[PATCH] D23925: [ELF] Default to entry address 0x0 in case start symbol is not defined and entry point is not specified in command line

[Eugene Leviant via llvm-commits]

evgeny777 created this revision.
evgeny777 added a reviewer: ruiu.
evgeny777 added subscribers: grimar, ikudrin, llvm-commits.
evgeny777 set the repository for this revision to rL LLVM.
evgeny777 added a project: lld.

The default behavior of GNU linkers is to set entry address to 0x0 when there is no start symbol (_start) and entry point is not passed to linker in command line arguments. Unfortunately lld reports undefined symbol error in this case (undefined symbol _start) and fails to link the program. An alternative is to always pass -Wl,-e,0 (or "-e 0"), which is not very convenient.

Repository:
  rL LLVM

https://reviews.llvm.org/D23925

Files:
  ELF/Config.h
  ELF/Driver.cpp
  ELF/Writer.cpp
  test/ELF/entry.s
  test/ELF/reproduce-windows.s
  test/ELF/undef-start.s

Index: test/ELF/undef-start.s
===================================================================
--- test/ELF/undef-start.s
+++ test/ELF/undef-start.s
@@ -1,4 +1,3 @@
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
-# RUN: not ld.lld %t -o %t2 2>&1 | FileCheck %s
-# CHECK: undefined symbol: _start
+# RUN: ld.lld %t -o %t2 2>&1
 # REQUIRES: x86
Index: test/ELF/reproduce-windows.s
===================================================================
--- test/ELF/reproduce-windows.s
+++ test/ELF/reproduce-windows.s
@@ -5,7 +5,7 @@
 # RUN: mkdir -p %t.dir/build
 # RUN: llvm-mc %s -o %t.dir/build/foo.o -filetype=obj -triple=x86_64-pc-linux
 # RUN: cd %t.dir
-# RUN: not ld.lld build/foo.o --reproduce repro
+# RUN: ld.lld build/foo.o --reproduce repro
 # RUN: cpio -t < repro.cpio | FileCheck %s
 
 # CHECK: repro/response.txt
Index: test/ELF/entry.s
===================================================================
--- test/ELF/entry.s
+++ test/ELF/entry.s
@@ -1,5 +1,6 @@
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t1
-# RUN: not ld.lld %t1 -o %t2
+# RUN: ld.lld %t1 -o %t2
+# RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=NOENTRY %s
 # RUN: ld.lld %t1 -o %t2 -e entry
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=SYM %s
 # RUN: ld.lld %t1 -shared -o %t2 -e entry
@@ -11,6 +12,7 @@
 # RUN: ld.lld %t1 -o %t2 -e 0777
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=OCT %s
 
+# NOENTRY: Entry: 0x0
 # SYM: Entry: 0x11000
 # DSO: Entry: 0x1000
 # DEC: Entry: 0x1000
Index: ELF/Writer.cpp
===================================================================
--- ELF/Writer.cpp
+++ ELF/Writer.cpp
@@ -740,8 +740,11 @@
 
     // We only report undefined symbols in regular objects. This means that we
     // will accept an undefined reference in bitcode if it can be optimized out.
-    if (S->IsUsedInRegularObj && Body->isUndefined() && !S->isWeak())
+    if (S->IsUsedInRegularObj && Body->isUndefined() && !S->isWeak()) {
+      if (S == Config->EntrySym && Config->ForcedEntry)
+        continue;
       reportUndefined<ELFT>(Body);
+    }
 
     if (!includeInSymtab<ELFT>(*Body))
       continue;
Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -551,8 +551,10 @@
 
   // Add entry symbol. Note that AMDGPU binaries have no entry points.
   if (Config->Entry.empty() && !Config->Shared && !Config->Relocatable &&
-      Config->EMachine != EM_AMDGPU)
+      Config->EMachine != EM_AMDGPU) {
     Config->Entry = (Config->EMachine == EM_MIPS) ? "__start" : "_start";
+    Config->ForcedEntry = true;
+  }
 
   // Default output filename is "a.out" by the Unix tradition.
   if (Config->OutputFile.empty())
Index: ELF/Config.h
===================================================================
--- ELF/Config.h
+++ ELF/Config.h
@@ -115,6 +115,7 @@
   bool ZNow;
   bool ZOrigin;
   bool ZRelro;
+  bool ForcedEntry = false;
   UnresolvedPolicy UnresolvedSymbols;
   BuildIdKind BuildId = BuildIdKind::None;
   ELFKind EKind = ELFNoneKind;


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