[ELF] Add -z pack-relative-relocs

[MaskRay]

Summary

GNU ld 2.38 added -z pack-relative-relocs which is similar to
--pack-dyn-relocs=relr but synthesizes the GLIBC_ABI_DT_RELR version
dependency if a shared object named libc.so.* has a GLIBC_2.* version
dependency.

This is used to implement the (as some glibc folks call) version lockout
mechanism. Add this option, because glibc does not want to support
--pack-dyn-relocs=relr which does not add GLIBC_ABI_DT_RELR.
See https://maskray.me/blog/2021-10-31-relative-relocations-and-relr for
detail.

Close https://github.com/llvm/llvm-project/issues/53775

[peter.smith]

Code changes LGTM. Will be worth release noting and updating the ld.lld.1 man page as I think we've got an entry for pack-dyn-relocs.

[MaskRay]

release note / manpage

[emaste]

lld/docs/ReleaseNotes.rst

29 ↗

(On Diff #412193)

"supported to support" sounds odd. maybe "is now available to support DT_RELR" or something similar

[peter.smith]

I agree with emaste's comment on the release notes. I'm guessing that can be fixed up prior to committing. LGTM otherwise.

[MaskRay]

Thanks for the review and the release note suggestion.
Updated.

I'll wait on glibc side discussion https://sourceware.org/pipermail/libc-alpha/2022-March/

[MaskRay]

Check libc.so.*

lld/ELF/Config.h

struct Configuration {
   std::vector<llvm::StringRef> thinLTOModulesToCompile;
   std::vector<llvm::StringRef> undefined;
   std::vector<SymbolVersion> dynamicList;
   std::vector<uint8_t> buildIdVector;
   llvm::MapVector<std::pair<const InputSectionBase *, const InputSectionBase *>,
                   uint64_t>
       callGraphProfile;
   bool allowMultipleDefinition;
-  bool androidPackDynRelocs;
+  bool androidPackDynRelocs = false;
   bool armHasBlx = false;
   bool armHasMovtMovw = false;
   bool armJ1J2BranchEncoding = false;
   bool asNeeded = false;
   BsymbolicKind bsymbolic = BsymbolicKind::None;
   bool callGraphProfileSort;
   bool checkSections;
   bool checkDynamicRelocs;
struct Configuration {
   bool optimizeBBJumps;
   bool optRemarksWithHotness;
   bool picThunk;
   bool pie;
   bool printGcSections;
   bool printIcfSections;
   bool relax;
   bool relocatable;
-  bool relrPackDynRelocs;
+  bool relrGlibc = false;
+  bool relrPackDynRelocs = false;
   bool saveTemps;
   std::vector<std::pair<llvm::GlobPattern, uint32_t>> shuffleSections;
   bool singleRoRx;
   bool shared;
   bool symbolic;
   bool isStatic = false;
   bool sysvHash = false;
   bool target1Rel;

lld/ELF/Driver.cpp

constexpr const char *knownZFlags[] = {
     "nocombreloc",
     "nocopyreloc",
     "nodefaultlib",
     "nodelete",
     "nodlopen",
     "noexecstack",
     "nognustack",
     "nokeep-text-section-prefix",
+    "nopack-relative-relocs",
     "norelro",
     "noseparate-code",
     "nostart-stop-gc",
     "notext",
     "now",
     "origin",
     "pac-plt",
+    "pack-relative-relocs",
     "rel",
     "rela",
     "relro",
     "retpolineplt",
     "rodynamic",
     "separate-code",
     "separate-loadable-segments",
     "shstk",
static void readConfigs(opt::InputArgList &args) {
   // Page alignment can be disabled by the -n (--nmagic) and -N (--omagic).
   // As PT_GNU_RELRO relies on Paging, do not create it when we have disabled
   // it.
   if (config->nmagic || config->omagic)
     config->zRelro = false;
 
   std::tie(config->buildId, config->buildIdVector) = getBuildId(args);
 
+  if (getZFlag(args, "pack-relative-relocs", "nopack-relative-relocs", false)) {
+    config->relrGlibc = true;
+    config->relrPackDynRelocs = true;
+  } else {
     std::tie(config->androidPackDynRelocs, config->relrPackDynRelocs) =
         getPackDynRelocs(args);
+  }
 
   if (auto *arg = args.getLastArg(OPT_symbol_ordering_file)){
     if (args.hasArg(OPT_call_graph_ordering_file))
       error("--symbol-ordering-file and --call-graph-order-file "
             "may not be used together");
     if (Optional<MemoryBufferRef> buffer = readFile(arg->getValue())){
       config->symbolOrderingFile = getSymbolOrderingFile(*buffer);
       // Also need to disable CallGraphProfileSort to prevent

