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[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

llvm-svn: 363399
This commit is contained in:
Fangrui Song 2019-06-14 14:09:15 +00:00
parent a59bc6e589
commit e05ca38101
2 changed files with 16 additions and 10 deletions
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18
lld/ELF/Relocations.cpp
View File

@ -924,10 +924,19 @@ template <class ELFT, class RelTy>
static void processRelocAux(InputSectionBase &Sec, RelExpr Expr, RelType Type, static void processRelocAux(InputSectionBase &Sec, RelExpr Expr, RelType Type,
uint64_t Offset, Symbol &Sym, const RelTy &Rel, uint64_t Offset, Symbol &Sym, const RelTy &Rel,
int64_t Addend) { 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}); Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
return; return;
} }
bool CanWrite = (Sec.Flags & SHF_WRITE) || !Config->ZText; bool CanWrite = (Sec.Flags & SHF_WRITE) || !Config->ZText;
if (CanWrite) { if (CanWrite) {
// FIXME Improve the way we handle absolute relocation types that will // FIXME Improve the way we handle absolute relocation types that will
@ -967,13 +976,6 @@ static void processRelocAux(InputSectionBase &Sec, RelExpr Expr, RelType Type,
} }
} }
// 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))) { if (!CanWrite && (Config->Pic && !isRelExpr(Expr))) {
error( error(
"can't create dynamic relocation " + toString(Type) + " against " + "can't create dynamic relocation " + toString(Type) + " against " +

8
lld/test/ELF/weak-undef-rw.s
View File

@ -1,9 +1,13 @@
# REQUIRES: x86 # REQUIRES: x86
# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
# RUN: ld.lld %t.o -o %t --export-dynamic # 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 .global _start
_start: _start: