[LLD] Add support for .openbsd.mutable

(rebaser's note) adapted from:
openbsd/src@bd249b5

In the linkers, collect objects in section "openbsd.mutable" and place them into a page-aligned region in the bss, with the right markers for kernel/ld.so to identify the region and skip making it immutable. While here, fix readelf/objdump versions to show all of this. ok miod kettenis

Author: deraadt

lld/ELF/LinkerScript.cpp

@@ -107,7 +107,8 @@ static StringRef getOutputSectionName(const InputSectionBase *s) {
   for (StringRef v :
        {".data.rel.ro", ".data", ".rodata", ".bss.rel.ro", ".bss", ".ldata",
         ".lrodata", ".lbss", ".gcc_except_table", ".init_array", ".fini_array",
-        ".tbss", ".tdata", ".ARM.exidx", ".ARM.extab", ".ctors", ".dtors"})
+        ".tbss", ".tdata", ".ARM.exidx", ".ARM.extab", ".ctors", ".dtors",
+        ".openbsd.randomdata", ".openbsd.mutable"})
     if (isSectionPrefix(v, s->name))
       return v;
 

lld/ELF/ScriptParser.cpp

@@ -1628,6 +1628,7 @@ unsigned ScriptParser::readPhdrType() {
                      .Case("PT_GNU_EH_FRAME", PT_GNU_EH_FRAME)
                      .Case("PT_GNU_STACK", PT_GNU_STACK)
                      .Case("PT_GNU_RELRO", PT_GNU_RELRO)
+                     .Case("PT_OPENBSD_MUTABLE", PT_OPENBSD_MUTABLE)
                      .Case("PT_OPENBSD_RANDOMIZE", PT_OPENBSD_RANDOMIZE)
                      .Case("PT_OPENBSD_WXNEEDED", PT_OPENBSD_WXNEEDED)
                      .Case("PT_OPENBSD_BOOTDATA", PT_OPENBSD_BOOTDATA)

lld/ELF/Writer.cpp

@@ -2245,6 +2245,12 @@ SmallVector<PhdrEntry *, 0> Writer<ELFT>::createPhdrs(Partition &part) {
     addHdr(PT_GNU_EH_FRAME, part.ehFrameHdr->getParent()->getPhdrFlags())
         ->add(part.ehFrameHdr->getParent());
 
+  // PT_OPENBSD_MUTABLE is an OpenBSD-specific feature. That makes
+  // the dynamic linker fill the segment with zero data, like bss, but
+  // it can be treated differently.
+  if (OutputSection *cmd = findSection(".openbsd.mutable", partNo))
+    addHdr(PT_OPENBSD_MUTABLE, cmd->getPhdrFlags())->add(cmd);
+
   // PT_OPENBSD_RANDOMIZE is an OpenBSD-specific feature. That makes
   // the dynamic linker fill the segment with random data.
   if (OutputSection *cmd = findSection(".openbsd.randomdata", partNo))