[ELF] - Linkerscript: do not define _edata,_end,_etext if SECTIONS is used.

[grimar]

Summary

I faced that when tried to link FreeBSD kernel. It was "duplicate symbol: _edata in (internal) and (internal)" error.
_data was a shared symbol that came from hack.so. At first it was replaced with DefinedRegular by the code
disabled in this patch and later when script tried to define the same symbol - the error was shown.

I think when SECTIONS command is used we should not define these symbols automatically.
In the same situation (as given in testcase) ld defines them as UND. gold defines as ABS with zero value.
I suggest just do not do anything in this case and let script handle everything.

[grimar]

With this + D23201, + D23165 + D22683 + slightly modified kernel linker script I was able to link FreeBSD kernel.
Though I am observing other problems now:

objcopy --only-keep-debug kernel.debug kernel.symbols
BFD: kernel.symbols: section `.eh_frame' can't be allocated in segment 3
objcopy: kernel.symbols: Bad value
Investigating it.

btw difference between my script and latest available is:

1. It does not contain AT(). Actually that what is currently commited in FreeBSD release 10.3 head I think.
2. Assignment to symbols inside output sections declarations (in *bss for example) I moved outside.

After that I verified that kernel works fine when linked with ld.

[evgeny777]

That's great news )

[grimar]

In D23206#506947, @evgeny777 wrote:

That's great news )

But link != it works. Just means lld did not crash :)

[ruiu]

LGTM

ELF/Writer.cpp

602 ↗

(On Diff #66947)

Can you inline this function? I think it is already well factored out, so we don't need to split it even more.

lld/trunk/ELF/Writer.cpp

template <class ELFT> void Writer<ELFT>::addReservedSymbols() {
 
   // __tls_get_addr is defined by the dynamic linker for dynamic ELFs. For
   // static linking the linker is required to optimize away any references to
   // __tls_get_addr, so it's not defined anywhere. Create a hidden definition
   // to avoid the undefined symbol error.
   if (!isOutputDynamic<ELFT>())
     Symtab.addIgnored("__tls_get_addr");
 
+  // If linker script do layout we do not need to create any standart symbols.
+  if (ScriptConfig->HasContents)
+    return;
+
   auto Define = [this](StringRef S, DefinedRegular<ELFT> *&Sym1,
                        DefinedRegular<ELFT> *&Sym2) {
     Sym1 = Symtab.addIgnored(S, STV_DEFAULT);
 
     // The name without the underscore is not a reserved name,
     // so it is defined only when there is a reference against it.
     assert(S.startswith("_"));
     S = S.substr(1);

lld/trunk/test/ELF/end.s

 // DEFAULT-NEXT:     Offset:
 // DEFAULT-NEXT:     Size: 6
 // DEFAULT: ]
 // DEFAULT: Symbols [
 // DEFAULT:     Name: _end
 // DEFAULT-NEXT:     Value: 0x12008
 // DEFAULT: ]
 
-// If there is no .bss section, "_end" should point to the end of the .data section.
-// RUN: echo "SECTIONS { \
-// RUN:          /DISCARD/ : { *(.bss) } }" > %t.script
-// RUN: ld.lld %t.o --script %t.script -o %t
-// RUN: llvm-readobj -sections -symbols %t | FileCheck %s --check-prefix=NOBSS
-
-// NOBSS: Sections [
-// NOBSS:     Name: .data
-// NOBSS-NEXT:     Type:
-// NOBSS-NEXT:     Flags [
-// NOBSS-NEXT:       SHF_ALLOC
-// NOBSS-NEXT:       SHF_WRITE
-// NOBSS-NEXT:     ]
-// NOBSS-NEXT:     Address: 0x159
-// NOBSS-NEXT:     Offset:
-// NOBSS-NEXT:     Size: 2
-// NOBSS: ]
-// NOBSS: Symbols [
-// NOBSS:     Name: _end
-// NOBSS-NEXT:     Value: 0x15B
-// NOBSS: ]
-
-// If the layout of the sections is changed, "_end" should point to the end of allocated address space.
-// RUN: echo "SECTIONS { \
-// RUN:          .bss : { *(.bss) } \
-// RUN:          .data : { *(.data) } \
-// RUN:          .text : { *(.text) } }" > %t.script
-// RUN: ld.lld %t.o --script %t.script -o %t
-// RUN: llvm-readobj -sections -symbols %t | FileCheck %s --check-prefix=TEXTATEND
-
-// TEXTATEND: Sections [
-// TEXTATEND:     Name: .text
-// TEXTATEND-NEXT:     Type:
-// TEXTATEND-NEXT:     Flags [
-// TEXTATEND-NEXT:       SHF_ALLOC
-// TEXTATEND-NEXT:       SHF_EXECINSTR
-// TEXTATEND-NEXT:     ]
-// TEXTATEND-NEXT:     Address: 0x160
-// TEXTATEND-NEXT:     Offset:
-// TEXTATEND-NEXT:     Size: 1
-// TEXTATEND: ]
-// TEXTATEND: Symbols [
-// TEXTATEND:     Name: _end
-// TEXTATEND-NEXT:     Value: 0x161
-// TEXTATEND: ]
-
 .global _start,_end
 .text
 _start:
     nop
 .data
     .word 1
 .bss
     .space 6

lld/trunk/test/ELF/linkerscript/linkerscript-edata-etext.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: echo "SECTIONS { }" > %t.script
+# RUN: not ld.lld %t.o -script %t.script -o %t 2>&1 | FileCheck %s
+# CHECK: undefined symbol: _edata
+# CHECK: undefined symbol: _end
+# CHECK: undefined symbol: _etext
+
+.global _start,_end,_etext,_edata
+.text
+_start:
+ .quad _edata + 0x1
+ .quad _etext + 0x1
+ .quad _end + 0x1
+
+.data
+  .word 1
+.bss
+  .align 4
+  .space 6