[PATCH] D41551: [ELF] - Allow relocation to a weak undefined symbol when -z notext is given.

[Rafael Avila de Espindola via llvm-commits]

LGTM

George Rimar via Phabricator <reviews at reviews.llvm.org> writes:

> grimar updated this revision to Diff 128158.
> grimar added a comment.
>
> - Updated in according to previous comments.
>
>
> https://reviews.llvm.org/D41551
>
> Files:
>   ELF/Relocations.cpp
>   test/ELF/znotext-weak-undef.s
>
>
> Index: test/ELF/znotext-weak-undef.s
> ===================================================================
> --- test/ELF/znotext-weak-undef.s
> +++ test/ELF/znotext-weak-undef.s
> @@ -0,0 +1,16 @@
> +# REQUIRES: x86
> +# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
> +# RUN: not ld.lld -z notext -shared %t.o -o %t 2>&1 | FileCheck %s
> +# CHECK: relocation R_X86_64_32 cannot be used against shared object; recompile with -fPIC
> +
> +# RUN: ld.lld -z notext %t.o -o %t
> +# RUN: llvm-readobj -r %t | FileCheck %s --check-prefix=EXE
> +# EXE:      Relocations [
> +# EXE-NEXT: ]
> +
> +.text
> +.global foo
> +.weak foo
> +
> +_start:
> +mov $foo,%eax
> Index: ELF/Relocations.cpp
> ===================================================================
> --- ELF/Relocations.cpp
> +++ ELF/Relocations.cpp
> @@ -585,6 +585,19 @@
>  static RelExpr adjustExpr(Symbol &Sym, RelExpr Expr, RelType Type,
>                            InputSectionBase &S, uint64_t RelOff,
>                            bool &IsConstant) {
> +  // If a relocation can be applied at link-time, we don't need to
> +  // create a dynamic relocation in the first place.
> +  if (IsConstant)
> +    return Expr;
> +
> +  // If the relocation is to a weak undef, and we producing executable,
> +  // give up on it and produce a non preemptible 0.
> +  if (!Config->Shared && Sym.isUndefWeak()) {
> +    Sym.IsPreemptible = false;
> +    IsConstant = true;
> +    return Expr;
> +  }
> +
>    // If a section writable or if we are allowed to create dynamic relocations
>    // against read-only sections (i.e. when "-z notext" is given), we can create
>    // any dynamic relocation the dynamic linker knows how to handle.
> @@ -596,22 +609,9 @@
>      return Expr;
>    }
>  
> -  // If a relocation can be applied at link-time, we don't need to
> -  // create a dynamic relocation in the first place.
> -  if (IsConstant)
> -    return Expr;
> -
>    // If we got here we know that this relocation would require the dynamic
>    // linker to write a value to read only memory.
>  
> -  // If the relocation is to a weak undef, give up on it and produce a
> -  // non preemptible 0.
> -  if (Sym.isUndefWeak()) {
> -    Sym.IsPreemptible = false;
> -    IsConstant = true;
> -    return Expr;
> -  }
> -
>    // We can hack around it if we are producing an executable and
>    // the refered symbol can be preemepted to refer to the executable.
>    if (Config->Shared || (Config->Pic && !isRelExpr(Expr))) {
>
>
> Index: test/ELF/znotext-weak-undef.s
> ===================================================================
> --- test/ELF/znotext-weak-undef.s
> +++ test/ELF/znotext-weak-undef.s
> @@ -0,0 +1,16 @@
> +# REQUIRES: x86
> +# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
> +# RUN: not ld.lld -z notext -shared %t.o -o %t 2>&1 | FileCheck %s
> +# CHECK: relocation R_X86_64_32 cannot be used against shared object; recompile with -fPIC
> +
> +# RUN: ld.lld -z notext %t.o -o %t
> +# RUN: llvm-readobj -r %t | FileCheck %s --check-prefix=EXE
> +# EXE:      Relocations [
> +# EXE-NEXT: ]
> +
> +.text
> +.global foo
> +.weak foo
> +
> +_start:
> +mov $foo,%eax
> Index: ELF/Relocations.cpp
> ===================================================================
> --- ELF/Relocations.cpp
> +++ ELF/Relocations.cpp
> @@ -585,6 +585,19 @@
>  static RelExpr adjustExpr(Symbol &Sym, RelExpr Expr, RelType Type,
>                            InputSectionBase &S, uint64_t RelOff,
>                            bool &IsConstant) {
> +  // If a relocation can be applied at link-time, we don't need to
> +  // create a dynamic relocation in the first place.
> +  if (IsConstant)
> +    return Expr;
> +
> +  // If the relocation is to a weak undef, and we producing executable,
> +  // give up on it and produce a non preemptible 0.
> +  if (!Config->Shared && Sym.isUndefWeak()) {
> +    Sym.IsPreemptible = false;
> +    IsConstant = true;
> +    return Expr;
> +  }
> +
>    // If a section writable or if we are allowed to create dynamic relocations
>    // against read-only sections (i.e. when "-z notext" is given), we can create
>    // any dynamic relocation the dynamic linker knows how to handle.
> @@ -596,22 +609,9 @@
>      return Expr;
>    }
>  
> -  // If a relocation can be applied at link-time, we don't need to
> -  // create a dynamic relocation in the first place.
> -  if (IsConstant)
> -    return Expr;
> -
>    // If we got here we know that this relocation would require the dynamic
>    // linker to write a value to read only memory.
>  
> -  // If the relocation is to a weak undef, give up on it and produce a
> -  // non preemptible 0.
> -  if (Sym.isUndefWeak()) {
> -    Sym.IsPreemptible = false;
> -    IsConstant = true;
> -    return Expr;
> -  }
> -
>    // We can hack around it if we are producing an executable and
>    // the refered symbol can be preemepted to refer to the executable.
>    if (Config->Shared || (Config->Pic && !isRelExpr(Expr))) {