[ELF] Add support for PT_OPENBSD_NOBTCFI

Author: brad0

lld/ELF/Config.h

@@ -363,6 +363,7 @@ struct Config {
   bool zInterpose;
   bool zKeepTextSectionPrefix;
   bool zLrodataAfterBss;
+  bool zNoBtCfi;
   bool zNodefaultlib;
   bool zNodelete;
   bool zNodlopen;

lld/ELF/Driver.cpp

@@ -1487,6 +1487,7 @@ static void readConfigs(Ctx &ctx, opt::InputArgList &args) {
       args, "keep-text-section-prefix", "nokeep-text-section-prefix", false);
   ctx.arg.zLrodataAfterBss =
       getZFlag(args, "lrodata-after-bss", "nolrodata-after-bss", false);
+  ctx.arg.zNoBtCfi = hasZOption(args, "nobtcfi");
   ctx.arg.zNodefaultlib = hasZOption(args, "nodefaultlib");
   ctx.arg.zNodelete = hasZOption(args, "nodelete");
   ctx.arg.zNodlopen = hasZOption(args, "nodlopen");

lld/ELF/Writer.cpp

@@ -2412,6 +2412,11 @@ Writer<ELFT>::createPhdrs(Partition &part) {
   if (ctx.arg.zWxneeded)
     addHdr(PT_OPENBSD_WXNEEDED, PF_X);
 
+  // PT_OPENBSD_NOBTCFI is an OpenBSD-specific header to mark that the
+  // executable is expected to violate branch-target CFI checks.
+  if (ctx.arg.zNoBtCfi)
+    addHdr(PT_OPENBSD_NOBTCFI, PF_X);
+
   if (OutputSection *cmd = findSection(ctx, ".note.gnu.property", partNo))
     addHdr(PT_GNU_PROPERTY, PF_R)->add(cmd);
 

lld/docs/ld.lld.1

@@ -952,6 +952,11 @@ Let __start_/__stop_ references retain the associated C identifier name sections
 Do not allow relocations against read-only segments.
 This is the default.
 .Pp
+.It Cm nobtcfi
+Create a
+.Dv PT_OPENBSD_NOBTCFI
+segment.
+.Pp
 .It Cm wxneeded
 Create a
 .Dv PT_OPENBSD_WXNEEDED

lld/test/ELF/openbsd-phdr.s

@@ -4,6 +4,10 @@
 # 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-unknown-openbsd /dev/null -o nobtcfi.o
+# RUN: ld.lld -z nobtcfi nobtcfi.o -o nobtcfi
+# RUN: llvm-readelf -l nobtcfi | FileCheck %s --check-prefix=NOBTCFI
+
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-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
@@ -14,6 +18,9 @@
 # RANDOMDATA: Name                Type     Address            Off             Size   ES Flg Lk Inf Al
 # RANDOMDATA: .openbsd.randomdata PROGBITS [[ADDR:[0-9a-f]+]] [[O:[0-9a-f]+]] 000008 00   A  0   0  1
 
+# NOBTCFI:    Type              Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
+# NOBTCFI:    OPENBSD_NOBTCFI   0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 E   0
+
 # WXNEEDED:   Type              Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
 # WXNEEDED:   OPENBSD_WXNEEDED  0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 E   0