *a)
 {
   struct dlopen_args *args = (struct dlopen_args *) a;
 
-  args->new = GLRO(dl_open) (args->file ?: "", args->mode | __RTLD_DLOPEN,
+  args->new = GLRO(dl_open) (args->file ?: "", 0, args->mode | __RTLD_DLOPEN,
 			     args->caller,
 			     args->file == NULL ? LM_ID_BASE : NS,
 			     __dlfcn_argc, __dlfcn_argv, __environ);
diff --git a/elf/dl-deps.c b/elf/dl-deps.c
index b5a43232a7..c2d008fb63 100644
--- a/elf/dl-deps.c
+++ b/elf/dl-deps.c
@@ -61,7 +61,7 @@ openaux (void *a)
 {
   struct openaux_args *args = (struct openaux_args *) a;
 
-  args->aux = _dl_map_object (args->map, args->name,
+  args->aux = _dl_map_object (args->map, args->name, 0,
 			      (args->map->l_type == lt_executable
 			       ? lt_library : args->map->l_type),
 			      args->trace_mode, args->open_mode,
diff --git a/elf/dl-libc.c b/elf/dl-libc.c
index 24a715511a..51c167d4a3 100644
--- a/elf/dl-libc.c
+++ b/elf/dl-libc.c
@@ -59,6 +59,7 @@ struct do_dlopen_args
 {
   /* Argument to do_dlopen.  */
   const char *name;
+  off_t offset;
   /* Opening mode.  */
   int mode;
   /* This is the caller of the dlopen() function.  */
@@ -93,7 +94,7 @@ do_dlopen (void *ptr)
 {
   struct do_dlopen_args *args = (struct do_dlopen_args *) ptr;
   /* Open and relocate the shared object.  */
-  args->map = GLRO(dl_open) (args->name, args->mode, args->caller_dlopen,
+  args->map = GLRO(dl_open) (args->name, args->offset, args->mode, args->caller_dlopen,
 			     __LM_ID_CALLER, __libc_argc, __libc_argv,
 			     __environ);
 }
@@ -186,6 +187,7 @@ __libc_dlopen_mode (const char *name, int mode)
 {
   struct do_dlopen_args args;
   args.name = name;
+  args.offset = 0;
   args.mode = mode;
   args.caller_dlopen = RETURN_ADDRESS (0);
 
diff --git a/elf/dl-load.c b/elf/dl-load.c
index 71867e7c1a..54f6b7b032 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -61,6 +61,7 @@ struct filebuf
 #include <abi-tag.h>
 #include <stackinfo.h>
 #include <sysdep.h>
+#include <_itoa.h>
 #include <stap-probe.h>
 #include <libc-pointer-arith.h>
 #include <array_length.h>
@@ -933,7 +934,7 @@ _dl_process_pt_gnu_property (struct link_map *l, int fd, const ElfW(Phdr) *ph)
 static
 #endif
 struct link_map *
-_dl_map_object_from_fd (const char *name, const char *origname, int fd,
+_dl_map_object_from_fd (const char *name, const char *origname, int fd, off_t offset,
 			struct filebuf *fbp, char *realname,
 			struct link_map *loader, int l_type, int mode,
 			void **stack_endp, Lmid_t nsid)
@@ -973,7 +974,8 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
 
       /* Look again to see if the real name matched another already loaded.  */
       for (l = GL(dl_ns)[nsid]._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_removed && _dl_file_id_match_p (&l->l_file_id, &id)
+	    && l->l_off == offset)
 	  {
 	    /* The object is already loaded.
 	       Just bump its reference count and return it.  */
@@ -982,7 +984,10 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
 	    /* If the name is not in the list of names for this object add
 	       it.  */
 	    free (realname);
-	    add_name_to_object (l, name);
+	    if (offset == 0)
+	      /* If offset!=0, foo.so/@0x<offset> should be the *only*
+	         name for this object. b/20141439.  */
+	      add_name_to_object (l, name);
 
 	    return l;
 	  }
@@ -1067,8 +1072,29 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
   else
     assert (r->r_state == RT_ADD);
 
