[PATCH] D55682: [ELF] Support defining __start/__stop symbols as hidden

[Petr Hosek via Phabricator via llvm-commits]

phosek created this revision.
phosek added a reviewer: ruiu.
Herald added subscribers: llvm-commits, arichardson, emaste.
Herald added a reviewer: espindola.

We cannot define these symbols as hidden by default because some
users apparently rely on using dlsym to look them up, see
https://sourceware.org/bugzilla/show_bug.cgi?id=21964

However, it's still useful to define them as hidden in many contexts.
To do so, this change introduces the -z hidden-start-stop-symbols
option that provides that ability.


Repository:
  rLLD LLVM Linker

https://reviews.llvm.org/D55682

Files:
  lld/ELF/Config.h
  lld/ELF/Driver.cpp
  lld/ELF/Writer.cpp
  lld/test/ELF/startstop-hidden.s


Index: lld/test/ELF/startstop-hidden.s
===================================================================
--- /dev/null
+++ lld/test/ELF/startstop-hidden.s
@@ -0,0 +1,26 @@
+// REQUIRES: x86
+// RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+// RUN: ld.lld -z hidden-start-stop-symbols %t.o -o %t
+// RUN: llvm-readobj -symbols %t | FileCheck %s
+
+// CHECK: Symbol {
+// CHECK:   Name: __start_bar
+// CHECK:   STV_HIDDEN
+// CHECK:   Section: bar
+// CHECK: }
+// CHECK: Symbol {
+// CHECK:   Name: __start_foo
+// CHECK:   STV_HIDDEN
+// CHECK:   Section: foo
+// CHECK: }
+
+.quad __start_foo
+.section foo,"ax"
+
+.quad __start_bar
+.section bar,"ax"
+
+.global _start
+.text
+_start:
+	nop
Index: lld/ELF/Writer.cpp
===================================================================
--- lld/ELF/Writer.cpp
+++ lld/ELF/Writer.cpp
@@ -1846,8 +1846,9 @@
   StringRef S = Sec->Name;
   if (!isValidCIdentifier(S))
     return;
-  addOptionalRegular(Saver.save("__start_" + S), Sec, 0, STV_PROTECTED);
-  addOptionalRegular(Saver.save("__stop_" + S), Sec, -1, STV_PROTECTED);
+  uint8_t B = Config->ZHiddenStartStopSymbols ? STV_HIDDEN : STV_PROTECTED;
+  addOptionalRegular(Saver.save("__start_" + S), Sec, 0, B);
+  addOptionalRegular(Saver.save("__stop_" + S), Sec, -1, B);
 }
 
 static bool needsPtLoad(OutputSection *Sec) {
Index: lld/ELF/Driver.cpp
===================================================================
--- lld/ELF/Driver.cpp
+++ lld/ELF/Driver.cpp
@@ -355,6 +355,7 @@
          S == "nokeep-text-section-prefix" || S == "norelro" || S == "notext" ||
          S == "now" || S == "origin" || S == "relro" || S == "retpolineplt" ||
          S == "rodynamic" || S == "text" || S == "wxneeded" ||
+         S == "hidden-start-stop-symbols" ||
          S.startswith("max-page-size=") || S.startswith("stack-size=");
 }
 
@@ -872,6 +873,7 @@
   Config->ZExecstack = getZFlag(Args, "execstack", "noexecstack", false);
   Config->ZGlobal = hasZOption(Args, "global");
   Config->ZHazardplt = hasZOption(Args, "hazardplt");
+  Config->ZHiddenStartStopSymbols = hasZOption(Args, "hidden-start-stop-symbols");
   Config->ZInitfirst = hasZOption(Args, "initfirst");
   Config->ZInterpose = hasZOption(Args, "interpose");
   Config->ZKeepTextSectionPrefix = getZFlag(
Index: lld/ELF/Config.h
===================================================================
--- lld/ELF/Config.h
+++ lld/ELF/Config.h
@@ -186,6 +186,7 @@
   bool ZExecstack;
   bool ZGlobal;
   bool ZHazardplt;
+  bool ZHiddenStartStopSymbols;
   bool ZInitfirst;
   bool ZInterpose;
   bool ZKeepTextSectionPrefix;


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