elf: Implement --preload option for the dynamic linker

ChangeLog

@@ -1,3 +1,16 @@
+2019-02-04  David Newall  <glibc@davidnewall.com>
+
+	elf: Implement --preload option for the dynamic linker.
+	* elf/rtld.c (preloadarg): New variable.
+	(handle_preload_list): Pass through “where” argument to
+	do_preload.
+	(dl_main): Handle "--preload" and add second call to
+	handle_preload_list.
+	* elf/Makefile (tests-special): Add tst-rtld-preload.out.
+	(tst-rtld-preload-OBJS): Set variable.
+	(tst-rtld-preload.out): New target.
+	* elf/tst-rtld-preload.sh: New file.
+
 2019-02-04  Matthew Malcomson  <matthew.malcomson@arm.com>
 
 	* Makefile (testrun.sh): Exit in case of incorrect argument.

NEWS

@@ -9,7 +9,8 @@ Version 2.30
 
 Major new features:
 
-  [Add new features here]
+* The dynamic linker accepts the --preload argument to preload shared
+  objects, in addition to the LD_PRELOAD environment variable.
 
 Deprecated and removed features, and other changes affecting compatibility:
 

elf/Makefile

@@ -354,7 +354,8 @@ endif
 
 ifeq (yes,$(build-shared))
 ifeq ($(run-built-tests),yes)
-tests-special += $(objpfx)tst-pathopt.out $(objpfx)tst-rtld-load-self.out
+tests-special += $(objpfx)tst-pathopt.out $(objpfx)tst-rtld-load-self.out \
+		 $(objpfx)tst-rtld-preload.out
 endif
 tests-special += $(objpfx)check-textrel.out $(objpfx)check-execstack.out \
 		 $(objpfx)check-localplt.out $(objpfx)check-initfini.out
@@ -883,6 +884,15 @@ $(objpfx)tst-rtld-load-self.out: tst-rtld-load-self.sh $(objpfx)ld.so
 	$(SHELL) $^ '$(test-wrapper)' '$(test-wrapper-env)' > $@; \
 	$(evaluate-test)
 
+tst-rtld-preload-OBJS = $(subst $(empty) ,:,$(strip $(preloadtest-preloads:=.so)))
+$(objpfx)tst-rtld-preload.out: tst-rtld-preload.sh $(objpfx)ld.so \
+			       $(objpfx)preloadtest \
+			       $(preloadtest-preloads:%=$(objpfx)%.so)
+	$(SHELL) $< $(objpfx)ld.so $(objpfx)preloadtest \
+		    '$(test-wrapper)' '$(test-wrapper-env)' '$(run_program_env)' \
+		    '$(rpath-link)' '$(tst-rtld-preload-OBJS)' > $@; \
+	$(evaluate-test)
+
 $(objpfx)initfirst: $(libdl)
 $(objpfx)initfirst.out: $(objpfx)firstobj.so
 

elf/rtld.c

@@ -826,15 +826,18 @@ static const char *library_path attribute_relro;
 static const char *preloadlist attribute_relro;
 /* Nonzero if information about versions has to be printed.  */
 static int version_info attribute_relro;
+/* The preload list passed as a command argument.  */
+static const char *preloadarg attribute_relro;
 
 /* The LD_PRELOAD environment variable gives list of libraries
    separated by white space or colons that are loaded before the
    executable's dependencies and prepended to the global scope list.
    (If the binary is running setuid all elements containing a '/' are
    ignored since it is insecure.)  Return the number of preloads
-   performed.  */
+   performed.   Ditto for --preload command argument.  */
 unsigned int
-handle_ld_preload (const char *preloadlist, struct link_map *main_map)
+handle_preload_list (const char *preloadlist, struct link_map *main_map,
+		     const char *where)
 {
   unsigned int npreloads = 0;
   const char *p = preloadlist;
@@ -858,7 +861,7 @@ handle_ld_preload (const char *preloadlist, struct link_map *main_map)
 	++p;
 
       if (dso_name_valid_for_suid (fname))
-	npreloads += do_preload (fname, main_map, "LD_PRELOAD");
+	npreloads += do_preload (fname, main_map, where);
     }
   return npreloads;
 }
@@ -974,6 +977,13 @@ dl_main (const ElfW(Phdr) *phdr,
 	  {
 	    process_dl_audit (_dl_argv[2]);
 
+	    _dl_skip_args += 2;
+	    _dl_argc -= 2;
+	    _dl_argv += 2;
+	  }
+	else if (! strcmp (_dl_argv[1], "--preload") && _dl_argc > 2)
+	  {
+	    preloadarg = _dl_argv[2];
 	    _dl_skip_args += 2;
 	    _dl_argc -= 2;
 	    _dl_argv += 2;
@@ -1006,7 +1016,8 @@ of this helper program; chances are you did not intend to run this program.\n\
 			variable LD_LIBRARY_PATH\n\
   --inhibit-rpath LIST  ignore RUNPATH and RPATH information in object names\n\
 			in LIST\n\
-  --audit LIST          use objects named in LIST as auditors\n");
+  --audit LIST          use objects named in LIST as auditors\n\
+  --preload LIST        preload objects named in LIST\n");
 
       ++_dl_skip_args;
       --_dl_argc;
@@ -1620,7 +1631,16 @@ ERROR: ld.so: object '%s' cannot be loaded as audit interface: %s; ignored.\n",
   if (__glibc_unlikely (preloadlist != NULL))
     {
       HP_TIMING_NOW (start);
-      npreloads += handle_ld_preload (preloadlist, main_map);
+      npreloads += handle_preload_list (preloadlist, main_map, "LD_PRELOAD");
+      HP_TIMING_NOW (stop);
+      HP_TIMING_DIFF (diff, start, stop);
+      HP_TIMING_ACCUM_NT (load_time, diff);
+    }
+
+  if (__glibc_unlikely (preloadarg != NULL))
+    {
+      HP_TIMING_NOW (start);
+      npreloads += handle_preload_list (preloadarg, main_map, "--preload");
       HP_TIMING_NOW (stop);
       HP_TIMING_DIFF (diff, start, stop);
       HP_TIMING_ACCUM_NT (load_time, diff);

elf/tst-rtld-preload.sh

@@ -0,0 +1,38 @@
+#!/bin/sh
+# Test --preload argument ld.so.
+# Copyright (C) 2019 Free Software Foundation, Inc.
+# This file is part of the GNU C Library.
+#
+# The GNU C Library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2.1 of the License, or (at your option) any later version.
+#
+# The GNU C Library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with the GNU C Library; if not, see
+# <http://www.gnu.org/licenses/>.
+
+set -e
+
+rtld=$1
+test_program=$2
+test_wrapper=$3
+test_wrapper_env=$4
+run_program_env=$5
+library_path=$6
+preload=$7
+
+echo "# [${test_wrapper}] [$rtld] [--library-path] [$library_path]" \
+     "[--preload] [$preload] [$test_program]"
+${test_wrapper_env} \
+${run_program_env} \
+${test_wrapper} $rtld --library-path "$library_path" \
+  --preload "$preload" $test_program 2>&1 && rc=0 || rc=$?
+echo "# exit status $rc"
+
+exit $rc