+#ifdef SHARED
+  // This code could be linked into 'sln', which does not have _itoa.
+  // We only care about this when this is linked into ld-linux.
+  if (offset != 0)
+    {
+      /* Google-specific: to help GDB, and for b/18243822, turn realname
+         into "realname/@0x<offset>"  */
+      realname = realloc (realname, strlen(realname) + 16 + 4 /* "/@0x" */);
+      if (realname == NULL)
+	{
+	  errstring = N_("unable to realloc");
+	  goto call_lose_errno;
+	}
+      strcat(realname, "/@0x");
+
+      char tmp[20];
+      tmp[19] = '\0';
+      strcat(realname, _itoa(offset, &tmp[19], 16, 0));
+    }
+#endif
+
   /* Enter the new object in the list of loaded objects.  */
-  l = _dl_new_object (realname, name, l_type, loader, mode, nsid);
+  l = _dl_new_object (realname, (offset ? realname : name), l_type, loader, mode, nsid);
   if (__glibc_unlikely (l == NULL))
     {
 #ifdef SHARED
@@ -1155,7 +1181,12 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
 	  c->mapend = ALIGN_UP (ph->p_vaddr + ph->p_filesz, GLRO(dl_pagesize));
 	  c->dataend = ph->p_vaddr + ph->p_filesz;
 	  c->allocend = ph->p_vaddr + ph->p_memsz;
-	  c->mapoff = ALIGN_DOWN (ph->p_offset, GLRO(dl_pagesize));
+	  if (offset & (GLRO(dl_pagesize) - 1))
+	    {
+	      errstring = N_("invalid offset");
+	      goto call_lose;
+	    }
+	  c->mapoff = ALIGN_DOWN(offset + ph->p_offset, GLRO(dl_pagesize));
 
 	  /* Determine whether there is a gap between the last segment
 	     and this one.  */
@@ -1448,6 +1479,8 @@ cannot enable executable stack as shared object requires");
   assert (origname == NULL);
 #endif
 
+  l->l_off = offset;
+
   /* When we profile the SONAME might be needed for something else but
      loading.  Add it right away.  */
   if (__glibc_unlikely (GLRO(dl_profile) != NULL)
@@ -1552,7 +1585,7 @@ print_search_path (struct r_search_path_elem **list,
    If FD is not -1, then the file is already open and FD refers to it.
    In that case, FD is consumed for both successful and error returns.  */
 static int
-open_verify (const char *name, int fd,
+open_verify (const char *name, int fd, off_t offset,
              struct filebuf *fbp, struct link_map *loader,
 	     int whatcode, int mode, bool *found_other_class, bool free_name)
 {
@@ -1633,6 +1666,9 @@ open_verify (const char *name, int fd,
       unsigned int osversion;
       size_t maplength;
 
+      if (offset > 0 && __lseek (fd, offset, SEEK_SET) == -1)
+	goto close_and_out;
+
       /* We successfully opened the file.  Now verify it is a file
 	 we can use.  */
       __set_errno (0);
@@ -1846,7 +1882,7 @@ open_verify (const char *name, int fd,
    if MAY_FREE_DIRS is true.  */
 
 static int
-open_path (const char *name, size_t namelen, int mode,
+open_path (const char *name, size_t namelen, off_t offset, int mode,
 	   struct r_search_path_struct *sps, char **realname,
 	   struct filebuf *fbp, struct link_map *loader, int whatcode,
 	   bool *found_other_class)
@@ -1898,7 +1934,7 @@ open_path (const char *name, size_t namelen, int mode,
 	  if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_LIBS))
 	    _dl_debug_printf ("  trying file=%s\n", buf);
 
-	  fd = open_verify (buf, -1, fbp, loader, whatcode, mode,
+	  fd = open_verify (buf, -1, offset, fbp, loader, whatcode, mode,
 			    found_other_class, false);
 	  if (this_dir->status[cnt] == unknown)
 	    {
@@ -1996,7 +2032,7 @@ open_path (const char *name, size_t namelen, int mode,
 /* Map in the shared object file NAME.  */
 
