[ELF] Allow SHF_LINK_ORDER sections to have sh_link=0

Part of https://bugs.llvm.org/show_bug.cgi?id=41734

The semantics of SHF_LINK_ORDER have been extended to represent metadata
sections associated with some other sections (usually text).

The associated text section may be discarded (e.g. LTO) and we want the
metadata section to have sh_link=0 (D72899, D76802).

Normally the metadata section is only referenced by the associated text
section. sh_link=0 means the associated text section is discarded, and
the metadata section will be garbage collected. If there is another
section (.gc_root) referencing the metadata section, the metadata
section will be retained. It's the .gc_root consumer's job to validate
the metadata sections.

  # This creates a SHF_LINK_ORDER .meta with sh_link=0
  .section .meta,"awo",@progbits,0
  1:
  .section .meta,"awo",@progbits,foo
  2:

  .section .gc_root,"a",@progbits
  .quad 1b
  .quad 2b

Reviewed By: pcc, jhenderson

Differential Revision: https://reviews.llvm.org/D72904

diff --git a/lld/ELF/InputFiles.cpp b/lld/ELF/InputFiles.cpp
index 6199f43..fbb9ac7 100644 (file)
--- a/lld/ELF/InputFiles.cpp
+++ b/lld/ELF/InputFiles.cpp
@@ -657,17 +657,19 @@ void ObjFile<ELFT>::initializeSections(bool ignoreComdats) {
     if (sec.sh_type == SHT_REL || sec.sh_type == SHT_RELA)
       this->sections[i] = createInputSection(sec);
 
-    if (!(sec.sh_flags & SHF_LINK_ORDER))
+    // A SHF_LINK_ORDER section with sh_link=0 is handled as if it did not have
+    // the flag.
+    if (!(sec.sh_flags & SHF_LINK_ORDER) || !sec.sh_link)
       continue;
 
-    // .ARM.exidx sections have a reverse dependency on the InputSection they
-    // have a SHF_LINK_ORDER dependency, this is identified by the sh_link.
     InputSectionBase *linkSec = nullptr;
     if (sec.sh_link < this->sections.size())
       linkSec = this->sections[sec.sh_link];
     if (!linkSec)
       fatal(toString(this) + ": invalid sh_link index: " + Twine(sec.sh_link));
 
+    // A SHF_LINK_ORDER section is discarded if its linked-to section is
+    // discarded.
     InputSection *isec = cast<InputSection>(this->sections[i]);
     linkSec->dependentSections.push_back(isec);
     if (!isa<InputSection>(linkSec))

diff --git a/lld/ELF/InputSection.cpp b/lld/ELF/InputSection.cpp
index 0a97b6a..b5b3522 100644 (file)
--- a/lld/ELF/InputSection.cpp
+++ b/lld/ELF/InputSection.cpp
@@ -276,8 +276,9 @@ void InputSectionBase::parseCompressedHeader() {
 }
 
 InputSection *InputSectionBase::getLinkOrderDep() const {
-  assert(link);
   assert(flags & SHF_LINK_ORDER);
+  if (!link)
+    return nullptr;
   return cast<InputSection>(file->getSections()[link]);
 }
 

diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index 68d64ba..1bd6fb6 100644 (file)
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -1612,6 +1612,9 @@ template <class ELFT> void Writer<ELFT>::sortSections() {
 static bool compareByFilePosition(InputSection *a, InputSection *b) {
   InputSection *la = a->getLinkOrderDep();
   InputSection *lb = b->getLinkOrderDep();
+  // SHF_LINK_ORDER sections with non-zero sh_link are ordered before others.
+  if (!la || !lb)
+    return la && !lb;
   OutputSection *aOut = la->getParent();
   OutputSection *bOut = lb->getParent();
 
@@ -1652,7 +1655,7 @@ template <class ELFT> void Writer<ELFT>::resolveShfLinkOrder() {
             sections.push_back(isec);
 
             InputSection *link = isec->getLinkOrderDep();
-            if (!link->getParent())
+            if (link && !link->getParent())
               error(toString(isec) + ": sh_link points to discarded section " +
                     toString(link));
           }

diff --git a/lld/test/ELF/invalid/linkorder-invalid-sec2.test b/lld/test/ELF/invalid/linkorder-invalid-sec2.test
deleted file mode 100644 (file)
index f78df3f..0000000
--- a/lld/test/ELF/invalid/linkorder-invalid-sec2.test
+++ /dev/null
@@ -1,16 +0,0 @@
-# REQUIRES: x86
-# RUN: yaml2obj %s -o %t.o
-# RUN: not ld.lld %t.o -o /dev/null 2>&1 | FileCheck %s
-# CHECK: invalid sh_link index: 0
-
---- !ELF
-FileHeader:
-  Class:           ELFCLASS64
-  Data:            ELFDATA2LSB
-  Type:            ET_REL
-  Machine:         EM_X86_64
-Sections:
-  - Name:          .linkorder
-    Type:          SHT_PROGBITS
-    Flags:         [ SHF_ALLOC, SHF_EXECINSTR, SHF_LINK_ORDER ]
-    Link:          0

diff --git a/lld/test/ELF/linkorder-mixed.s b/lld/test/ELF/linkorder-mixed.s
new file mode 100644 (file)
index 0000000..37f1f64
--- /dev/null
+++ b/lld/test/ELF/linkorder-mixed.s
@@ -0,0 +1,39 @@
+## Test that we allow SHF_LINK_ORDER sections with sh_link=0.
+## SHF_LINK_ORDER sections with sh_link!=0 are ordered before others.
+# RUN: llvm-mc -filetype=obj %s -o %t.o
+# RUN: ld.lld %t.o -o %t
+# RUN: llvm-readelf -S -x .linkorder %t | FileCheck %s
+
+# CHECK:      [Nr] Name       {{.*}} Size   ES Flg Lk Inf
+# CHECK-NEXT: [ 0]            {{.*}}
+# CHECK-NEXT: [ 1] .linkorder {{.*}} 000004 00  AL  3   0
+# CHECK-NEXT: [ 2] .ignore    {{.*}}
+# CHECK-NEXT: [ 3] .text      {{.*}}
+
+# CHECK:      Hex dump of section '.linkorder':
+# CHECK-NEXT:   [[#%x,ADDR:]] 01020003
+
+## TODO Allow non-contiguous SHF_LINK_ORDER sections in an output section.
+# RUN: llvm-mc --filetype=obj --defsym EXTRA=1 %s -o %t.o
+# RUN: not ld.lld %t.o -o /dev/null
+
+.section .text,"ax",@progbits,unique,0
+.Ltext0:
+.section .text,"ax",@progbits,unique,1
+.Ltext1:
+.section .linkorder,"ao",@progbits,0,unique,0
+  .byte 0
+.section .linkorder,"ao",@progbits,.Ltext0
+  .byte 1
+.section .linkorder,"ao",@progbits,.Ltext1
+  .byte 2
+
+.ifdef EXTRA
+.section .linkorder,"a",@progbits
+  .byte 4
+.else
+.section .ignore,"ao",@progbits,.Ltext1
+.endif
+
+.section .linkorder,"ao",@progbits,0,unique,3
+  .byte 3