[ELF] Report an error for an unrepresentable reloc against abs symbol

PC-relative relocations against absolute symbols are not representable
in position-independent code. In order to support such relocations,
someone has to compute `S + A - P` or subtract `B` as a base relocation,
but such dynamic relocations don't exist.

This is contrary to non-PC-relative relocations against non-absolute
symbols. In this case, we can simply use the base relocation (e.g.
R_X86_64_RELATIVE).

#348

elf/arch-i386.cc

@@ -410,8 +410,8 @@ void InputSection<E>::scan_relocations(Context<E> &ctx) {
     case R_386_PC32: {
       Action table[][4] = {
         // Absolute  Local  Imported data  Imported code
-        {  BASEREL,  NONE,  DYNREL,        DYNREL },     // DSO
-        {  BASEREL,  NONE,  COPYREL,       PLT    },     // PIE
+        {  ERROR,    NONE,  DYNREL,        DYNREL },     // DSO
+        {  ERROR,    NONE,  COPYREL,       PLT    },     // PIE
         {  NONE,     NONE,  COPYREL,       PLT    },     // PDE
       };
       dispatch(ctx, table, i, rel, sym);
@@ -422,10 +422,16 @@ void InputSection<E>::scan_relocations(Context<E> &ctx) {
     case R_386_GOTPC:
       sym.flags |= NEEDS_GOT;
       break;
-    case R_386_PLT32:
-      if (sym.is_imported)
-        sym.flags |= NEEDS_PLT;
+    case R_386_PLT32: {
+      Action table[][4] = {
+        // Absolute  Local  Imported data  Imported code
+        {  ERROR,    NONE,  PLT,           PLT    },     // DSO
+        {  ERROR,    NONE,  PLT,           PLT    },     // PIE
+        {  NONE,     NONE,  PLT,           PLT    },     // PDE
+      };
+      dispatch(ctx, table, i, rel, sym);
       break;
+    }
     case R_386_TLS_GOTIE:
     case R_386_TLS_LE:
     case R_386_TLS_IE:

elf/arch-x86-64.cc

@@ -629,8 +629,8 @@ void InputSection<E>::scan_relocations(Context<E> &ctx) {
     case R_X86_64_PC64: {
       Action table[][4] = {
         // Absolute  Local  Imported data  Imported code
-        {  BASEREL,  NONE,  DYNREL,        DYNREL },     // DSO
-        {  BASEREL,  NONE,  COPYREL,       PLT    },     // PIE
+        {  ERROR,    NONE,  DYNREL,        DYNREL },     // DSO
+        {  ERROR,    NONE,  COPYREL,       PLT    },     // PIE
         {  NONE,     NONE,  COPYREL,       PLT    },     // PDE
       };
       dispatch(ctx, table, i, rel, sym);
@@ -665,10 +665,16 @@ void InputSection<E>::scan_relocations(Context<E> &ctx) {
       break;
     }
     case R_X86_64_PLT32:
-    case R_X86_64_PLTOFF64:
-      if (sym.is_imported)
-        sym.flags |= NEEDS_PLT;
+    case R_X86_64_PLTOFF64: {
+      Action table[][4] = {
+        // Absolute  Local  Imported data  Imported code
+        {  ERROR,    NONE,  PLT,           PLT    },     // DSO
+        {  ERROR,    NONE,  PLT,           PLT    },     // PIE
+        {  NONE,     NONE,  PLT,           PLT    },     // PDE
+      };
+      dispatch(ctx, table, i, rel, sym);
       break;
+    }
     case R_X86_64_TLSGD:
       if (i + 1 == rels.size())
         Fatal(ctx) << *this

elf/input-sections.cc

@@ -116,8 +116,12 @@ void InputSection<E>::dispatch(Context<E> &ctx, Action table[3][4], i64 i,
   bool is_writable = (shdr().sh_flags & SHF_WRITE);
 
   auto error = [&] {
-    Error(ctx) << *this << ": " << rel << " relocation against symbol `"
-               << sym << "' can not be used; recompile with -fPIC";
+    if (sym.is_absolute())
+      Error(ctx) << *this << ": " << rel << " relocation against symbol `"
+                 << sym << "' can not be used; recompile with -fno-PIC";
+    else
+      Error(ctx) << *this << ": " << rel << " relocation against symbol `"
+                 << sym << "' can not be used; recompile with -fPIC";
   };
 
   auto warn_textrel = [&] {

test/elf/absolute-symbols.sh

@@ -12,14 +12,37 @@ mkdir -p $t
 
 [ "$(uname -m)" = x86_64 ] || { echo skipped; exit; }
 
-cat <<EOF | $CC -o $t/a.o -c -fno-PIC -x assembler -
-.globl foo, main
-foo = 0x8080
-main:
-  call foo
+cat <<EOF | $CC -o $t/a.o -c -x assembler -
+.globl foo
+foo = 0x800000
 EOF
 
-$CC -B. -o $t/exe -no-pie $t/a.o
-objdump -d $t/exe | grep -qE 'callq\s+8080'
+cat <<EOF | $CC -o $t/b.o -c -fno-PIC -xc -
+#define _GNU_SOURCE 1
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <ucontext.h>
+
+void handler(int signum, siginfo_t *info, void *ptr) {
+  ucontext_t *u = (ucontext_t *)ptr;
+  printf("ip=0x%llx\n", u->uc_mcontext.gregs[REG_RIP]);
+  exit(0);
+}
+
+void foo();
+
+int main() {
+  struct sigaction act;
+  act.sa_flags = SA_SIGINFO | SA_RESETHAND;
+  act.sa_sigaction = handler;
+  sigemptyset(&act.sa_mask);
+  sigaction(SIGSEGV, &act, 0);
+  foo();
+}
+EOF
+
+$CC -B. -o $t/exe -no-pie $t/a.o $t/b.o
+$t/exe | grep -q '^ip=0x800000$'
 
 echo OK

test/elf/reloc-abs-error.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+export LANG=
+set -e
+CC="${CC:-cc}"
+CXX="${CXX:-c++}"
+testname=$(basename "$0" .sh)
+echo -n "Testing $testname ... "
+cd "$(dirname "$0")"/../..
+mold="$(pwd)/mold"
+t=out/test/elf/$testname
+mkdir -p $t
+
+cat <<EOF | $CC -o $t/a.o -c -x assembler -
+.globl foo
+foo = 0x800000
+EOF
+
+cat <<EOF | $CC -o $t/b.o -c -fPIC -xc -
+void foo();
+int main() { foo(); }
+EOF
+
+! $CC -B. -o $t/exe -pie $t/a.o $t/b.o >& $t/log
+grep -q 'recompile with -fno-PIC' $t/log
+
+echo OK