[PATCH] D24388: [ELF] Merge sections with different access attributes

[Eugene Leviant via llvm-commits]

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

There was a long story here of merging output sections when linker script is used and reverting it back to original scheme with multiple output sections having the same name. Now when we returned to original scheme, I'm getting troubles linking boot loader, where all code and data should be packed to a single section and then extracted from result image using objcopy.

To deal with this problem, I suggest dropping section access attributes from output section key and merging them in addSection(). This solves my problem and doesn't break unit tests, except repsection-va.s, which is part of linker script processor test suite.

I'd like to add that such behavior conforms to GNU linkers. Both gold and ld merge section attributes in such case.

Repository:
  rL LLVM

https://reviews.llvm.org/D24388

Files:
  ELF/OutputSections.cpp
  ELF/OutputSections.h
  test/ELF/linkerscript/repsection-va.s

Index: test/ELF/linkerscript/repsection-va.s
===================================================================
--- test/ELF/linkerscript/repsection-va.s
+++ test/ELF/linkerscript/repsection-va.s
@@ -7,9 +7,8 @@
 # CHECK:      Sections:
 # CHECK-NEXT: Idx Name          Size      Address          Type
 # CHECK-NEXT:   0               00000000 0000000000000000
-# CHECK-NEXT:   1 .foo          00000004 0000000000000158 DATA
-# CHECK-NEXT:   2 .foo          00000004 000000000000015c DATA
-# CHECK-NEXT:   3 .text         00000001 0000000000000160 TEXT DATA
+# CHECK-NEXT:   1 .foo          00000008 0000000000000158 DATA
+# CHECK-NEXT:   2 .text         00000001 0000000000000160 TEXT DATA
 
 .global _start
 _start:
Index: ELF/OutputSections.h
===================================================================
--- ELF/OutputSections.h
+++ ELF/OutputSections.h
@@ -102,6 +102,7 @@
     if (Alignment > Header.sh_addralign)
       Header.sh_addralign = Alignment;
   }
+  void updateFlags(InputSectionBase<ELFT> *C);
 
   // If true, this section will be page aligned on disk.
   // Typically the first section of each PT_LOAD segment has this flag.
Index: ELF/OutputSections.cpp
===================================================================
--- ELF/OutputSections.cpp
+++ ELF/OutputSections.cpp
@@ -56,6 +56,12 @@
 }
 
 template <class ELFT>
+void OutputSectionBase<ELFT>::updateFlags(InputSectionBase<ELFT> *C) {
+  this->Header.sh_flags |=
+      C->getSectionHdr()->sh_flags & (SHF_EXECINSTR | SHF_WRITE);
+}
+
+template <class ELFT>
 GotPltSection<ELFT>::GotPltSection()
     : OutputSectionBase<ELFT>(".got.plt", SHT_PROGBITS, SHF_ALLOC | SHF_WRITE) {
   this->Header.sh_addralign = Target->GotPltEntrySize;
@@ -894,6 +900,7 @@
   Sections.push_back(S);
   S->OutSec = this;
   this->updateAlignment(S->Alignment);
+  this->updateFlags(C);
 }
 
 // If an input string is in the form of "foo.N" where N is a number,
@@ -1252,6 +1259,7 @@
   auto *Sec = cast<MergeInputSection<ELFT>>(C);
   Sec->OutSec = this;
   this->updateAlignment(Sec->Alignment);
+  this->updateFlags(C);
   this->Header.sh_entsize = Sec->getSectionHdr()->sh_entsize;
   Sections.push_back(Sec);
 
@@ -1868,7 +1876,8 @@
 OutputSectionFactory<ELFT>::createKey(InputSectionBase<ELFT> *C,
                                       StringRef OutsecName) {
   const Elf_Shdr *H = C->getSectionHdr();
-  uintX_t Flags = H->sh_flags & ~SHF_GROUP & ~SHF_COMPRESSED;
+  uintX_t Flags =
+      H->sh_flags & ~(SHF_GROUP | SHF_COMPRESSED | SHF_EXECINSTR | SHF_WRITE);
 
   // For SHF_MERGE we create different output sections for each alignment.
   // This makes each output section simple and keeps a single level mapping from


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