[ELF] Don't emit dynamic relocations with weak undef in writable sections
In processRelocAux(), our handling of 1) link-time constant and 2) weak
undef is the same, so put them together to simplify the logic.
This moves the weak undef code around. The result is that: in a writable
section (or -z notext), we will no longer emit dynamic relocations for
weak undefined symbols.
The new behavior seems to match GNU linkers, and improves consistency
with the case of a readonly section.
The condition `!Config->Shared` was there probably because it is common
for a -shared link not to specify full dependencies. Keep it now but we
may revisit the decision in the future.
gABI says:
> The behavior of weak symbols in areas not specified by this document is
> implementation defined. Weak symbols are intended primarily for use in
> system software. Applications using weak symbols are unreliable since
> changes in the runtime environment might cause the execution to fail.
Reviewed By: ruiu
Differential Revision: https://reviews.llvm.org/D63003
git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@363399 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/ELF/Relocations.cpp b/ELF/Relocations.cpp
index 1398d33..6a1c0fa 100644
--- a/ELF/Relocations.cpp
+++ b/ELF/Relocations.cpp
@@ -924,10 +924,19 @@
static void processRelocAux(InputSectionBase &Sec, RelExpr Expr, RelType Type,
uint64_t Offset, Symbol &Sym, const RelTy &Rel,
int64_t Addend) {
- if (isStaticLinkTimeConstant(Expr, Type, Sym, Sec, Offset)) {
+ // If the relocation is known to be a link-time constant, we know no dynamic
+ // relocation will be created, pass the control to relocateAlloc() or
+ // relocateNonAlloc() to resolve it.
+ //
+ // The behavior of an undefined weak reference is implementation defined. If
+ // the relocation is to a weak undef, and we are producing an executable, let
+ // relocate{,Non}Alloc() resolve it.
+ if (isStaticLinkTimeConstant(Expr, Type, Sym, Sec, Offset) ||
+ (!Config->Shared && Sym.isUndefWeak())) {
Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
return;
}
+
bool CanWrite = (Sec.Flags & SHF_WRITE) || !Config->ZText;
if (CanWrite) {
// FIXME Improve the way we handle absolute relocation types that will
@@ -967,13 +976,6 @@
}
}
- // If the relocation is to a weak undef, and we are producing
- // executable, give up on it and produce a non preemptible 0.
- if (!Config->Shared && Sym.isUndefWeak()) {
- Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
- return;
- }
-
if (!CanWrite && (Config->Pic && !isRelExpr(Expr))) {
error(
"can't create dynamic relocation " + toString(Type) + " against " +
diff --git a/test/ELF/weak-undef-rw.s b/test/ELF/weak-undef-rw.s
index c75e7d6..e8d9515 100644
--- a/test/ELF/weak-undef-rw.s
+++ b/test/ELF/weak-undef-rw.s
@@ -1,9 +1,13 @@
# REQUIRES: x86
# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
# RUN: ld.lld %t.o -o %t --export-dynamic
-# RUN: llvm-readobj -r %t | FileCheck %s
+# RUN: llvm-readelf -r %t | FileCheck %s
-# CHECK: R_X86_64_64 foobar 0x0
+## gABI leaves the behavior of weak undefined references implementation defined.
+## We choose to resolve it statically and not create a dynamic relocation for
+## implementation simplicity. This also matches ld.bfd and gold.
+
+# CHECK: no relocations
.global _start
_start:
