elf: Implement ld.so --help

--help processing is deferred to the point where the executable has
been loaded, so that it is possible to eventually include information
from the main executable in the help output.

As suggested in the GNU command-line interface guidelines, the help
message is printed to standard output, and the exit status is
successful.

Handle usage errors closer to the GNU command-line interface
guidelines.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

elf/dl-main.h

@@ -63,6 +63,7 @@ struct audit_list
 enum rtld_mode
   {
     rtld_mode_normal, rtld_mode_list, rtld_mode_verify, rtld_mode_trace,
+    rtld_mode_help,
   };
 
 /* Aggregated state information extracted from environment variables
@@ -101,6 +102,11 @@ call_init_paths (const struct dl_main_state *state)
 }
 
 /* Print ld.so usage information and exit.  */
-_Noreturn void _dl_usage (void) attribute_hidden;
+_Noreturn void _dl_usage (const char *argv0, const char *wrong_option)
+  attribute_hidden;
+
+/* Print ld.so --help output and exit.  */
+_Noreturn void _dl_help (const char *argv0, struct dl_main_state *state)
+  attribute_hidden;
 
 #endif /* _DL_MAIN */

elf/dl-usage.c

@@ -19,12 +19,24 @@
 #include <dl-cache.h>
 #include <dl-main.h>
 #include <ldsodefs.h>
+#include <unistd.h>
 
 void
-_dl_usage (void)
+_dl_usage (const char *argv0, const char *wrong_option)
 {
-  _dl_fatal_printf ("\
-Usage: ld.so [OPTION]... EXECUTABLE-FILE [ARGS-FOR-PROGRAM...]\n\
+  if (wrong_option != NULL)
+    _dl_error_printf ("%s: unrecognized option '%s'\n", argv0, wrong_option);
+  else
+    _dl_error_printf ("%s: missing program name\n", argv0);
+  _dl_error_printf ("Try '%s --help' for more information.\n", argv0);
+  _exit (EXIT_FAILURE);
+}
+
+void
+_dl_help (const char *argv0, struct dl_main_state *state)
+{
+  _dl_printf ("\
+Usage: %s [OPTION]... EXECUTABLE-FILE [ARGS-FOR-PROGRAM...]\n\
 You have invoked `ld.so', the helper program for shared library executables.\n\
 This program usually lives in the file `/lib/ld.so', and special directives\n\
 in executable files using ELF shared libraries tell the system's program\n\
@@ -47,5 +59,9 @@ of this helper program; chances are you did not intend to run this program.\n\
                         in LIST\n\
   --audit LIST          use objects named in LIST as auditors\n\
   --preload LIST        preload objects named in LIST\n\
-  --argv0 STRING        set argv[0] to STRING before running\n");
+  --argv0 STRING        set argv[0] to STRING before running\n\
+  --help                display this help and exit\n\
+",
+              argv0);
+  _exit (EXIT_SUCCESS);
 }

elf/rtld.c

@@ -1151,6 +1151,7 @@ dl_main (const ElfW(Phdr) *phdr,
   _dl_starting_up = 1;
 #endif
 
+  const char *ld_so_name = _dl_argv[0];
   if (*user_entry == (ElfW(Addr)) ENTRY_POINT)
     {
       /* Ho ho.  We are not the program interpreter!  We are the program
@@ -1178,8 +1179,12 @@ dl_main (const ElfW(Phdr) *phdr,
       while (_dl_argc > 1)
 	if (! strcmp (_dl_argv[1], "--list"))
 	  {
-	    state.mode = rtld_mode_list;
-	    GLRO(dl_lazy) = -1;	/* This means do no dependency analysis.  */
+	    if (state.mode != rtld_mode_help)
+	      {
+	       state.mode = rtld_mode_list;
+		/* This means do no dependency analysis.  */
+		GLRO(dl_lazy) = -1;
+	      }
 
 	    ++_dl_skip_args;
 	    --_dl_argc;
@@ -1187,7 +1192,8 @@ dl_main (const ElfW(Phdr) *phdr,
 	  }
 	else if (! strcmp (_dl_argv[1], "--verify"))
 	  {
-	    state.mode = rtld_mode_verify;
+	    if (state.mode != rtld_mode_help)
+	      state.mode = rtld_mode_verify;
 
 	    ++_dl_skip_args;
 	    --_dl_argc;
@@ -1242,13 +1248,34 @@ dl_main (const ElfW(Phdr) *phdr,
 	    _dl_argc -= 2;
 	    _dl_argv += 2;
 	  }
+	else if (strcmp (_dl_argv[1], "--help") == 0)
+	  {
+	    state.mode = rtld_mode_help;
+	    --_dl_argc;
+	    ++_dl_argv;
+	  }
+	else if (_dl_argv[1][0] == '-' && _dl_argv[1][1] == '-')
+	  {
+	   if (_dl_argv[1][1] == '\0')
+	     /* End of option list.  */
+	     break;
+	   else
+	     /* Unrecognized option.  */
+	     _dl_usage (ld_so_name, _dl_argv[1]);
+	  }
 	else
 	  break;
 
       /* If we have no further argument the program was called incorrectly.
 	 Grant the user some education.  */
       if (_dl_argc < 2)
-	_dl_usage ();
+	{
+	  if (state.mode == rtld_mode_help)
+	    /* --help without an executable is not an error.  */
+	    _dl_help (ld_so_name, &state);
+	  else
+	    _dl_usage (ld_so_name, NULL);
+	}
 
       ++_dl_skip_args;
       --_dl_argc;
@@ -1273,7 +1300,8 @@ dl_main (const ElfW(Phdr) *phdr,
 	    break;
 	  }
 
-      if (__glibc_unlikely (state.mode == rtld_mode_verify))
+      if (__glibc_unlikely (state.mode == rtld_mode_verify
+			    || state.mode == rtld_mode_help))
 	{
 	  const char *objname;
 	  const char *err_str = NULL;
@@ -1286,9 +1314,16 @@ dl_main (const ElfW(Phdr) *phdr,
 	  (void) _dl_catch_error (&objname, &err_str, &malloced, map_doit,
 				  &args);
 	  if (__glibc_unlikely (err_str != NULL))
-	    /* We don't free the returned string, the programs stops
-	       anyway.  */
-	    _exit (EXIT_FAILURE);
+	    {
+	      /* We don't free the returned string, the programs stops
+		 anyway.  */
+	      if (state.mode == rtld_mode_help)
+		/* Mask the failure to load the main object.  The help
+		   message contains less information in this case.  */
+		_dl_help (ld_so_name, &state);
+	      else
+		_exit (EXIT_FAILURE);
+	    }
 	}
       else
 	{
@@ -1647,6 +1682,11 @@ dl_main (const ElfW(Phdr) *phdr,
   audit_list_add_dynamic_tag (&state.audit_list, main_map, DT_AUDIT);
   audit_list_add_dynamic_tag (&state.audit_list, main_map, DT_DEPAUDIT);
 
+  /* At this point, all data has been obtained that is included in the
+     --help output.  */
+  if (__glibc_unlikely (state.mode == rtld_mode_help))
+    _dl_help (ld_so_name, &state);
+
   /* If we have auditing DSOs to load, do it now.  */
   bool need_security_init = true;
   if (state.audit_list.length > 0)