[ELF] - Make __start_/__stop_<section_name> symbols STV_PROTECTED

[grimar]

Summary

Recently bfd did this change and it seems reasonable to me,
I suggest doing the same.

Relative threads in binutils:
https://sourceware.org/ml/binutils/2017-08/msg00207.html
https://sourceware.org/bugzilla/show_bug.cgi?id=21964

[• espindola]

LGTM
I am actually surprised these are not hidden.

[ruiu]

LGTM

[grimar]

espindola accepted this revision.
espindola added a comment.
This revision is now accepted and ready to land.

LGTM
I am actually surprised these are not hidden.

https://reviews.llvm.org/D44566

Yes, Binutils initially made them hidden but had to change to protected
because of https://sourceware.org/bugzilla/show_bug.cgi?id=21964.
(http://lists.gnu.org/archive/html/bug-binutils/2017-08/msg00195.html)

George.

ELF/Writer.cpp

 // __stop_<secname> symbols. They are at beginning and end of the section,
 // respectively. This is not requested by the ELF standard, but GNU ld and
 // gold provide the feature, and used by many programs.
 template <class ELFT>
 void Writer<ELFT>::addStartStopSymbols(OutputSection *Sec) {
   StringRef S = Sec->Name;
   if (!isValidCIdentifier(S))
     return;
-  addOptionalRegular(Saver.save("__start_" + S), Sec, 0, STV_DEFAULT);
-  addOptionalRegular(Saver.save("__stop_" + S), Sec, -1, STV_DEFAULT);
+  addOptionalRegular(Saver.save("__start_" + S), Sec, 0, STV_PROTECTED);
+  addOptionalRegular(Saver.save("__stop_" + S), Sec, -1, STV_PROTECTED);
 }
 
 static bool needsPtLoad(OutputSection *Sec) {
   if (!(Sec->Flags & SHF_ALLOC))
     return false;
 
   // Don't allocate VA space for TLS NOBITS sections. The PT_TLS PHDR is
   // responsible for allocating space for them, not the PT_LOAD that

test/ELF/startstop-shared.s

 // REQUIRES: x86
 // RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
 // RUN: ld.lld %t.o -o %t.so -shared
 // RUN: llvm-readobj -r -t %t.so | FileCheck  %s
 
         .data
         .quad __start_foo
         .section foo,"aw"
 
         .hidden __start_bar
         .quad __start_bar
         .section bar,"a"
 
-// Test that we are able to hide the symbol.
-// CHECK:      R_X86_64_RELATIVE - 0x[[ADDR:.*]]
+// CHECK:      Relocations [
+// CHECK-NEXT:   Section {{.*}} .rela.dyn {
+// CHECK-NEXT:     R_X86_64_RELATIVE
+// CHECK-NEXT:     R_X86_64_RELATIVE
+// CHECK-NEXT:   }
+// CHECK-NEXT: ]
 
-// By default the symbol is visible and we need a dynamic reloc.
-// CHECK:  R_X86_64_64 __start_foo 0x0
+// Test that we are able to hide the symbol.
+// By default the symbol is protected.
 
 // CHECK:      Name: __start_bar
-// CHECK-NEXT: Value: 0x[[ADDR]]
+// CHECK-NEXT: Value:
 // CHECK-NEXT: Size:
 // CHECK-NEXT: Binding: Local
+// CHECK-NEXT: Type: None
+// CHECK-NEXT: Other [
+// CHECK-NEXT:   STV_HIDDEN
+// CHECK-NEXT: ]
 
 // CHECK:      Name: __start_foo
 // CHECK-NEXT: Value:
 // CHECK-NEXT: Size:
 // CHECK-NEXT: Binding: Global
+// CHECK-NEXT: Type: None
+// CHECK-NEXT: Other [
+// CHECK-NEXT:   STV_PROTECTED
+// CHECK-NEXT: ]
+// CHECK-NEXT: Section: foo

test/ELF/startstop.s

 // DISASM:    1010:       90      nop
 // DISASM:    1011:       90      nop
 // DISASM: Disassembly of section bar:
 // DISASM: __start_bar:
 // DISASM:    1012:       90      nop
 // DISASM:    1013:       90      nop
 // DISASM:    1014:       90      nop
 
-
 // SYMBOL:      Relocations [
 // SYMBOL-NEXT:   Section ({{.*}}) .rela.dyn {
-// SYMBOL-NEXT:     0x2010 R_X86_64_64 __stop_zed1 0x0
-// SYMBOL-NEXT:     0x2018 R_X86_64_64 __stop_zed1 0x1
-// SYMBOL-NEXT:     0x2000 R_X86_64_64 __stop_zed2 0x0
-// SYMBOL-NEXT:     0x2008 R_X86_64_64 __stop_zed2 0x1
+// SYMBOL-NEXT:     R_X86_64_RELATIVE
+// SYMBOL-NEXT:     R_X86_64_RELATIVE
+// SYMBOL-NEXT:     R_X86_64_RELATIVE
+// SYMBOL-NEXT:     R_X86_64_RELATIVE
 // SYMBOL-NEXT:   }
 // SYMBOL-NEXT: ]
 
 // SYMBOL: Symbol {
 // SYMBOL:   Name: __start_bar
 // SYMBOL:   Value: 0x1012
 // SYMBOL:   STV_HIDDEN
 // SYMBOL:   Section: bar
 // SYMBOL: }
 // SYMBOL-NOT:   Section: __stop_bar
 // SYMBOL: Symbol {
 // SYMBOL:   Name: __start_foo
 // SYMBOL:   Value: 0x100F
 // SYMBOL:   STV_HIDDEN
 // SYMBOL:   Section: foo
 // SYMBOL: }
 // SYMBOL: Symbol {
 // SYMBOL:   Name: __stop_foo
 // SYMBOL:   Value: 0x1012
-// STMBOL:   STV_HIDDEN
+// SYMBOL:   STV_HIDDEN
 // SYMBOL:   Section: foo
 // SYMBOL: }
 
 // SYMBOL: Symbol {
 // SYMBOL:   Name: __stop_zed1
 // SYMBOL:   Value: 0x2010
-// STMBOL:   Other: 0
+// SYMBOL:   STV_PROTECTED
 // SYMBOL:   Section: zed1
 // SYMBOL: }
 // SYMBOL: Symbol {
 // SYMBOL:   Name: __stop_zed2
 // SYMBOL:   Value: 0x2020
-// STMBOL:   Other: 0
+// SYMBOL:   STV_PROTECTED
 // SYMBOL:   Section: zed2
 // SYMBOL: }
 
 .hidden __start_foo
 .hidden __stop_foo
 .hidden __start_bar
 .global _start
 .text