[ELF] Fix "recompile with -fPIC" error

mold used to emit a spurious "recompile with -fPIC" error when creating
a position-dependent executable. This is because we alwyas create
dynamic relocations for importe symbols. If the relocated place is
read-only, we can't apply a dynamic relocation, and we printed out the
error message.

This patch fixes the issue by creating copy relocations and PLTs for
such symbols.

Fixes #116

elf/arch-aarch64.cc

@@ -398,7 +398,7 @@ void InputSection<AARCH64>::scan_relocations(Context<AARCH64> &ctx) {
         // Absolute  Local    Imported data  Imported code
         {  NONE,     BASEREL, DYNREL,        DYNREL },     // DSO
         {  NONE,     BASEREL, DYNREL,        DYNREL },     // PIE
-        {  NONE,     NONE,    DYNREL,        DYNREL },     // PDE
+        {  NONE,     NONE,    COPYREL,       PLT    },     // PDE
       };
       dispatch(ctx, table, i);
       break;

elf/arch-i386.cc

@@ -404,7 +404,7 @@ void InputSection<I386>::scan_relocations(Context<I386> &ctx) {
         // Absolute  Local    Imported data  Imported code
         {  NONE,     BASEREL, DYNREL,        DYNREL },     // DSO
         {  NONE,     BASEREL, DYNREL,        DYNREL },     // PIE
-        {  NONE,     NONE,    DYNREL,        DYNREL },     // PDE
+        {  NONE,     NONE,    COPYREL,       PLT },        // PDE
       };
       dispatch(ctx, table, i);
       break;

elf/arch-x86-64.cc

@@ -556,7 +556,7 @@ void InputSection<X86_64>::scan_relocations(Context<X86_64> &ctx) {
         // Absolute  Local    Imported data  Imported code
         {  NONE,     BASEREL, DYNREL,        DYNREL },     // DSO
         {  NONE,     BASEREL, DYNREL,        DYNREL },     // PIE
-        {  NONE,     NONE,    DYNREL,        DYNREL },     // PDE
+        {  NONE,     NONE,    COPYREL,       PLT    },     // PDE
       };
       dispatch(ctx, table, i);
       break;

elf/output-chunks.cc

@@ -861,7 +861,6 @@ void GotSection<E>::copy_buf(Context<E> &ctx) {
 template <typename E>
 void PltSection<E>::add_symbol(Context<E> &ctx, Symbol<E> *sym) {
   ASSERT(!sym->has_plt(ctx));
-  ASSERT(!sym->has_got(ctx));
 
   if (this->shdr.sh_size == 0) {
     this->shdr.sh_size = E::plt_hdr_size;
@@ -1005,7 +1004,7 @@ void DynsymSection<E>::copy_buf(Context<E> &ctx) {
     } else if (sym.file->is_dso || sym.esym().is_undef()) {
       esym.st_shndx = SHN_UNDEF;
       esym.st_size = 0;
-      if (!ctx.arg.pic && sym.has_plt(ctx) && !sym.has_got(ctx)) {
+      if (sym.has_plt(ctx) && !ctx.arg.pic) {
         // Emit an address for a canonical PLT
         esym.st_value = sym.get_plt_addr(ctx);
       }

elf/passes.cc

@@ -531,7 +531,10 @@ void scan_rels(Context<E> &ctx) {
       ctx.got->add_got_symbol(ctx, sym);
 
     if (sym->flags & NEEDS_PLT) {
-      if (sym->flags & NEEDS_GOT)
+      // If we need to create a canonical PLT (i.e. ctx.arg.pic is false),
+      // we can't use .plt.got because otherwise .plt.got and .plt would
+      // refer each other, resulting in an infinite loop at runtime.
+      if ((sym->flags & NEEDS_GOT) && !ctx.arg.pic)
         ctx.pltgot->add_symbol(ctx, sym);
       else
         ctx.plt->add_symbol(ctx, sym);

test/func-addr.sh

@@ -10,11 +10,13 @@ cat <<EOF | cc -shared -o $t/a.so -xc -
 void fn() {}
 EOF
 
-cat <<EOF | cc -o $t/b.o -c -xc -
+cat <<EOF | cc -o $t/b.o -c -xc -fno-PIC -
 #include <stdio.h>
 
+typedef void Func();
+
 void fn();
-void (*ptr)() = fn;
+Func *const ptr = fn;
 
 int main() {
   printf("%d\n", fn == ptr);