lld/ELF/SyntheticSections.cpp

 
 template <class ELFT> void VersionNeedSection<ELFT>::finalizeContents() {
   for (SharedFile *f : sharedFiles) {
     if (f->vernauxs.empty())
       continue;
     verneeds.emplace_back();
     Verneed &vn = verneeds.back();
     vn.nameStrTab = getPartition().dynStrTab->addString(f->soName);
+    bool isGlibc2 = false;
     for (unsigned i = 0; i != f->vernauxs.size(); ++i) {
       if (f->vernauxs[i] == 0)
         continue;
       auto *verdef =
           reinterpret_cast<const typename ELFT::Verdef *>(f->verdefs[i]);
-      vn.vernauxs.push_back(
-          {verdef->vd_hash, f->vernauxs[i],
-           getPartition().dynStrTab->addString(f->getStringTable().data() +
-                                               verdef->getAux()->vda_name)});
+      StringRef ver(f->getStringTable().data() + verdef->getAux()->vda_name);
+      if (config->relrGlibc && ver.startswith("GLIBC_2."))
+        isGlibc2 = true;
+      vn.vernauxs.push_back({verdef->vd_hash, f->vernauxs[i],
+                             getPartition().dynStrTab->addString(ver)});
+    }
+    if (isGlibc2) {
+      const char *ver = "GLIBC_ABI_DT_RELR";
+      vn.vernauxs.push_back({hashSysV(ver),
+                             ++SharedFile::vernauxNum + getVerDefNum(),
+                             getPartition().dynStrTab->addString(ver)});
     }
   }
 
   if (OutputSection *sec = getPartition().dynStrTab->getParent())
     getParent()->link = sec->sectionIndex;
   getParent()->info = verneeds.size();
 }
 

lld/test/ELF/pack-dyn-relocs-glibc.s

+# REQUIRES: x86
+## -z pack-relative-relocs is a variant of --pack-dyn-relocs=relr: add
+## GLIBC_ABI_DT_RELR verneed if there is a verneed named "GLIBC_2.*".
+
+# RUN: rm -rf %t && split-file %s %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %t/a.s -o %t/a.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %t/libc.s -o %t/libc.o
+# RUN: ld.lld -shared --soname=libc.so.6 --version-script=%t/glibc.ver %t/libc.o -o %t/libc.so.6
+
+# RUN: ld.lld -pie %t/a.o %t/libc.so.6 -z pack-relative-relocs -o %t/glibc 2>&1 | count 0
+# RUN: llvm-readelf -r -V %t/glibc | FileCheck %s --check-prefix=GLIBC
+## Arbitrarily let -z pack-relative-relocs win.
+# RUN: ld.lld -pie %t/a.o %t/libc.so.6 -z pack-relative-relocs --pack-dyn-relocs=relr -o %t/glibc2
+# RUN: cmp %t/glibc %t/glibc2
+
+# GLIBC:      Relocation section '.relr.dyn' at offset {{.*}} contains 1 entries:
+# GLIBC:      Version needs section '.gnu.version_r' contains 1 entries:
+# GLIBC-NEXT:  Addr: {{.*}}
+# GLIBC-NEXT:   0x0000: Version: 1  File: libc.so.6  Cnt: 2
+# GLIBC-NEXT:   0x0010:   Name: GLIBC_2.33  Flags: none  Version: 2
+# GLIBC-NEXT:   0x0020:   Name: GLIBC_ABI_DT_RELR  Flags: none  Version: 3
+# GLIBC-EMPTY:
+
+# RUN: ld.lld -pie %t/a.o %t/libc.so.6 -z pack-relative-relocs -z nopack-relative-relocs -o %t/notrelr 2>&1 | count 0
+# RUN: llvm-readelf -r -V %t/notrelr | FileCheck %s --check-prefix=REGULAR
+
+# REGULAR-NOT: Relocation section '.relr.dyn'
+# REGULAR-NOT: Name: GLIBC_ABI_DT_RELR
+
+## There is no GLIBC_2.* verneed. Don't add GLIBC_ABI_DT_RELR verneed.
+# RUN: ld.lld -shared --soname=libc.so.6 --version-script=%t/other.ver %t/libc.o -o %t/libc.so.6
+# RUN: ld.lld -pie %t/a.o %t/libc.so.6 -z pack-relative-relocs -o %t/other
+# RUN: llvm-readelf -r -V %t/other | FileCheck %s --check-prefix=OTHER
+
+# OTHER:      Relocation section '.relr.dyn' at offset {{.*}} contains 1 entries:
+# OTHER:      Version needs section '.gnu.version_r' contains 1 entries:
+# OTHER-NEXT:  Addr: {{.*}}
+# OTHER-NEXT:   0x0000: Version: 1  File: libc.so.6  Cnt: 1
+# OTHER-NEXT:   0x0010:   Name: GLIBC_3  Flags: none  Version: 2
+# OTHER-EMPTY:
+
+#--- a.s
+.globl _start
+_start:
+  call stat
+
+.data
+.balign 8
+.dc.a .data
+
+#--- libc.s
+.weak stat
+stat:
+
+#--- glibc.ver
+GLIBC_2.33 {
+  stat;
+};
+
+#--- other.ver
+GLIBC_3 {
+  stat;
+};