[ELF] Add -z nosectionheader

GNU ld since 2.41 supports this option, which is mildly useful. It omits
the section header table and non-ALLOC sections (including
.symtab/.strtab (--strip-all)).

This option is simple to implement and might be used by LLDB to test
program headers parsing without the section header table (#100900).

-z sectionheader, which is the default, is also added.

Pull Request: #101286

Author: MaskRay

lld/ELF/Config.h

@@ -333,6 +333,7 @@ struct Config {
   bool zPacPlt;
   bool zRelro;
   bool zRodynamic;
+  bool zSectionHeader;
   bool zShstk;
   bool zStartStopGC;
   uint8_t zStartStopVisibility;

lld/ELF/Driver.cpp

@@ -447,6 +447,8 @@ static void checkOptions() {
       error("-r and --export-dynamic may not be used together");
     if (config->debugNames)
       error("-r and --debug-names may not be used together");
+    if (!config->zSectionHeader)
+      error("-r and -z nosectionheader may not be used together");
   }
 
   if (config->executeOnly) {
@@ -836,6 +838,8 @@ static ICFLevel getICF(opt::InputArgList &args) {
 static StripPolicy getStrip(opt::InputArgList &args) {
   if (args.hasArg(OPT_relocatable))
     return StripPolicy::None;
+  if (!config->zSectionHeader)
+    return StripPolicy::All;
 
   auto *arg = args.getLastArg(OPT_strip_all, OPT_strip_debug);
   if (!arg)
@@ -1411,7 +1415,9 @@ static void readConfigs(opt::InputArgList &args) {
   config->soName = args.getLastArgValue(OPT_soname);
   config->sortSection = getSortSection(args);
   config->splitStackAdjustSize = args::getInteger(args, OPT_split_stack_adjust_size, 16384);
-  config->strip = getStrip(args);
+  config->zSectionHeader =
+      getZFlag(args, "sectionheader", "nosectionheader", true);
+  config->strip = getStrip(args); // needs zSectionHeader
   config->sysroot = args.getLastArgValue(OPT_sysroot);
   config->target1Rel = args.hasFlag(OPT_target1_rel, OPT_target1_abs, false);
   config->target2 = getTarget2(args);

lld/ELF/SyntheticSections.cpp

@@ -4665,7 +4665,8 @@ template <class ELFT> void elf::createSyntheticSections() {
 
   auto add = [](SyntheticSection &sec) { ctx.inputSections.push_back(&sec); };
 
-  in.shStrTab = std::make_unique<StringTableSection>(".shstrtab", false);
+  if (config->zSectionHeader)
+    in.shStrTab = std::make_unique<StringTableSection>(".shstrtab", false);
 
   Out::programHeaders = make<OutputSection>("", 0, SHF_ALLOC);
   Out::programHeaders->addralign = config->wordsize;
@@ -4917,7 +4918,8 @@ template <class ELFT> void elf::createSyntheticSections() {
     add(*in.symTab);
   if (in.symTabShndx)
     add(*in.symTabShndx);
-  add(*in.shStrTab);
+  if (in.shStrTab)
+    add(*in.shStrTab);
   if (in.strTab)
     add(*in.strTab);
 }

lld/ELF/Writer.cpp

@@ -1875,13 +1875,16 @@ template <class ELFT> void Writer<ELFT>::finalizeSections() {
   sortSections();
 
   // Create a list of OutputSections, assign sectionIndex, and populate
-  // in.shStrTab.
+  // in.shStrTab. If -z nosectionheader is specified, drop non-ALLOC sections.
   for (SectionCommand *cmd : script->sectionCommands)
     if (auto *osd = dyn_cast<OutputDesc>(cmd)) {
       OutputSection *osec = &osd->osec;
+      if (!in.shStrTab && !(osec->flags & SHF_ALLOC))
+        continue;
       outputSections.push_back(osec);
       osec->sectionIndex = outputSections.size();
-      osec->shName = in.shStrTab->addString(osec->name);
+      if (in.shStrTab)
+        osec->shName = in.shStrTab->addString(osec->name);
     }
 
   // Prefer command line supplied address over other constraints.
@@ -2703,6 +2706,10 @@ template <class ELFT> void Writer<ELFT>::writeHeader() {
   auto *eHdr = reinterpret_cast<Elf_Ehdr *>(Out::bufferStart);
   eHdr->e_type = getELFType();
   eHdr->e_entry = getEntryAddr();
+
+  // If -z nosectionheader is specified, omit the section header table.
+  if (!in.shStrTab)
+    return;
   eHdr->e_shoff = sectionHeaderOff;
 
   // Write the section header table.

lld/docs/ReleaseNotes.rst

@@ -26,6 +26,10 @@ Non-comprehensive list of changes in this release
 ELF Improvements
 ----------------
 
+* ``-z nosectionheader`` has been implemented to omit the section header table.
+  The operation is similar to ``llvm-objcopy --strip-sections``.
+  (`#101286 <https://github.com/llvm/llvm-project/pull/101286>`_)
+
 Breaking changes
 ----------------
 

lld/docs/ld.lld.1

@@ -857,6 +857,9 @@ The object will omit the
 .Dv PT_GNU_RELRO
 segment.
 .Pp
+.It Cm nosectionheader
+Don't generate the section header table.
+.Pp
 .It Cm notext
 Allow relocations against read-only segments.
 Sets the

lld/test/CMakeLists.txt

@@ -64,6 +64,7 @@ if (NOT LLD_BUILT_STANDALONE)
     llvm-profdata
     llvm-readelf
     llvm-readobj
+    llvm-strings
     llvm-strip
     llvm-symbolizer
     not

lld/test/ELF/zsectionheader.s

@@ -0,0 +1,36 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+# RUN: ld.lld -shared -z nosectionheader -z sectionheader %t.o -o %t.so 2>&1 | count 0
+# RUN: llvm-readelf -hS %t.so | FileCheck %s --check-prefixes=CHECK,SHDR
+
+# RUN: ld.lld -shared -z nosectionheader %t.o -o %t0.so
+# RUN: llvm-readelf -h --dyn-syms %t0.so | FileCheck %s --check-prefixes=CHECK,NOSHDR
+# RUN: llvm-strings %t0.so | FileCheck %s --check-prefixes=NOSHDR-STR
+
+# CHECK:       Size of this header:               64 (bytes)
+# CHECK-NEXT:  Size of program headers:           56 (bytes)
+# CHECK-NEXT:  Number of program headers:         6
+# CHECK-NEXT:  Size of section headers:           64 (bytes)
+# SHDR-NEXT:   Number of section headers:         13
+# SHDR-NEXT:   Section header string table index: 11
+# NOSHDR-NEXT: Number of section headers:         0
+# NOSHDR-NEXT: Section header string table index: 0
+
+# SHDR:        Section Headers:
+# NOSHDR:      Symbol table for image contains 2 entries:
+# NOSHDR:        _start
+
+## _start occurs as a dynamic string table entry. There is no static string table
+## entry. `nonalloc` is not in the output.
+# NOSHDR-STR:      _start
+# NOSHDR-STR-NOT:  _start
+
+# RUN: not ld.lld -r -z nosectionheader %t.o -o /dev/null 2>&1 | FileCheck %s --check-prefix=ERR
+
+# ERR: error: -r and -z nosectionheader may not be used together
+
+.globl _start
+_start:
+
+.section nonalloc,""
+.asciz "_start"