 struct link_map *
-_dl_map_object (struct link_map *loader, const char *name,
+_dl_map_object (struct link_map *loader, const char *name, off_t offset,
 		int type, int trace_mode, int mode, Lmid_t nsid)
 {
   int fd;
@@ -2111,7 +2147,7 @@ _dl_map_object (struct link_map *loader, const char *name,
 	  for (l = loader; l; l = l->l_loader)
 	    if (cache_rpath (l, &l->l_rpath_dirs, DT_RPATH, "RPATH"))
 	      {
-		fd = open_path (name, namelen, mode,
+		fd = open_path (name, namelen, offset, mode,
 				&l->l_rpath_dirs,
 				&realname, &fb, loader, LA_SER_RUNPATH,
 				&found_other_class);
@@ -2127,7 +2163,7 @@ _dl_map_object (struct link_map *loader, const char *name,
 	      && main_map != NULL && main_map->l_type != lt_loaded
 	      && cache_rpath (main_map, &main_map->l_rpath_dirs, DT_RPATH,
 			      "RPATH"))
-	    fd = open_path (name, namelen, mode,
+	    fd = open_path (name, namelen, offset, mode,
 			    &main_map->l_rpath_dirs,
 			    &realname, &fb, loader ?: main_map, LA_SER_RUNPATH,
 			    &found_other_class);
@@ -2135,7 +2171,7 @@ _dl_map_object (struct link_map *loader, const char *name,
 
       /* Try the LD_LIBRARY_PATH environment variable.  */
       if (fd == -1 && env_path_list.dirs != (void *) -1)
-	fd = open_path (name, namelen, mode, &env_path_list,
+	fd = open_path (name, namelen, offset, mode, &env_path_list,
 			&realname, &fb,
 			loader ?: GL(dl_ns)[LM_ID_BASE]._ns_loaded,
 			LA_SER_LIBPATH, &found_other_class);
@@ -2144,7 +2180,7 @@ _dl_map_object (struct link_map *loader, const char *name,
       if (fd == -1 && loader != NULL
 	  && cache_rpath (loader, &loader->l_runpath_dirs,
 			  DT_RUNPATH, "RUNPATH"))
-	fd = open_path (name, namelen, mode,
+	fd = open_path (name, namelen, offset, mode,
 			&loader->l_runpath_dirs, &realname, &fb, loader,
 			LA_SER_RUNPATH, &found_other_class);
 
@@ -2153,7 +2189,7 @@ _dl_map_object (struct link_map *loader, const char *name,
           realname = _dl_sysdep_open_object (name, namelen, &fd);
           if (realname != NULL)
             {
-              fd = open_verify (realname, fd,
+              fd = open_verify (realname, fd, offset,
                                 &fb, loader ?: GL(dl_ns)[nsid]._ns_loaded,
                                 LA_SER_CONFIG, mode, &found_other_class,
                                 false);
@@ -2207,7 +2243,7 @@ _dl_map_object (struct link_map *loader, const char *name,
 
 	      if (cached != NULL)
 		{
-		  fd = open_verify (cached, -1,
+		  fd = open_verify (cached, -1, 0,
 				    &fb, loader ?: GL(dl_ns)[nsid]._ns_loaded,
 				    LA_SER_CONFIG, mode, &found_other_class,
 				    false);
@@ -2225,7 +2261,7 @@ _dl_map_object (struct link_map *loader, const char *name,
 	  && ((l = loader ?: GL(dl_ns)[nsid]._ns_loaded) == NULL
 	      || __glibc_likely (!(l->l_flags_1 & DF_1_NODEFLIB)))
 	  && rtld_search_dirs.dirs != (void *) -1)
-	fd = open_path (name, namelen, mode, &rtld_search_dirs,
+	fd = open_path (name, namelen, offset, mode, &rtld_search_dirs,
 			&realname, &fb, l, LA_SER_DEFAULT, &found_other_class);
 
