[PATCH] D26133: [ELF] Allow relative relocations to absolute symbols in PIC

[Petr Hosek via Phabricator via llvm-commits]

phosek updated this revision to Diff 80864.
phosek added a comment.

This appears to be working, please take a look. The only question is whether we should be also generating the relocation in the weak case or keep it as a special case same as before?


Repository:
  rL LLVM

https://reviews.llvm.org/D26133

Files:
  ELF/Relocations.cpp
  test/ELF/linkerscript/symbol-absolute.s
  test/ELF/relocation-relative-absolute.s
  test/ELF/relocation-relative-weak.s


Index: test/ELF/relocation-relative-weak.s
===================================================================
--- test/ELF/relocation-relative-weak.s
+++ test/ELF/relocation-relative-weak.s
@@ -4,6 +4,7 @@
 # RUN: llvm-readobj -dyn-relocations %t | FileCheck %s
 
 # CHECK:      Dynamic Relocations {
+# CHECK-NEXT:   0x3018 R_X86_64_JUMP_SLOT - 0x0
 # CHECK-NEXT: }
 
 .globl _start
Index: test/ELF/relocation-relative-absolute.s
===================================================================
--- test/ELF/relocation-relative-absolute.s
+++ test/ELF/relocation-relative-absolute.s
@@ -2,11 +2,14 @@
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %tinput1.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux \
 # RUN:   %S/Inputs/relocation-relative-absolute.s -o %tinput2.o
-# RUN: not ld.lld %tinput1.o %tinput2.o -o %t -pie 2>&1 | FileCheck %s
+# RUN: ld.lld %tinput1.o %tinput2.o -o %t -pie
+# RUN: llvm-readobj -dyn-relocations %t | FileCheck %s
+
+# CHECK:      Dynamic Relocations {
+# CHECK-NEXT:   0x3018 R_X86_64_JUMP_SLOT - 0x2A
+# CHECK-NEXT: }
 
 .globl _start
 _start:
 
-# CHECK: {{.*}}input1.o:(.text+0x1): relocation R_X86_64_PLT32 cannot refer to absolute symbol 'answer' defined in {{.*}}input2.o
-
 call answer at PLT
Index: test/ELF/linkerscript/symbol-absolute.s
===================================================================
--- /dev/null
+++ test/ELF/linkerscript/symbol-absolute.s
@@ -0,0 +1,12 @@
+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: echo "PROVIDE_HIDDEN(base = 0);" > %t.script
+# RUN: ld.lld -shared --gc-sections --script %t.script -o %t1 %t
+# RUN: llvm-readobj --elf-output-style=GNU --file-headers --symbols %t1 | FileCheck %s
+# CHECK: 0000000000000000     0 NOTYPE  LOCAL  HIDDEN   ABS base
+
+.text
+.globl _start
+_start:
+	lea base(%rip),%rax
Index: ELF/Relocations.cpp
===================================================================
--- ELF/Relocations.cpp
+++ ELF/Relocations.cpp
@@ -338,30 +338,9 @@
 
   bool AbsVal = isAbsoluteValue<ELFT>(Body);
   bool RelE = isRelExpr(E);
-  if (AbsVal && !RelE)
-    return true;
-  if (!AbsVal && RelE)
+  if (AbsVal || RelE)
     return true;
 
-  // Relative relocation to an absolute value. This is normally unrepresentable,
-  // but if the relocation refers to a weak undefined symbol, we allow it to
-  // resolve to the image base. This is a little strange, but it allows us to
-  // link function calls to such symbols. Normally such a call will be guarded
-  // with a comparison, which will load a zero from the GOT.
-  // Another special case is MIPS _gp_disp symbol which represents offset
-  // between start of a function and '_gp' value and defined as absolute just
-  // to simplify the code.
-  if (AbsVal && RelE) {
-    if (Body.isUndefined() && !Body.isLocal() && Body.symbol()->isWeak())
-      return true;
-    if (&Body == ElfSym<ELFT>::MipsGpDisp)
-      return true;
-    error(S.getLocation(RelOff) + ": relocation " + toString(Type) +
-          " cannot refer to absolute symbol '" + toString(Body) +
-          "' defined in " + toString(Body.File));
-    return true;
-  }
-
   return Target->usesOnlyLowPageBits(Type);
 }
 
@@ -442,8 +421,11 @@
   if (Body.isGnuIFunc()) {
     Expr = toPlt(Expr);
   } else if (!Preemptible) {
-    if (needsPlt(Expr))
+    if (needsPlt(Expr)) {
+      if (isAbsoluteValue<ELFT>(Body))
+        return Expr;
       Expr = fromPlt(Expr);
+    }
     if (Expr == R_GOT_PC && !isAbsoluteValue<ELFT>(Body))
       Expr = Target->adjustRelaxExpr(Type, Data, Expr);
   }


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D26133.80864.patch
Type: text/x-patch
Size: 3598 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20161209/f237e81e/attachment-0001.bin>