[ELF] Default to --no-allow-shlib-undefined for executables

[MaskRay]

Summary

This follows the ld.bfd/gold behavior.

The error check is useful as it turns a common type of ld.so undefined symbol errors into link-time errors:

// a.cc => a.so (not linked with -z defs)
void f(); // f is undefined
void g() { f(); }

// b.cc => executable with a DT_NEEDED entry on a.so
void g();
int main() { g(); }

// ld.so errors when g() is executed (lazy binding) or when the program is started (LD_BIND_NOW)
// symbol lookup error: ... undefined symbol: f

[ruiu]

Could you also update the man page? The file is lld/docs/ld.lld.1.

[MaskRay]

Update docs/ld.lld.1

[ruiu]

LGTM

docs/ld.lld.1

51

an -> a

[MaskRay]

an -> a

ELF/Driver.cpp

void LinkerDriver::readConfigs(opt::InputArgList &Args) {
   errorHandler().FatalWarnings =
       Args.hasFlag(OPT_fatal_warnings, OPT_no_fatal_warnings, false);
   ThreadsEnabled = Args.hasFlag(OPT_threads, OPT_no_threads, true);
 
   Config->AllowMultipleDefinition =
       Args.hasFlag(OPT_allow_multiple_definition,
                    OPT_no_allow_multiple_definition, false) ||
       hasZOption(Args, "muldefs");
-  Config->AllowShlibUndefined = Args.hasFlag(
-      OPT_allow_shlib_undefined, OPT_no_allow_shlib_undefined, true);
+  Config->AllowShlibUndefined =
+      Args.hasFlag(OPT_allow_shlib_undefined, OPT_no_allow_shlib_undefined,
+                   Args.hasArg(OPT_shared));
   Config->AuxiliaryList = args::getStrings(Args, OPT_auxiliary);
   Config->Bsymbolic = Args.hasArg(OPT_Bsymbolic);
   Config->BsymbolicFunctions = Args.hasArg(OPT_Bsymbolic_functions);
   Config->CheckSections =
       Args.hasFlag(OPT_check_sections, OPT_no_check_sections, true);
   Config->Chroot = Args.getLastArgValue(OPT_chroot);
   Config->CompressDebugSections = getCompressDebugSections(Args);
   Config->Cref = Args.hasFlag(OPT_cref, OPT_no_cref, false);

docs/ld.lld.1

 .Nm
 as a native linker as well as a cross linker.
 
 .Sh OPTIONS
 .Bl -tag -width indent
 .It Fl -allow-multiple-definition
 Do not error if a symbol is defined multiple times.
 The first definition will be used.
+.It Fl -allow-shlib-undefined
+Allow unresolved references in shared libraries.
+This option is enabled by default when linking a shared library.
 .It Fl -apply-dynamic-relocs
 Apply link-time values for dynamic relocations.
 .It Fl -as-needed
 Only set
 .Dv DT_NEEDED
 for shared libraries if used.
 .It Fl -auxiliary Ns = Ns Ar value
 Set the

 Optimization level for LTO.
 .It Fl -lto-partitions Ns = Ns Ar value
 Number of LTO codegen partitions.
 .It Fl m Ar value
 Set target emulation.
 .It Fl -Map Ns = Ns Ar file , Fl M Ar file
 Print a link map to
 .Ar file .
+.It Fl -no-allow-shlib-undefined
+Do not allow unresolved references in shared libraries.
+This option is enabled by default when linking an executable.
 .It Fl -no-as-needed
 Always set
 .Dv DT_NEEDED
 for shared libraries.
 .It Fl -no-color-diagnostics
 Do not use colors in diagnostics.
 .It Fl -no-define-common
 Do not assign space to common symbols.

test/ELF/allow-shlib-undefined.s

 # REQUIRES: x86
 # --allow-shlib-undefined is set by default
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux \
 # RUN:   %p/Inputs/allow-shlib-undefined.s -o %t1.o
 # RUN: ld.lld -shared %t1.o -o %t.so
 
-# RUN: ld.lld %t.o %t.so -o /dev/null
 # RUN: ld.lld --allow-shlib-undefined %t.o %t.so -o /dev/null
 # RUN: not ld.lld --no-allow-shlib-undefined %t.o %t.so -o /dev/null 2>&1 | FileCheck %s
+# Executable defaults to --no-allow-shlib-undefined
+# RUN: not ld.lld %t.o %t.so -o /dev/null 2>&1 | FileCheck %s
+# -shared defaults to --allow-shlib-undefined
+# RUN: ld.lld -shared %t.o %t.so -o /dev/null
 
 # RUN: echo | llvm-mc -filetype=obj -triple=x86_64-unknown-linux -o %tempty.o
 # RUN: ld.lld -shared %tempty.o -o %tempty.so
 # RUN: ld.lld -shared %t1.o %tempty.so -o %t2.so
 # RUN: ld.lld --no-allow-shlib-undefined %t.o %t2.so -o /dev/null
 
 # DSO with undefines:
 # should link with or without any of these options.

test/ELF/unresolved-symbols.s

 # RUN: ld.lld %t2.o -o %t1_1 --unresolved-symbols=ignore-all
 # RUN: llvm-readobj %t1_1 > /dev/null 2>&1
 # RUN: not ld.lld %t2.o -o /dev/null --unresolved-symbols=ignore-all --no-undefined 2>&1 | \
 # RUN:   FileCheck -check-prefix=ERRUND %s
 # ERRUND: error: undefined symbol: undef
 # ERRUND: >>> referenced by {{.*}}:(.text+0x1)
 
 ## Also ignore all should not produce error for symbols from DSOs.
-# RUN: ld.lld %t1.o %t.so -o %t1_3 --unresolved-symbols=ignore-all
+# RUN: ld.lld %t1.o %t.so -o %t1_3 --allow-shlib-undefined --unresolved-symbols=ignore-all
 # RUN: llvm-readobj %t1_3 > /dev/null 2>&1
 
 ## Ignoring undefines in objects should not produce error for symbol from object.
 # RUN: ld.lld %t1.o %t2.o -o %t2 --unresolved-symbols=ignore-in-object-files
 # RUN: llvm-readobj %t2 > /dev/null 2>&1
 ## And still should not should produce for undefines from DSOs.
-# RUN: ld.lld %t1.o %t.so -o /dev/null --unresolved-symbols=ignore-in-object-files
+# RUN: ld.lld %t1.o %t.so -o /dev/null --allow-shlib-undefined --unresolved-symbols=ignore-in-object-files
 # RUN: llvm-readobj %t2 > /dev/null 2>&1
 
 ## Ignoring undefines in shared should produce error for symbol from object.
 # RUN: not ld.lld %t2.o -o /dev/null --unresolved-symbols=ignore-in-shared-libs 2>&1 | \
 # RUN:   FileCheck -check-prefix=ERRUND %s
 ## And should not produce errors for symbols from DSO.
-# RUN: ld.lld %t1.o %t.so -o %t3_1 --unresolved-symbols=ignore-in-shared-libs
+# RUN: ld.lld %t1.o %t.so -o %t3_1 --allow-shlib-undefined --unresolved-symbols=ignore-in-shared-libs
 # RUN: llvm-readobj %t3_1 > /dev/null 2>&1
 
 ## Ignoring undefines in shared libs should not produce error for symbol from object
 ## if we are linking DSO.
 # RUN: ld.lld -shared %t1.o -o %t4 --unresolved-symbols=ignore-in-shared-libs
 # RUN: llvm-readobj %t4 > /dev/null 2>&1
 
 ## Do not report undefines if linking relocatable.