       /* Add another newline when we are tracing the library loading.  */
@@ -2242,7 +2278,7 @@ _dl_map_object (struct link_map *loader, const char *name,
 	fd = -1;
       else
 	{
-	  fd = open_verify (realname, -1, &fb,
+	  fd = open_verify (realname, -1, offset, &fb,
 			    loader ?: GL(dl_ns)[nsid]._ns_loaded, 0, mode,
 			    &found_other_class, true);
 	  if (__glibc_unlikely (fd == -1))
@@ -2304,7 +2340,7 @@ _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,
+  return _dl_map_object_from_fd (name, origname, fd, offset, &fb, realname, loader,
 				 type, mode, &stack_end, nsid);
 }
 
diff --git a/elf/dl-open.c b/elf/dl-open.c
index 4bc5e3b6f1..2773ef2364 100644
--- a/elf/dl-open.c
+++ b/elf/dl-open.c
@@ -46,6 +46,8 @@
 struct dl_open_args
 {
   const char *file;
+  /* ELF header at offset in file.  */
+  off_t offset;
   int mode;
   /* This is the caller of the dlopen() function.  */
   const void *caller_dlopen;
@@ -528,7 +530,7 @@ dl_open_worker (void *a)
 
   /* Load the named object.  */
   struct link_map *new;
-  args->map = new = _dl_map_object (call_map, file, lt_loaded, 0,
+  args->map = new = _dl_map_object (call_map, file, args->offset, lt_loaded, 0,
 				    mode | __RTLD_CALLMAP, args->nsid);
 
   /* If the pointer returned is NULL this means the RTLD_NOLOAD flag is
@@ -805,7 +807,7 @@ dl_open_worker (void *a)
 }
 
 void *
-_dl_open (const char *file, int mode, const void *caller_dlopen, Lmid_t nsid,
+_dl_open (const char *file, off_t offset, int mode, const void *caller_dlopen, Lmid_t nsid,
 	  int argc, char *argv[], char *env[])
 {
   if ((mode & RTLD_BINDING_MASK) == 0)
@@ -855,6 +857,7 @@ no more namespaces available for dlmopen()"));
 
   struct dl_open_args args;
   args.file = file;
+  args.offset = offset;
   args.mode = mode;
   args.caller_dlopen = caller_dlopen;
   args.map = NULL;
diff --git a/elf/rtld.c b/elf/rtld.c
index 14a42ed00a..a829895b48 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -666,7 +666,7 @@ map_doit (void *a)
 {
   struct map_args *args = (struct map_args *) a;
   int type = (args->mode == __RTLD_OPENEXEC) ? lt_executable : lt_library;
-  args->map = _dl_map_object (args->loader, args->str, type, 0,
+  args->map = _dl_map_object (args->loader, args->str, 0, type, 0,
 			      args->mode, LM_ID_BASE);
 }
 
@@ -674,7 +674,7 @@ static void
 dlmopen_doit (void *a)
 {
   struct dlmopen_args *args = (struct dlmopen_args *) a;
-  args->map = _dl_open (args->fname,
+  args->map = _dl_open (args->fname, 0,
 			(RTLD_LAZY | __RTLD_DLOPEN | __RTLD_AUDIT
 			 | __RTLD_SECURE),
 			dl_main, LM_ID_NEWLM, _dl_argc, _dl_argv,
@@ -1338,7 +1338,7 @@ of this helper program; chances are you did not intend to run this program.\n\
 	{
 	  RTLD_TIMING_VAR (start);
 	  rtld_timer_start (&start);
-	  _dl_map_object (NULL, rtld_progname, lt_executable, 0,
+	  _dl_map_object (NULL, rtld_progname, 0, lt_executable, 0,
 			  __RTLD_OPENEXEC, LM_ID_BASE);
 	  rtld_timer_stop (&load_time, start);
 	}
diff --git a/include/dlfcn.h b/include/dlfcn.h
index a1816e4991..eea8de1e1a 100644
--- a/include/dlfcn.h
+++ b/include/dlfcn.h
@@ -1,5 +1,6 @@
 #ifndef _DLFCN_H
 #include <dlfcn/dlfcn.h>
+#include <sys/types.h>
 #ifndef _ISOMAC
 #include <link.h>		/* For ElfW.  */
 #include <stdbool.h>
@@ -110,6 +111,8 @@ extern int _dlerror_run (void (*operate) (void *), void *args)
 struct dlfcn_hook
 {
   void *(*dlopen) (const char *file, int mode, void *dl_caller);
+  void *(*dlopen_with_offset) (const char *file, off_t offset,
+			       int mode, void *dl_caller);
   int (*dlclose) (void *handle);
   void *(*dlsym) (void *handle, const char *name, void *dl_caller);
   void *(*dlvsym) (void *handle, const char *name, const char *version,
@@ -120,6 +123,8 @@ struct dlfcn_hook
 		  void **extra_info, int flags);
   int (*dlinfo) (void *handle, int request, void *arg);
   void *(*dlmopen) (Lmid_t nsid, const char *file, int mode, void *dl_caller);
+  void *(*dlmopen_with_offset) (Lmid_t nsid, const char *file, off_t offset,
+				int mode, void *dl_caller);
   void *pad[4];
 };
 
