Add linker warning for when it creates an executable stack.

   PR 29072

Author: nickclifton

bfd/elflink.c

@@ -7124,13 +7124,23 @@ bfd_elf_size_dynamic_sections (bfd *output_bfd,
   /* Determine any GNU_STACK segment requirements, after the backend
      has had a chance to set a default segment size.  */
   if (info->execstack)
-    elf_stack_flags (output_bfd) = PF_R | PF_W | PF_X;
+    {
+      /* If the user has explicitly requested warnings, then generate one even
+	 though the choice is the result of another command line option.  */
+      if (info->warn_execstack == 1)
+	_bfd_error_handler
+	  (_("\
+warning: enabling an executable stack because of -z execstack command line option"));
+      elf_stack_flags (output_bfd) = PF_R | PF_W | PF_X;
+    }
   else if (info->noexecstack)
     elf_stack_flags (output_bfd) = PF_R | PF_W;
   else
     {
       bfd *inputobj;
       asection *notesec = NULL;
+      bfd *noteobj = NULL;
+      bfd *emptyobj = NULL;
       int exec = 0;
 
       for (inputobj = info->input_bfds;
@@ -7149,15 +7159,45 @@ bfd_elf_size_dynamic_sections (bfd *output_bfd,
 	  s = bfd_get_section_by_name (inputobj, ".note.GNU-stack");
 	  if (s)
 	    {
-	      if (s->flags & SEC_CODE)
-		exec = PF_X;
 	      notesec = s;
+	      if (s->flags & SEC_CODE)
+		{
+		  noteobj = inputobj;
+		  exec = PF_X;
+		  /* There is no point in scanning the remaining bfds.  */
+		  break;
+		}
 	    }
 	  else if (bed->default_execstack)
-	    exec = PF_X;
+	    {
+	      exec = PF_X;
+	      emptyobj = inputobj;
+	    }
 	}
+
       if (notesec || info->stacksize > 0)
-	elf_stack_flags (output_bfd) = PF_R | PF_W | exec;
+	{
+	  if (exec)
+	    {
+	      if (info->warn_execstack != 2)
+		{
+		  /* PR 29072: Because an executable stack is a serious
+		     security risk, make sure that the user knows that it is
+		     being enabled despite the fact that it was not requested
+		     on the command line.  */
+		  if (noteobj)
+		    _bfd_error_handler (_("\
+warning: %s: requires executable stack (because the .note.GNU-stack section is executable)"),
+		       bfd_get_filename (noteobj));
+		  else if (emptyobj)
+		    _bfd_error_handler (_("\
+warning: %s: missing .note.GNU-stack section implies executable stack"),
+		       bfd_get_filename (emptyobj));
+		}
+	    }
+	  elf_stack_flags (output_bfd) = PF_R | PF_W | exec;
+	}
+
       if (notesec && exec && bfd_link_relocatable (info)
 	  && notesec->output_section != bfd_abs_section_ptr)
 	notesec->output_section->flags |= SEC_CODE;

include/bfdlink.h

@@ -489,6 +489,10 @@ struct bfd_link_info
      flags.  */
   unsigned int noexecstack: 1;
 
+  /* Tri-state variable: 0 => not set by user; 1 => set, warnings
+     enabled; 2 => warnings disabled; 3 => unused.  */
+  unsigned int warn_execstack: 2;
+
   /* TRUE if we want to produced optimized output files.  This might
      need much more time and therefore must be explicitly selected.  */
   unsigned int optimize: 1;

ld/NEWS

@@ -1,5 +1,13 @@
 -*- text -*-
 
+* The linker will now generate a warning message if the stack is made
+  executable.  By default this warning is not issued if the user has
+  specifically requested an executable stack via the "-z execstack"
+  command line option, but the warning can be forced via the new
+  "--warn-execstack" option.  Alternatively all warnings about creating
+  an executable stack can be suppressed via the "--no-warn-execstack"
+  option.
+
 * TYPE=<type> is now supported in an output section description to set the
   section type value.
 

ld/emultempl/avrelf.em

@@ -134,6 +134,7 @@ avr_elf_create_output_section_statements (void)
       einfo (_("%X%P: can not create stub BFD: %E\n"));
       return;
     }
+  stub_file->the_bfd->flags |= BFD_LINKER_CREATED;
 
   /* Now we add the stub section.  */
 

ld/ld.texi

@@ -2653,6 +2653,33 @@ Warn if any global constructors are used.  This is only useful for a few
 object file formats.  For formats like COFF or ELF, the linker can not
 detect the use of global constructors.
 
