Generate Position-Independent Executables (PIE) if possible

The same ELF target platforms that support PIC also use PIE by default.
PIE code generation is a small variation on PIC code generation.
Controllable via `-fpie` / `-fno-pie` (for code generation)
and `-pie` / `-no-pie` (for linking).

Author: xavierleroy

cfrontend/C2C.ml

@@ -64,17 +64,19 @@ let atom_is_external a =
   | _ -> true
   | exception Not_found -> true
 
-(* In ELF PIC code, the rules are different: all non-static symbols
-   must be accessed through the GOT, even if they are defined in the
-   current compilation unit.  (This is to allow symbol interposition by
-   the dynamic loader.)  *)
+(* In ELF PIC code, all non-static symbols must be accessed through
+   the GOT, even if they are defined in the current compilation unit.
+   (This is to allow symbol interposition by the dynamic loader.)
+   In ELF PIE code, there is no interposition, so locally-defined
+   symbols do not need GOT access.
+   In non-PIC, non-PIE mode, the GOT is unused. *)
 let atom_needs_GOT_access a =
-  !Clflags.option_fpic &&
-  begin match Hashtbl.find decl_atom a with
-  | { a_storage = C.Storage_static } -> false
-  | _ -> true
-  | exception Not_found -> true
-  end
+  if !Clflags.option_fpic then
+    not (atom_is_static a)
+  else if !Clflags.option_fpie then
+    atom_is_external a
+  else
+    false
 
 let atom_alignof a =
   try

configure

@@ -353,6 +353,7 @@ if test "$arch" = "x86" -a "$bitsize" = "64"; then
         casm_options="-m64 -c"
         clinker="${toolprefix}cc"
         clinker_options="-m64"
+        clinker_needs_no_pie=false
         cprepro="${toolprefix}cc"
         cprepro_options="-m64 -U__GNUC__ -U__SIZEOF_INT128__ -E"
         system="bsd"
@@ -363,6 +364,7 @@ if test "$arch" = "x86" -a "$bitsize" = "64"; then
         cc_options="-m64"
         casm_options="-m64 -c"
         clinker_options="-m64"
+        clinker_needs_no_pie=false
         cprepro_options="-m64 -U__GNUC__ -U__SIZEOF_INT128__ -E"
         system="linux"
         pic_supported=true
@@ -407,6 +409,7 @@ if test "$arch" = "riscV"; then
   cc_options="$model_options"
   casm_options="$model_options -c"
   clinker_options="$model_options"
+  clinker_needs_no_pie=false
   cprepro_options="$model_options -U__GNUC__ -E"
   system="linux"
   pic_supported=true

driver/Clflags.ml

@@ -23,6 +23,7 @@ let option_fpacked_structs = ref false
 let option_funstructured_switch = ref false
 let option_ffpu = ref true
 let option_fpic = ref false
+let option_fpie = ref Configuration.pic_supported
 let option_ffloatconstprop = ref 2
 let option_ftailcalls = ref true
 let option_fconstprop = ref true

driver/Configuration.mli

@@ -61,4 +61,4 @@ val elf_target: bool
   (** Is the target binary format ELF? *)
 
 val pic_supported: bool
-  (** Are we able to generate PIC code? *)
+  (** Are we able to generate PIC and PIE code? *)

driver/Driver.ml

@@ -201,6 +201,7 @@ Processing options:
   -fif-conversion Perform if-conversion (generation of conditional moves) [on]
 Code generation options: (use -fno-<opt> to turn off -f<opt>)
   -fpic / -fPIC  Generate position-independent code [off]
+  -fpie / -fPIE  Generate position-independent executables [on if supported]
   -ffpu          Use FP registers for some integer operations [on]
   -fsmall-data <n>  Set maximal size <n> for allocation in small data area
   -fsmall-const <n>  Set maximal size <n> for allocation in small constant area
@@ -272,6 +273,11 @@ let cmdline_actions =
     then option_fpic := true
     else warning no_loc Unnamed
          "option -fpic not supported on this platform, ignored" in
+  let set_pie_mode () =
+    if Configuration.pic_supported
+    then option_fpie := true
+    else warning no_loc Unnamed
+         "option -fpie not supported on this platform, ignored" in
   [
 (* Getting help *)
   Exact "-help", Unit print_usage_and_exit;
@@ -311,7 +317,11 @@ let cmdline_actions =
   Exact "-fpic", Unit set_pic_mode;
   Exact "-fPIC", Unit set_pic_mode;
   Exact "-fno-pic", Unset option_fpic;
-  Exact "-fno-PIC", Unset option_fpic ] @
+  Exact "-fno-PIC", Unset option_fpic;
+  Exact "-fpie", Unit set_pie_mode;
+  Exact "-fPIE", Unit set_pie_mode;
+  Exact "-fno-pie", Unset option_fpie;
+  Exact "-fno-PIE", Unset option_fpie ] @
       f_opt "common" option_fcommon @
 (* Target processor options *)
   (if Configuration.arch = "arm" then

