[ELF] Implement --rosegment

By default linker would not create a separate segment to hold read only data.

This option overrides that behavior by creating the a separate read only segment
for read only data.

llvm-svn: 217358

8 files changed, 49 insertions, 3 deletions

diff --git a/lld/include/lld/ReaderWriter/ELFLinkingContext.h b/lld/include/lld/ReaderWriter/ELFLinkingContext.h
index f551a3cbfb8..32f7a7b7ab4 100644
--- a/lld/include/lld/ReaderWriter/ELFLinkingContext.h
+++ b/lld/include/lld/ReaderWriter/ELFLinkingContext.h
@@ -251,6 +251,13 @@ public:
     return true;
   }
 
+  // By default, the linker would merge sections that are read only with
+  // segments that have read and execute permissions. When the user specifies a
+  // flag --rosegment, a separate segment needs to be created.
+  bool mergeRODataToTextSegment() const { return _mergeRODataToTextSegment; }
+
+  void setCreateSeparateROSegment() { _mergeRODataToTextSegment = false; }
+
 private:
   ELFLinkingContext() LLVM_DELETED_FUNCTION;
 
@@ -273,6 +280,7 @@ protected:
   bool _useShlibUndefines;
   bool _dynamicLinkerArg;
   bool _noAllowDynamicLibraries;
+  bool _mergeRODataToTextSegment;
   OutputMagic _outputMagic;
   StringRefVector _inputSearchPaths;
   std::unique_ptr<Writer> _writer;
diff --git a/lld/lib/Driver/GnuLdDriver.cpp b/lld/lib/Driver/GnuLdDriver.cpp
index 623a74f8af2..c6dda412921 100644
--- a/lld/lib/Driver/GnuLdDriver.cpp
+++ b/lld/lib/Driver/GnuLdDriver.cpp
@@ -537,6 +537,10 @@ bool GnuLdDriver::parse(int argc, const char *argv[],
       ctx->setSharedObjectName(inputArg->getValue());
       break;
 
+    case OPT_rosegment:
+      ctx->setCreateSeparateROSegment();
+      break;
+
     default:
       break;
     } // end switch on option ID
diff --git a/lld/lib/Driver/GnuLdOptions.td b/lld/lib/Driver/GnuLdOptions.td
index d062181bd3c..08adcc4b6ab 100644
--- a/lld/lib/Driver/GnuLdOptions.td
+++ b/lld/lib/Driver/GnuLdOptions.td
@@ -201,6 +201,9 @@ def defsym : Joined<["--"], "defsym=">,
 //===----------------------------------------------------------------------===//
 def grp_customopts : OptionGroup<"opts">,
      HelpText<"CUSTOM OPTIONS">;
+def rosegment: Flag<["--"], "rosegment">,
+     HelpText<"Put read-only non-executable sections in their own segment">,
+     Group<grp_customopts>;
 def z : Separate<["-"], "z">,
      HelpText<"Linker Option extensions">,
      Group<grp_customopts>;
diff --git a/lld/lib/ReaderWriter/ELF/DefaultLayout.h b/lld/lib/ReaderWriter/ELF/DefaultLayout.h
index beef4a45ea5..4030526b10a 100644
--- a/lld/lib/ReaderWriter/ELF/DefaultLayout.h
+++ b/lld/lib/ReaderWriter/ELF/DefaultLayout.h
@@ -634,7 +634,8 @@ template <class ELFT> void DefaultLayout<ELFT>::assignSectionsToSegments() {
         StringRef segmentName = section->segmentKindToStr();
 
         int64_t lookupSectionFlag = msi->flags();
-        if (!(lookupSectionFlag & llvm::ELF::SHF_WRITE))
+        if ((!(lookupSectionFlag & llvm::ELF::SHF_WRITE)) &&
+            (_context.mergeRODataToTextSegment()))
           lookupSectionFlag &= ~llvm::ELF::SHF_EXECINSTR;
 
         // Merge string sections into Data segment itself
diff --git a/lld/lib/ReaderWriter/ELF/ELFLinkingContext.cpp b/lld/lib/ReaderWriter/ELF/ELFLinkingContext.cpp
index dbcee258a25..60237928920 100644
--- a/lld/lib/ReaderWriter/ELF/ELFLinkingContext.cpp
+++ b/lld/lib/ReaderWriter/ELF/ELFLinkingContext.cpp
@@ -59,8 +59,8 @@ ELFLinkingContext::ELFLinkingContext(
       _isStaticExecutable(false), _noInhibitExec(false),
       _mergeCommonStrings(false), _runLayoutPass(true),
       _useShlibUndefines(true), _dynamicLinkerArg(false),
-      _noAllowDynamicLibraries(false), _outputMagic(OutputMagic::DEFAULT),
-      _sysrootPath("") {}
+      _noAllowDynamicLibraries(false), _mergeRODataToTextSegment(true),
+      _outputMagic(OutputMagic::DEFAULT), _sysrootPath("") {}
 
 bool ELFLinkingContext::is64Bits() const { return getTriple().isArch64Bit(); }
 
diff --git a/lld/test/elf/Inputs/rodata.c b/lld/test/elf/Inputs/rodata.c
new file mode 100644
index 00000000000..b43c9c94589
--- /dev/null
+++ b/lld/test/elf/Inputs/rodata.c
@@ -0,0 +1,4 @@
+const unsigned char *str = "llvm";
+int foo() {
+  return str[0];
+}
diff --git a/lld/test/elf/Inputs/rodata.o b/lld/test/elf/Inputs/rodata.o
new file mode 100644
index 00000000000..f13ddc9bba9
--- /dev/null
+++ b/lld/test/elf/Inputs/rodata.o
diff --git a/lld/test/elf/rosegment.test b/lld/test/elf/rosegment.test
new file mode 100644
index 00000000000..eab2a904ee7
--- /dev/null
+++ b/lld/test/elf/rosegment.test
@@ -0,0 +1,26 @@
+# Tests that the option --rosegment produces an output file with a separate
+# segment created for read only data.
+RUN: lld -flavor gnu -target x86_64 %p/Inputs/rodata.o -o %t1.elf \
+RUN: --noinhibit-exec
+RUN: lld -flavor gnu -target x86_64 %p/Inputs/rodata.o --rosegment -o %t2.elf \
+RUN: --noinhibit-exec
+RUN: llvm-readobj -program-headers %t1.elf | FileCheck %s -check-prefix=NORO-SEGMENT
+RUN: llvm-readobj -program-headers %t2.elf | FileCheck %s -check-prefix=RO-SEGMENT
+
+#NORO-SEGMENT:    Type: PT_PHDR
+#NORO-SEGMENT:    Type: PT_INTERP
+#NORO-SEGMENT:    Type: PT_LOAD
+#NORO-SEGMENT:    Type: PT_LOAD
+#NORO-SEGMENT:    Type: PT_GNU_EH_FRAME
+#NORO-SEGMENT:    Type: PT_DYNAMIC
+
+#RO-SEGMENT:    Type: PT_PHDR
+#RO-SEGMENT:    Type: PT_INTERP
+#RO-SEGMENT:    Type: PT_LOAD
+#RO-SEGMENT:    Type: PT_LOAD
+#RO-SEGMENT:   Flags [
+#RO-SEGMENT:      PF_R (0x4)
+#RO-SEGMENT:    ]
+#RO-SEGMENT:    Type: PT_LOAD
+#RO-SEGMENT:    Type: PT_GNU_EH_FRAME
+#RO-SEGMENT:    Type: PT_DYNAMIC