[lld][ELF] TBSS section has to be seperately considered so that it doesnot occupy memory space

[shankarke]

Summary

This adds functionality to the ELF writer to not add memory size to the segment and checks to make sure that DATA starts at the same address where TBSS starts with a test.

[ruiu]

test/elf/tlsAddr.test

7

you may want to check the size of .tdata.

[shankarke]

test/elf/tlsAddr.test

7

This is a test known to produce this address for DATA. Should be sufficient. If the address of TDATA changes, data also changes.

[Bigcheese]

lgtm

[Eugene.Zelenko]

Committed in rLrL183815.

lib/ReaderWriter/ELF/DefaultLayout.h

for (auto ai : msi->sections()) {
 
         msi->setHasSegment();
         section->setSegmentType(segmentType);
         StringRef segmentName = section->segmentKindToStr();
 
         int64_t lookupSectionFlag = msi->flags();
         if (!(lookupSectionFlag & llvm::ELF::SHF_WRITE))
           lookupSectionFlag &= ~llvm::ELF::SHF_EXECINSTR;
+
+        // Merge string sections into Data segment itself
         lookupSectionFlag &= ~(llvm::ELF::SHF_STRINGS | llvm::ELF::SHF_MERGE);
 
+        // Merge the TLS section into the DATA segment itself
+        lookupSectionFlag &= ~(llvm::ELF::SHF_TLS);
+
         Segment<ELFT> *segment;
         // We need a seperate segment for sections that dont have
         // the segment type to be PT_LOAD
         if (segmentType != llvm::ELF::PT_LOAD) {
           const std::pair<AdditionalSegmentKey, Segment<ELFT> *>
           additionalSegment(segmentType, nullptr);
           std::pair<typename AdditionalSegmentMapT::iterator, bool>
           additionalSegmentInsert(

lib/ReaderWriter/ELF/SegmentChunks.h

 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ELF.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/FileOutputBuffer.h"
 
 namespace lld {
 namespace elf {
+
+template <typename ELFT> class DefaultLayout;
+
 /// \brief A segment can be divided into segment slices
 ///        depending on how the segments can be split
 template<class ELFT>
 class SegmentSlice {
 public:
   typedef typename std::vector<Chunk<ELFT> *>::iterator SectionIter;
 
   SegmentSlice() { }
for (auto section : slice->sections()) {
       if (auto s = dyn_cast<Section<ELFT> >(section)) {
         if (isTLSSegment)
           s->assignVirtualAddress(tlsStartAddr);
         else
           s->assignVirtualAddress(addr);
       }
       if (isTLSSegment)
         tlsStartAddr += section->memSize();
+      section->setMemSize(addr + section->memSize() - section->virtualAddr());
+      // TBSS section is special that it doesnot contribute to memory of any
+      // segment, If we see a tbss section, dont add memory size to addr
+      // The fileOffset is automatically taken care of since TBSS section does
+      // not endup using file size
+      if (section->order() != DefaultLayout<ELFT>::ORDER_TBSS)
         addr += section->memSize();
-      section->setMemSize(addr - section->virtualAddr());
     }
     slice->setMemSize(addr - slice->virtualAddr());
   }
 }
 
 // Write the Segment
 template <class ELFT>
 void Segment<ELFT>::write(ELFWriter *writer, llvm::FileOutputBuffer &buffer) {

test/elf/Inputs/tlsaddr.c

+__thread int tls0 = 0;
+__thread int tls1 = 0;
+__thread int tls2 = 1;
+__thread int tls3 = 2;
+
+int main() {
+  return tls0 + tls1 + tls2;
+}

test/elf/dynamic-segorder.test

 RUN: lld -flavor gnu -target x86_64-linux %p/Inputs/tls.x86-64 \
 RUN:   %p/Inputs/shared.so-x86-64 -o %t -e main --allow-shlib-undefined
 RUN: llvm-objdump -p %t | FileCheck %s
 
 CHECK: PHDR
 CHECK: flags r-x
 CHECK: INTERP
 CHECK: flags r--
 CHECK: LOAD
 CHECK: flags r-x
 CHECK: LOAD
 CHECK: flags rw-
-CHECK: LOAD
-CHECK: flags rw-
 CHECK: DYNAMIC
 CHECK: flags r--
 CHECK: TLS
 CHECK: flags rw-

test/elf/tlsAddr.test

+# This tests verifies that TLS variables have correct offsets
+# and that TBSS doesnot occupy memory 
+RUN: lld -flavor gnu -target x86_64-linux %p/Inputs/tlsAddr.x86-64 -static  \
+RUN: -o %t --noinhibit-exec
+RUN: llvm-objdump -section-headers %t | FileCheck -check-prefix=CHECKADDR %s
+
+CHECKADDR:   8 .data         00000000 0000000000401008 DATA