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

[Petr Hosek via llvm-commits]

phosek created this revision.
phosek added a reviewer: ruiu.
phosek added a subscriber: llvm-commits.
phosek set the repository for this revision to rL LLVM.
phosek added a project: lld.

While these might cause an overflow at runtime on some targets, lld shouldn't reject such a case, it's clearly meaningful and the technique is used on, at least, pre-VDSO GNU/Linux and AIX.

https://llvm.org/bugs/show_bug.cgi?id=30406


Repository:
  rL LLVM

https://reviews.llvm.org/D26133

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


Index: test/ELF/relocation-relative-absolute.s
===================================================================
--- test/ELF/relocation-relative-absolute.s
+++ test/ELF/relocation-relative-absolute.s
@@ -1,11 +1,14 @@
 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
-# RUN: not ld.lld %t.o -o %t -pie 2>&1 | FileCheck %s
+# RUN: ld.lld %t.o -o %t -pie
+# RUN: llvm-readobj -dyn-relocations %t | FileCheck %s
+
+# CHECK:      Dynamic Relocations {
+# CHECK-NEXT: }
 
 .globl _start
 _start:
 
-# CHECK: relocation R_X86_64_PLT32 cannot refer to absolute symbol answer
 call answer at PLT
 
 .globl answer
Index: ELF/Relocations.cpp
===================================================================
--- ELF/Relocations.cpp
+++ ELF/Relocations.cpp
@@ -328,23 +328,8 @@
 
   bool AbsVal = isAbsoluteValue<ELFT>(Body);
   bool RelE = isRelExpr(E);
-  if (AbsVal && !RelE)
+  if (AbsVal || RelE)
     return true;
-  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.
-  if (AbsVal && RelE) {
-    if (Body.isUndefined() && !Body.isLocal() && Body.symbol()->isWeak())
-      return true;
-    error("relocation " + getRelName(Type) +
-          " cannot refer to absolute symbol " + Body.getName());
-    return true;
-  }
 
   return Target->usesOnlyLowPageBits(Type);
 }


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