[ELF] Add support for PT_OPENBSD_NOBTCFI (#120005)

Author: brad0

lld/ELF/Config.h

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

lld/ELF/Driver.cpp

@@ -1897,6 +1897,9 @@ static void setConfigs(Ctx &ctx, opt::InputArgList &args) {
       ErrAlways(ctx) << "cannot open --why-extract= file " << ctx.arg.whyExtract
                      << ": " << e.message();
   }
+
+  if (ctx.arg.osabi == ELFOSABI_OPENBSD)
+    ctx.arg.zNoBtCfi = hasZOption(args, "nobtcfi");
 }
 
 static bool isFormatBinary(Ctx &ctx, StringRef s) {

lld/ELF/Writer.cpp

@@ -2405,6 +2405,11 @@ Writer<ELFT>::createPhdrs(Partition &part) {
     addHdr(PT_GNU_STACK, perm)->p_memsz = ctx.arg.zStackSize;
   }
 
+  // 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);
+
   // PT_OPENBSD_WXNEEDED is a OpenBSD-specific header to mark the executable
   // is expected to perform W^X violations, such as calling mprotect(2) or
   // mmap(2) with PROT_WRITE | PROT_EXEC, which is prohibited by default on

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

@@ -4,8 +4,11 @@
 # 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 wxneeded.o
-# RUN: ld.lld -z wxneeded wxneeded.o -o wxneeded
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-openbsd /dev/null -o empty.o
+# RUN: ld.lld -z nobtcfi empty.o -o nobtcfi
+# RUN: llvm-readelf -l nobtcfi | FileCheck %s --check-prefix=NOBTCFI
+
+# RUN: ld.lld -z wxneeded empty.o -o wxneeded
 # RUN: llvm-readelf -l wxneeded | FileCheck %s --check-prefix=WXNEEDED
 
 # RUN: ld.lld -T lds randomdata.o -o out
@@ -14,6 +17,9 @@
 # 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