+@kindex --warn-execstack
+@cindex warnings, on exectuable stack
+@cindex executable stack, warnings on
+@item --warn-execstack
+@itemx --no-warn-execstack
+On ELF platforms this option controls how the linker generates warning
+messages when it creates an output file with an executable stack.  By
+default the linker will not warn if the @command{-z execstack} command
+line option has been used, but this behaviour can be overridden by the
+@option{--warn-execstack} option.
+
+On the other hand the linker will normally warn if the stack is made
+executable because one or more of the input files need an execuable
+stack and neither of the @command{-z execstack} or @command{-z
+noexecstack} comman line options have been specified.  This warning
+can be disabled via the @command{--no-warn-execstack} option.
+
+Note: ELF format input files specify that they need an executable
+stack by having a @var{.note.GNU-stack} section with the executable
+bit set in its section flags.  They can specify that they do not need
+an executable stack by having that section, but without the executable
+flag bit set.  If an input file does not have a @var{.note.GNU-stack}
+section present then the default behaviour is target specific.  For
+some targets, then absence of such a section implies that an
+executable stack @emph{is} required.  This is often a problem for hand
+crafted assembler files.
+
 @kindex --warn-multiple-gp
 @item --warn-multiple-gp
 Warn if multiple global pointer values are required in the output file.

ld/ldlex.h

@@ -164,6 +164,8 @@ enum option_values
   OPTION_CTF_VARIABLES,
   OPTION_NO_CTF_VARIABLES,
   OPTION_CTF_SHARE_TYPES,
+  OPTION_WARN_EXECSTACK,
+  OPTION_NO_WARN_EXECSTACK,
 };
 
 /* The initial parser states.  */

ld/lexsup.c

@@ -536,6 +536,10 @@ static const struct ld_option ld_options[] =
   { {"warn-constructors", no_argument, NULL, OPTION_WARN_CONSTRUCTORS},
     '\0', NULL, N_("Warn if global constructors/destructors are seen"),
     TWO_DASHES },
+  { {"warn-execstack", no_argument, NULL, OPTION_WARN_EXECSTACK},
+    '\0', NULL, N_("Warn when creating an executable stack"), TWO_DASHES },
+  { {"no-warn-execstack", no_argument, NULL, OPTION_NO_WARN_EXECSTACK},
+    '\0', NULL, N_("Do not warn when creating an executable stack"), TWO_DASHES },
   { {"warn-multiple-gp", no_argument, NULL, OPTION_WARN_MULTIPLE_GP},
     '\0', NULL, N_("Warn if the multiple GP values are used"), TWO_DASHES },
   { {"warn-once", no_argument, NULL, OPTION_WARN_ONCE},
@@ -915,6 +919,12 @@ parse_args (unsigned argc, char **argv)
 	case OPTION_NON_CONTIGUOUS_REGIONS_WARNINGS:
 	  link_info.non_contiguous_regions_warnings = true;
 	  break;
+	case OPTION_WARN_EXECSTACK:
+	  link_info.warn_execstack = 1;
+	  break;
+	case OPTION_NO_WARN_EXECSTACK:
+	  link_info.warn_execstack = 2;
+	  break;
 	case 'e':
 	  lang_add_entry (optarg, true);
 	  break;
@@ -2150,6 +2160,10 @@ elf_static_list_options (FILE *file)
   fprintf (file, _("\
   -z noexecstack              Mark executable as not requiring executable stack\n"));
   fprintf (file, _("\
+  --warn-execstack            Generate a warning if the stack is executable\n"));
+  fprintf (file, _("\
+  --no-warn-execstack         Do not generate a warning if the stack is executable\n"));
+  fprintf (file, _("\
   -z unique-symbol            Avoid duplicated local symbol names\n"));
   fprintf (file, _("\
   -z nounique-symbol          Keep duplicated local symbol names (default)\n"));

ld/testsuite/ld-elf/eh4.d

@@ -1,7 +1,7 @@
 #source: eh4.s
 #source: eh4a.s
 #as: --64
-#ld: -melf_x86_64 -shared -Ttext 0x400 -z max-page-size=0x200000 -z noseparate-code
+#ld: -melf_x86_64 -shared -Ttext 0x400 -z max-page-size=0x200000 -z noseparate-code -z noexecstack
 #readelf: -wf
 #target: x86_64-*-*
 

ld/testsuite/ld-elf/elf.exp

@@ -180,9 +180,24 @@ if { [check_gc_sections_available] && ![istarget "v850-*-*"] } {
     }
 }
 
