[PATCH] D38360: [ELF] Set Dot initially to --image-base value when using linker scripts

[James Henderson via Phabricator via llvm-commits]

jhenderson created this revision.
Herald added a subscriber: emaste.

This is an alternative to https://reviews.llvm.org/D38238. When parsing linker scripts, LLD starts with a '.' value of 0, regardless of the internal default image base for the target, and regardless of switches such as --image-base. It seems reasonable to use a different image base value when using linker scripts and --image-base is specified, since otherwise the switch has no effect. However, the default image base should not be used when processing linker scripts, because this will change the behaviour for existing linker script users, and potentially result in invalid output being produced, as a subsequent assignment to dot could move the location counter backwards (see the comments in https://reviews.llvm.org/D38238 for examples).


https://reviews.llvm.org/D38360

Files:
  ELF/Config.h
  ELF/Driver.cpp
  ELF/LinkerScript.cpp
  test/ELF/linkerscript/image-base.s


Index: test/ELF/linkerscript/image-base.s
===================================================================
--- test/ELF/linkerscript/image-base.s
+++ test/ELF/linkerscript/image-base.s
@@ -0,0 +1,18 @@
+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: echo "SECTIONS { mysym = .; }" > %t.script
+
+# RUN: ld.lld %t.o -o %t-default.elf -T %t.script
+# RUN: llvm-readobj --symbols %t-default.elf | FileCheck %s --check-prefix=DEFAULT
+# DEFAULT: Name: mysym
+# DEFAULT-NEXT: Value: 0x0
+
+# RUN: ld.lld %t.o -o %t-switch.elf -T %t.script --image-base=0x100000
+# RUN: llvm-readobj --symbols %t-switch.elf | FileCheck %s --check-prefix=SWITCH
+# SWITCH: Name: mysym
+# SWITCH-NEXT: Value: 0x100000
+
+.global _start
+_start:
+    nop
Index: ELF/LinkerScript.cpp
===================================================================
--- ELF/LinkerScript.cpp
+++ ELF/LinkerScript.cpp
@@ -365,7 +365,7 @@
   // script parser.
   CurAddressState = State.get();
   CurAddressState->OutSec = Aether;
-  Dot = 0;
+  Dot = Config->InitialDot;
 
   for (size_t I = 0; I < Opt.Commands.size(); ++I) {
     // Handle symbol assignments outside of any output section.
@@ -776,7 +776,7 @@
 
 void LinkerScript::assignAddresses() {
   // Assign addresses as instructed by linker script SECTIONS sub-commands.
-  Dot = 0;
+  Dot = Config->InitialDot;
   auto State = make_unique<AddressState>(Opt);
   // CurAddressState captures the local AddressState and makes it accessible
   // deliberately. This is needed as there are some cases where we cannot just
Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -925,6 +925,8 @@
   }
   if ((V % Config->MaxPageSize) != 0)
     warn("-image-base: address isn't multiple of page size: " + S);
+
+  Config->InitialDot = V;
   return V;
 }
 
Index: ELF/Config.h
===================================================================
--- ELF/Config.h
+++ ELF/Config.h
@@ -174,6 +174,7 @@
   uint16_t EMachine = llvm::ELF::EM_NONE;
   uint64_t ErrorLimit = 20;
   uint64_t ImageBase;
+  uint64_t InitialDot = 0;
   uint64_t MaxPageSize;
   uint64_t ZStackSize;
   unsigned LTOPartitions;


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