elf: execve statically linked programs instead of crashing [BZ #28648]

Programs without dynamic dependencies and without a program
interpreter are now run via execve.

Previously, the dynamic linker either crashed while attempting to
read a non-existing dynamic segment (looking for DT_AUDIT/DT_DEPAUDIT
data), or the self-relocated in the static PIE executable crashed
because the outer dynamic linker had already applied RELRO protection.

<dl-execve.h> is needed because execve is not available in the
dynamic loader on Hurd.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

NEWS

@@ -184,6 +184,10 @@ Major new features:
   than or equal to a given integer.  This function is a GNU extension,
   although Solaris also provides a similar function.
 
+* When invoked explicitly, the dynamic linker now uses the kernel to
+  execute programs that do not have any dynamic dependency (that is,
+  they are statically linked).  This feature is Linux-specific.
+
 Deprecated and removed features, and other changes affecting compatibility:
 
 * The function pthread_mutex_consistent_np has been deprecated; programs

elf/Makefile

@@ -231,6 +231,7 @@ tests += restest1 preloadtest loadfail multiload origtest resolvfail \
 	 tst-tls20 tst-tls21 tst-dlmopen-dlerror tst-dlmopen-gethostbyname \
 	 tst-dl-is_dso tst-ro-dynamic \
 	 tst-audit18 \
+	 tst-rtld-run-static \
 #	 reldep9
 tests-internal += loadtest unload unload2 circleload1 \
 	 neededtest neededtest2 neededtest3 neededtest4 \
@@ -1977,3 +1978,5 @@ $(objpfx)tst-ro-dynamic-mod.so: $(objpfx)tst-ro-dynamic-mod.os \
 	$(LINK.o) -nostdlib -nostartfiles -shared -o $@ \
 		-Wl,--script=tst-ro-dynamic-mod.map \
 		$(objpfx)tst-ro-dynamic-mod.os
+
+$(objpfx)tst-rtld-run-static.out: $(objpfx)/ldconfig

elf/rtld.c

@@ -50,6 +50,7 @@
 #include <gnu/lib-names.h>
 #include <dl-tunables.h>
 #include <get-dynamic-info.h>
+#include <dl-execve.h>
 
 #include <assert.h>
 
@@ -1106,6 +1107,45 @@ load_audit_modules (struct link_map *main_map, struct audit_list *audit_list)
     }
 }
 
+/* Check if the executable is not actualy dynamically linked, and
+   invoke it directly in that case.  */
+static void
+rtld_chain_load (struct link_map *main_map, char *argv0)
+{
+  /* The dynamic loader run against itself.  */
+  const char *rtld_soname
+    = ((const char *) D_PTR (&GL(dl_rtld_map), l_info[DT_STRTAB])
+       + GL(dl_rtld_map).l_info[DT_SONAME]->d_un.d_val);
+  if (main_map->l_info[DT_SONAME] != NULL
+      && strcmp (rtld_soname,
+		 ((const char *) D_PTR (main_map, l_info[DT_STRTAB])
+		  + main_map->l_info[DT_SONAME]->d_un.d_val)) == 0)
+    _dl_fatal_printf ("%s: loader cannot load itself\n", rtld_soname);
+
+  /* With DT_NEEDED dependencies, the executable is dynamically
+     linked.  */
+  if (__glibc_unlikely (main_map->l_info[DT_NEEDED] != NULL))
+    return;
+
+  /* If the executable has program interpreter, it is dynamically
+     linked.  */
+  for (size_t i = 0; i < main_map->l_phnum; ++i)
+    if (main_map->l_phdr[i].p_type == PT_INTERP)
+      return;
+
+  const char *pathname = _dl_argv[0];
+  if (argv0 != NULL)
+    _dl_argv[0] = argv0;
+  int errcode = __rtld_execve (pathname, _dl_argv, _environ);
+  const char *errname = strerrorname_np (errcode);
+  if (errname != NULL)
+    _dl_fatal_printf("%s: cannot execute %s: %s\n",
+		     rtld_soname, pathname, errname);
+  else
+    _dl_fatal_printf("%s: cannot execute %s: %d\n",
+		     rtld_soname, pathname, errno);
+}
+
 static void
 dl_main (const ElfW(Phdr) *phdr,
 	 ElfW(Word) phnum,
@@ -1374,14 +1414,8 @@ dl_main (const ElfW(Phdr) *phdr,
       /* Now the map for the main executable is available.  */
       main_map = GL(dl_ns)[LM_ID_BASE]._ns_loaded;
 
