[LLD] Extend special OpenBSD support, but scope under ELFOSABI

- Add support for `.openbsd.mutable`

  (rebaser's note) adapted from:
  openbsd/src@bd249b5
  New auto-coalescing sections removed

  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

- Add support for `.openbsd.syscalls`

  (rebaser's note) adapted from:
  openbsd/src@42a61ac

  Collect .openbsd.syscalls sections into a new PT_OPENBSD_SYSCALLS
  segment. This will be used soon to pin system calls to designated call
  sites.

  ok deraadt@

- Scope OpenBSD special section handling under that ELFOSABI

  As a preexisting comment in `ELF/Writer.cpp` says:

  > section names shouldn't be significant in ELF in spirit.

  so scoping OSABI-specific magic name hacks to just the OSABI in
  question limits the degree to which we deviate from that "spirit" for
  all other OSABIs.

  OpenBSD in particular is very fast moving, having added a number of
  special sections, etc. in recent years. It is unclear how possible /
  reasonable it is for upstream to implement all these features in any
  event, but scoping like this at least mitigates the fallout for other
  OSABIs systems which wish to be more slow-moving.

Author: deraadt

lld/ELF/ScriptParser.cpp

@@ -1628,7 +1628,9 @@ 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_SYSCALLS", PT_OPENBSD_SYSCALLS)
                      .Case("PT_OPENBSD_WXNEEDED", PT_OPENBSD_WXNEEDED)
                      .Case("PT_OPENBSD_BOOTDATA", PT_OPENBSD_BOOTDATA)
                      .Default(-1);

lld/ELF/Writer.cpp

@@ -601,10 +601,16 @@ static bool isRelroSection(const OutputSection *sec) {
   // ELF in spirit. But in reality many linker features depend on
   // magic section names.
   StringRef s = sec->name;
-  return s == ".data.rel.ro" || s == ".bss.rel.ro" || s == ".ctors" ||
-         s == ".dtors" || s == ".jcr" || s == ".eh_frame" ||
-         s == ".fini_array" || s == ".init_array" ||
-         s == ".openbsd.randomdata" || s == ".preinit_array";
+
+  bool abiAgnostic = s == ".data.rel.ro" || s == ".bss.rel.ro" ||
+                     s == ".ctors" || s == ".dtors" || s == ".jcr" ||
+                     s == ".eh_frame" || s == ".fini_array" ||
+                     s == ".init_array" || s == ".preinit_array";
+
+  bool abiSpecific =
+      config->osabi == ELFOSABI_OPENBSD && s == ".openbsd.randomdata";
+
+  return abiAgnostic || abiSpecific;
 }
 
 // We compute a rank for each section. The rank indicates where the
@@ -2273,10 +2279,23 @@ SmallVector<PhdrEntry *, 0> Writer<ELFT>::createPhdrs(Partition &part) {
     addHdr(PT_GNU_EH_FRAME, part.ehFrameHdr->getParent()->getPhdrFlags())
         ->add(part.ehFrameHdr->getParent());
 
-  // 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))
-    addHdr(PT_OPENBSD_RANDOMIZE, cmd->getPhdrFlags())->add(cmd);
+  // Handle OpenBSD-specific section types for OpenBSD "OS ABI".
+  if (config->osabi == ELFOSABI_OPENBSD) {
+    // PT_OPENBSD_MUTABLE 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 makes the dynamic linker fill the segment
+    // with random data.
+    if (OutputSection *cmd = findSection(".openbsd.randomdata", partNo))
+      addHdr(PT_OPENBSD_RANDOMIZE, cmd->getPhdrFlags())->add(cmd);
+
+    // PT_OPENBSD_SYSCALLS makes the kernel and dynamic linker register
+    // system call sites.
+    if (OutputSection *cmd = findSection(".openbsd.syscalls", partNo))
+      addHdr(PT_OPENBSD_SYSCALLS, cmd->getPhdrFlags())->add(cmd);
+  }
 
   if (config->zGnustack != GnuStackKind::None) {
     // PT_GNU_STACK is a special section to tell the loader to make the

lld/test/ELF/openbsd-phdr.s

@@ -1,10 +1,10 @@
 # REQUIRES: x86
 # RUN: rm -rf %t && split-file %s %t && cd %t
-# RUN: llvm-mc -filetype=obj -triple=x86_64 randomdata.s -o randomdata.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-openbsd randomdata.s -o randomdata.o
 # RUN: ld.lld randomdata.o -o randomdata
 # RUN: llvm-readelf -S -l randomdata | FileCheck %s --check-prefix=RANDOMDATA
 
-# RUN: llvm-mc -filetype=obj -triple=x86_64 /dev/null -o wxneeded.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-openbsd /dev/null -o wxneeded.o
 # RUN: ld.lld -z wxneeded wxneeded.o -o wxneeded
 # RUN: llvm-readelf -l wxneeded | FileCheck %s --check-prefix=WXNEEDED
 
@@ -20,6 +20,8 @@
 # RANDOMDATA: Type              Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
 # RANDOMDATA: OPENBSD_RANDOMIZE 0x[[O]]  0x[[ADDR]]         0x[[ADDR]]         0x000008 0x000008 R   0x1
 # CHECK-NEXT: OPENBSD_BOOTDATA  0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 R   0
+# CHECK-NEXT: OPENBSD_MUTABLE   0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 R   0
+# CHECK-NEXT: OPENBSD_SYSCALLS  0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 R   0
 # CHECK-NEXT: OPENBSD_WXNEEDED  0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 R   0
 
 #--- randomdata.s
@@ -31,6 +33,8 @@ PHDRS {
   text PT_LOAD FILEHDR PHDRS;
   rand PT_OPENBSD_RANDOMIZE;
   boot PT_OPENBSD_BOOTDATA;
+  mutable PT_OPENBSD_MUTABLE;
+  syscalls PT_OPENBSD_SYSCALLS;
   wxneeded PT_OPENBSD_WXNEEDED;
 }
 SECTIONS {