Define __executable_start

[• rafael]

Summary

It is a bit odd to define two symbols pointing to the same location, but it is a trivial change, so it is probably ok.

This is defined by both gold and bfd.

[ruiu]

LGTM

ELF/Writer.cpp

916–917

Since this is just two iterations, I'd do

addOptionalRegular<ELFT>("__ehdr_start", Out::ElfHeader, 0, STV_HIDDEN);
addOptionalRegular<ELFT>("__executable_start", Out::ElfHeader, 0, STV_HIDDEN);

[• espindola]

304803

ELF/Writer.cpp

template <class ELFT> void Writer<ELFT>::addReservedSymbols() {
   // to avoid the undefined symbol error.
   if (!InX::DynSymTab)
     Symtab<ELFT>::X->addIgnored("__tls_get_addr");
 
   // __ehdr_start is the location of ELF file headers. Note that we define
   // this symbol unconditionally even when using a linker script, which
   // differs from the behavior implemented by GNU linker which only define
   // this symbol if ELF headers are in the memory mapped segment.
-  addOptionalRegular<ELFT>("__ehdr_start", Out::ElfHeader, 0, STV_HIDDEN);
+  // __executable_start is not documented, but the expectation of at
+  // least the android libc is that it points to the elf header too.
+  for (const char *Name : {"__ehdr_start", "__executable_start"})
+    addOptionalRegular<ELFT>(Name, Out::ElfHeader, 0, STV_HIDDEN);
 
   // If linker script do layout we do not need to create any standart symbols.
   if (Script->Opt.HasSections)
     return;
 
   auto Add = [](StringRef S) {
     return addOptionalRegular<ELFT>(S, Out::ElfHeader, 0, STV_DEFAULT);
   };

test/ELF/ehdr_start.s

 # REQUIRES: x86
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
 # RUN: ld.lld %t.o -o %t
 # RUN: llvm-readobj -symbols %t | FileCheck %s
 # CHECK:    Name: __ehdr_start (1)
 # CHECK-NEXT:    Value: 0x200000
 # CHECK-NEXT:    Size: 0
 # CHECK-NEXT:    Binding: Local (0x0)
 # CHECK-NEXT:    Type: None (0x0)
 # CHECK-NEXT:    Other [ (0x2)
 # CHECK-NEXT:      STV_HIDDEN (0x2)
 # CHECK-NEXT:    ]
 # CHECK-NEXT:    Section: .text (0x1)
 
+# CHECK:    Name: __executable_start
+# CHECK-NEXT:    Value: 0x200000
+# CHECK-NEXT:    Size: 0
+# CHECK-NEXT:    Binding: Local
+# CHECK-NEXT:    Type: None
+# CHECK-NEXT:    Other [
+# CHECK-NEXT:      STV_HIDDEN
+# CHECK-NEXT:    ]
+# CHECK-NEXT:    Section: .text
+
 .text
 .global _start, __ehdr_start
 _start:
   .quad __ehdr_start
+  .quad __executable_start
 
 # RUN: ld.lld -r %t.o -o %t.r
 # RUN: llvm-readobj -symbols %t.r | FileCheck %s --check-prefix=RELOCATABLE
 
 # RELOCATABLE:    Name: __ehdr_start (1)
 # RELOCATABLE-NEXT:    Value: 0x0
 # RELOCATABLE-NEXT:    Size: 0
 # RELOCATABLE-NEXT:    Binding: Global (0x1)
 # RELOCATABLE-NEXT:    Type: None (0x0)
 # RELOCATABLE-NEXT:    Other: 0
 # RELOCATABLE-NEXT:    Section: Undefined (0x0)