-      if (__glibc_likely (state.mode == rtld_mode_normal)
+      if (__glibc_likely (state.mode == rtld_mode_normal))
-	  && GL(dl_rtld_map).l_info[DT_SONAME] != NULL
+	rtld_chain_load (main_map, argv0);
-	  && main_map->l_info[DT_SONAME] != NULL
-	  && strcmp ((const char *) D_PTR (&GL(dl_rtld_map), l_info[DT_STRTAB])
-		     + GL(dl_rtld_map).l_info[DT_SONAME]->d_un.d_val,
-		     (const char *) D_PTR (main_map, l_info[DT_STRTAB])
-		     + main_map->l_info[DT_SONAME]->d_un.d_val) == 0)
-	_dl_fatal_printf ("loader cannot load itself\n");
 
       phdr = main_map->l_phdr;
       phnum = main_map->l_phnum;

elf/tst-rtld-run-static.c

@@ -0,0 +1,62 @@
+/* Test running statically linked programs using ld.so.
+   Copyright (C) 2021 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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <support/check.h>
+#include <support/support.h>
+#include <support/capture_subprocess.h>
+#include <string.h>
+#include <stdlib.h>
+
+static int
+do_test (void)
+{
+  char *ldconfig_path = xasprintf ("%s/elf/ldconfig", support_objdir_root);
+
+  {
+    char *argv[] = { (char *) "ld.so", ldconfig_path, (char *) "--help", NULL };
+    struct support_capture_subprocess cap
+      = support_capture_subprogram (support_objdir_elf_ldso, argv);
+    support_capture_subprocess_check (&cap, "no --argv0", 0, sc_allow_stdout);
+    puts ("info: output without --argv0:");
+    puts (cap.out.buffer);
+    TEST_VERIFY (strstr (cap.out.buffer, "Usage: ldconfig [OPTION...]\n")
+                 == cap.out.buffer);
+    support_capture_subprocess_free (&cap);
+  }
+
+  {
+    char *argv[] =
+      {
+        (char *) "ld.so", (char *) "--argv0", (char *) "ldconfig-argv0",
+        ldconfig_path, (char *) "--help", NULL
+      };
+    struct support_capture_subprocess cap
+      = support_capture_subprogram (support_objdir_elf_ldso, argv);
+    support_capture_subprocess_check (&cap, "with --argv0", 0, sc_allow_stdout);
+    puts ("info: output with --argv0:");
+    puts (cap.out.buffer);
+    TEST_VERIFY (strstr (cap.out.buffer, "Usage: ldconfig-argv0 [OPTION...]\n")
+                 == cap.out.buffer);
+    support_capture_subprocess_free (&cap);
+  }
+
+  free (ldconfig_path);
+  return 0;
+}
+
+#include <support/test-driver.c>

sysdeps/generic/dl-execve.h

@@ -0,0 +1,25 @@
+/* execve for the dynamic linker.  Generic stub version.
+   Copyright (C) 2021 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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+
+static int
+__rtld_execve (const char *path, char *const *argv, char *const *envp)
+{
+  return ENOSYS;
+}

sysdeps/unix/sysv/linux/dl-execve.h

@@ -0,0 +1,25 @@
+/* execve for the dynamic linker.  Linux version.
+   Copyright (C) 2021 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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+
+static inline int
+__rtld_execve (const char *path, char *const *argv, char *const *envp)
+{
+  return -INTERNAL_SYSCALL_CALL (execve, path, argv, envp);
+}