[ELF] Add support for PT_OPENBSD_NOBTCFI

[brad0]

No description provided.

[llvmbot]

@llvm/pr-subscribers-lld-elf

@llvm/pr-subscribers-lld

Author: Brad Smith (brad0)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/120005.diff

5 Files Affected:

• (modified) lld/ELF/Config.h (+1)
• (modified) lld/ELF/Driver.cpp (+1)
• (modified) lld/ELF/Writer.cpp (+5)
• (modified) lld/docs/ld.lld.1 (+5)
• (modified) lld/test/ELF/openbsd-phdr.s (+7)

diff --git a/lld/ELF/Config.h b/lld/ELF/Config.h
index 5b6b332cd597df..b2859486d58e93 100644
--- a/lld/ELF/Config.h
+++ b/lld/ELF/Config.h
@@ -363,6 +363,7 @@ struct Config {
   bool zInterpose;
   bool zKeepTextSectionPrefix;
   bool zLrodataAfterBss;
+  bool zNoBtCfi;
   bool zNodefaultlib;
   bool zNodelete;
   bool zNodlopen;
diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
index 9240f29d98d614..f573a8d3e19f3b 100644
--- a/lld/ELF/Driver.cpp
+++ b/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");
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index d5581ca3e1c921..0ae871f231bec9 100644
--- a/lld/ELF/Writer.cpp
+++ b/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);
 
diff --git a/lld/docs/ld.lld.1 b/lld/docs/ld.lld.1
index 2fa6f64b2d2032..b28c6082f68b09 100644
--- a/lld/docs/ld.lld.1
+++ b/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
diff --git a/lld/test/ELF/openbsd-phdr.s b/lld/test/ELF/openbsd-phdr.s
index 275f944511701e..097a499318a5bc 100644
--- a/lld/test/ELF/openbsd-phdr.s
+++ b/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
 

[MaskRay]

I think this is fine... but it is not a good idea to keep introducing new PT_OPENBSD program headers...

[MaskRay]

ld.lld -z nobtcfi empty.o has a false positive warning. Perhaps we need to move this variable back to readConfigs.

In driver.test, I use --implicit-check-not=warning: to catch such issues.