@@ -128,8 +133,14 @@ libdl_hidden_proto (_dlfcn_hook)
 
 extern void *__dlopen (const char *file, int mode DL_CALLER_DECL)
      attribute_hidden;
+extern void *__dlopen_with_offset (const char *file, off_t offset,
+				   int mode DL_CALLER_DECL)
+     attribute_hidden;
 extern void *__dlmopen (Lmid_t nsid, const char *file, int mode DL_CALLER_DECL)
      attribute_hidden;
+extern void *__dlmopen_with_offset (Lmid_t nsid, const char *file, off_t offset,
+				    int mode DL_CALLER_DECL)
+     attribute_hidden;
 extern int __dlclose (void *handle)
      attribute_hidden;
 extern void *__dlsym (void *handle, const char *name DL_CALLER_DECL)
diff --git a/include/link.h b/include/link.h
index aea268439c..a9431cdd35 100644
--- a/include/link.h
+++ b/include/link.h
@@ -261,6 +261,12 @@ struct link_map
        object is the same as one already loaded.  */
     struct r_file_id l_file_id;
 
+    /* Google-specific extension, intended to be part of public interface
+       to the debugger.  As such, it belongs right after l_prev... except
+       putting it there causes Google libunwind to crash due to its own
+       peeking into glibc internals (see grte_v1_glibc_link_map).  */
+    size_t l_off;  /* File offset to Elf_Ehdr.  */
+
     /* Collected information about own RUNPATH directories.  */
     struct r_search_path_struct l_runpath_dirs;
 
diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index 62ac40d81b..8a0e8e3802 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -644,8 +644,9 @@ struct rtld_global_ro
 				   const ElfW(Sym) **, struct r_scope_elem *[],
 				   const struct r_found_version *, int, int,
 				   struct link_map *);
-  void *(*_dl_open) (const char *file, int mode, const void *caller_dlopen,
-		     Lmid_t nsid, int argc, char *argv[], char *env[]);
+  void *(*_dl_open) (const char *file, off_t offset, int mode,
+		     const void *caller_dlopen, Lmid_t nsid,
+		     int argc, char *argv[], char *env[]);
   void (*_dl_close) (void *map);
   void *(*_dl_tls_get_addr_soft) (struct link_map *);
 #ifdef HAVE_DL_DISCOVER_OSVERSION
@@ -889,10 +890,12 @@ int _dl_catch_exception (struct dl_exception *exception,
 libc_hidden_proto (_dl_catch_exception)
 
 /* Open the shared object NAME and map in its segments.
+   ELF header is at OFFSET into the file.
    LOADER's DT_RPATH is used in searching for NAME.
    If the object is already opened, returns its existing map.  */
 extern struct link_map *_dl_map_object (struct link_map *loader,
 					const char *name,
+					off_t offset,
 					int type, int trace_mode, int mode,
 					Lmid_t nsid) attribute_hidden;
 
@@ -1170,8 +1173,9 @@ extern char *_dl_dst_substitute (struct link_map *l, const char *name,
 /* Open the shared object NAME, relocate it, and run its initializer if it
    hasn't already been run.  MODE is as for `dlopen' (see <dlfcn.h>).  If
    the object is already opened, returns its existing map.  */
-extern void *_dl_open (const char *name, int mode, const void *caller,
-		       Lmid_t nsid, int argc, char *argv[], char *env[])
+extern void *_dl_open (const char *name, off_t offset, int mode,
+		       const void *caller, Lmid_t nsid,
+		       int argc, char *argv[], char *env[])
      attribute_hidden;
 
 /* Free or queue for freeing scope OLD.  If other threads might be
-- 
2.30.2