-if { [istarget *-*-*linux*]
-     || [istarget *-*-nacl*]
-     || [istarget *-*-gnu*] } {
+proc target_defaults_to_execstack {} {
+    if {   [istarget "aarch64*-*-*"]
+	|| [istarget "arc*-*-*"]
+	|| [istarget "ia64*-*-*"]
+	|| [istarget "loongarch*-*-*"]
+	|| [istarget "nios2*-*-*"]
+	|| [istarget "powerpc64*-*-*"]
+	|| [istarget "riscv*-*-*"]
+	|| [istarget "tilegx*-*-*"]
+        || [istarget "tilepro*-*-*"] } {
+	  return 0
+    }
+    return 1
+}
+
+if {   [istarget *-*-*linux*]
+    || [istarget *-*-nacl*]
+    || [istarget *-*-gnu*] } {
     run_ld_link_tests [list \
 	[list "stack exec" \
 	    "-z execstack" \
@@ -199,7 +214,7 @@ if { [istarget *-*-*linux*]
 	    {{readelf {-Wl} stack-noexec.rd}} \
 	    "stack-noexec.exe"] \
 	[list "stack size" \
-	    "-z stack-size=0x123400" \
+	    "-z stack-size=0x123400 -z noexecstack" \
 	    "" \
 	    "" \
 	    {stack.s} \
@@ -212,7 +227,49 @@ if { [istarget *-*-*linux*]
 	    {pr23900-1.s} \
 	    [list [list "readelf" {-Wl} $pr23900_1_exp]] \
 	    "pr23900-1.exe"] \
+	[list "PR ld/29072 (warn about an executable .note-GNU-stack)" \
+	    "-e 0" \
+	    "" \
+	    "" \
+	    {pr29072-a.s} \
+	    {{ld pr29072.a.warn}} \
+	    "pr29072-a.exe"] \
+	[list "PR 29072 (warn about -z execstack)" \
+	    "-z execstack --warn-execstack" \
+	    "" \
+	    "" \
+	    {stack.s} \
+	     {{ld pr29072.c.warn}} \
+	    "pr29072-c.exe"] \
+	[list "PR ld/29072 (suppress warnings about executable stack)" \
+	    "-e 0 --no-warn-execstack" \
+	    "" \
+	    "" \
+	    {pr29072-a.s} \
+	    {} \
+	    "pr29072-d.exe"] \
+	]
+    if { [target_defaults_to_execstack] } {
+	run_ld_link_tests [list \
+          [list "PR ld/29072 (warn about absent .note-GNU-stack)" \
+	    "-e 0 -z stack-size=0x123400" \
+	    "" \
+	    "" \
+	    {pr29072-b.s} \
+	    {{ld pr29072.b.warn}} \
+	    "pr29072-b.exe"] \
 	]
+    } else {
+	run_ld_link_tests [list \
+          [list "PR ld/29072 (ignore absent .note-GNU-stackk)" \
+	    "-e 0 -z stack-size=0x123400" \
+	    "" \
+	    "" \
+	    {pr29072-b.s} \
+	    {} \
+	    "pr29072-b.exe"] \
+	]
+    }
 }
 
 if [check_gc_sections_available] {
@@ -365,7 +422,7 @@ if { [istarget *-*-linux*]
     run_ld_link_exec_tests [list \
 	[list \
 	    "Run mbind2a" \
-	    "$NOPIE_LDFLAGS -Wl,-z,common-page-size=0x4000" \
+	    "$NOPIE_LDFLAGS -Wl,-z,common-page-size=0x4000 -Wl,-z,noexecstack" \
 	    "" \
 	    { mbind2a.s mbind2b.c } \
 	    "mbind2a" \
@@ -374,7 +431,7 @@ if { [istarget *-*-linux*]
 	] \
 	[list \
 	    "Run mbind2b" \
-	    "-static -Wl,-z,common-page-size=0x4000" \
+	    "-static -Wl,-z,common-page-size=0x4000 -Wl,-z,noexecstack" \
 	    "" \
 	    { mbind2a.s mbind2b.c } \
 	    "mbind2b" \