driver/Linker.ml

@@ -35,11 +35,13 @@ let linker exe_name files =
 
 
 let gnu_linker_help =
-{|  -shared        Produce a shared library instead of an executable
-  -nodefaultlibs Do not use the standard system libraries when
+{|  -nodefaultlibs Do not use the standard system libraries when
                  linking
   -nostdlib      Do not use the standard system startup files or
                  libraries when linking
+  -no-pie        Do not produce a position-independent executable
+  -pie           Produce a position-independent executable
+  -shared        Produce a shared library instead of an executable
 |}
 
 let linker_help =
@@ -73,6 +75,8 @@ let linker_actions =
   (if Configuration.gnu_toolchain then
     [ Exact "-nodefaultlibs", Self push_linker_arg;
       Exact "-nostdlib", Self push_linker_arg;
+      Exact "-pie", Self push_linker_arg;
+      Exact "-no-pie", Self push_linker_arg;
       Exact "-shared", Self push_linker_arg]
   else []) @
   [ Exact "-s", Self push_linker_arg;

riscV/TargetPrinter.ml

@@ -147,7 +147,7 @@ module Target : TARGET =
 (* Emit the target of a call, with a `@plt` suffix in PIC mode. *)
 
     let symbol_plt oc s =
-      if !Clflags.option_fpic
+      if SelectOp.symbol_is_relocatable s
       then fprintf oc "%a@plt" symbol s
       else symbol oc s
 
@@ -604,7 +604,9 @@ module Target : TARGET =
 
     let print_prologue oc =
       fprintf oc "	.option %s\n"
-        (if !Clflags.option_fpic then "pic" else "nopic");
+        (if !Clflags.option_fpic || !Clflags.option_fpie
+         then "pic"
+         else "nopic");
       if !Clflags.option_g then begin
         section oc Section_text;
       end

tools/runner.sh

@@ -182,7 +182,7 @@ case "$target,$os" in
     case "$1" in
       1) Run_test "$simu_aarch64" "";;
       2) Run_test "$simu_aarch64" "-fpic";;
-      3) Run_test "$simu_aarch64" "-Os";;
+      3) Run_test "$simu_aarch64" "-Os -fno-pie -no-pie";;
     esac;;
   aarch64,macos)
     case "$1" in
@@ -206,7 +206,7 @@ case "$target,$os" in
     case "$1" in
       1) Run_test "$simu_rv64" "";;
       2) Run_test "$simu_rv64" "-fpic";;
-      3) Run_test "$simu_rv64" "-Os";;
+      3) Run_test "$simu_rv64" "-Os -fno-pie -no-pie";;
     esac;;
   x86_32,*)
     case "$1" in
@@ -217,7 +217,7 @@ case "$target,$os" in
     case "$1" in
       1) Run_test "" "";;
       2) Run_test "" "-fpic";;
-      3) Run_test "" "-Os";;
+      3) Run_test "" "-Os -fno-pie -no-pie";;
     esac;;
   *)
     Fatal "Unknown configuration \"$target\" - \"$os\""

x86/TargetPrinter.ml

@@ -140,7 +140,7 @@ module ELF_System : SYSTEM =
 
     let symbol_function oc symb =
       symbol_paren oc symb;
-      if !Clflags.option_fpic then fprintf oc "@PLT"
+      if SelectOp.symbol_is_relocatable symb then fprintf oc "@PLT"
 
     let label = elf_label