Set DF_TEXTREL instead of creating an empty DT_TEXTREL.

[ruiu]

Summary

Generic ABI as of 2000 [1] mentioned that "Its [DT_TEXTREL's] use
has been superseded by the DF_TEXTREL flag". We currently create only
DT_TEXTREL and don't set the DF_TEXTREL flag.

This patch is to let the linker to set the flag instead of creating
a DT_TEXTREL marker segment.

With this patch, lld no longer emits DT_TEXTREL. I believe that's not
an issue because it is a very old option and text relocation is rare.
If you want to keep it, we can set both DT_TEXTREL and DF_TEXTREL though.
But I'm not comfortable with supporting only the old way.

[1] http://www.sco.com/developers/gabi/2000-07-17/ch5.dynamic.html

[arichardson]

The FreeBSD rtld supports both so this LGTM. Emitting both is probably more compatible but I'm not sure it's required. What does ld.bfd do?

[ruiu]

GNU linkers seem to emit both since perhaps almost 20 years ago. I'm actually fine with emitting both, but since we've had such an extremely long transition period, I'd like to emit just DF_TEXTREL if all OSes are OK with that.

[ruiu]

• restore code to create DT_TEXTREL

[kettenis]

As @rafael noticed in a follow-up by mail, OpenBSD doesn't implement support for DF_TEXTREL. I don't see a reason why we couldn't so I've implemented support and expect it to be committed before OpenBSD 6.3 is released.

[ruiu]

Nice. Thanks!

[• rafael]

Thanks!

Mark Kettenis via Phabricator <[e-mail]> writes:

kettenis added a comment.

As @rafael noticed in a follow-up by mail, OpenBSD doesn't implement support for DF_TEXTREL. I don't see a reason why we couldn't so I've implemented support and expect it to be committed before OpenBSD 6.3 is released.

Repository:

rLLD LLVM Linker

https://reviews.llvm.org/D43920

ELF/SyntheticSections.cpp

template <class ELFT> void DynamicSection<ELFT>::finalizeContents() {
   if (Config->ZNow) {
     DtFlags |= DF_BIND_NOW;
     DtFlags1 |= DF_1_NOW;
   }
   if (Config->ZOrigin) {
     DtFlags |= DF_ORIGIN;
     DtFlags1 |= DF_1_ORIGIN;
   }
+  if (!Config->ZText)
+    DtFlags |= DF_TEXTREL;
 
   if (DtFlags)
     addInt(DT_FLAGS, DtFlags);
   if (DtFlags1)
     addInt(DT_FLAGS_1, DtFlags1);
 
   // DT_DEBUG is a pointer to debug informaion used by debuggers at runtime. We
   // need it for each process, so we don't write it for DSOs. The loader writes

test/ELF/ztext-text-notext.s

 
 # CHECK:      Relocations [
 # CHECK-NEXT:    Section {{.*}} .rela.dyn {
 # CHECK-NEXT:      0x1000 R_X86_64_RELATIVE - 0x1000
 # CHECK-NEXT:      0x1008 R_X86_64_64 bar 0x0
 # CHECK-NEXT:      0x1010 R_X86_64_PC64 zed 0x0
 # CHECK-NEXT:    }
 # CHECK-NEXT:  ]
+
 # CHECK: DynamicSection [
-# CHECK: 0x0000000000000016 TEXTREL 0x0
+# CHECK:   FLAGS TEXTREL
+# CHECK:   TEXTREL 0x0
 
 # STATIC:      Relocations [
 # STATIC-NEXT:    Section {{.*}} .rela.dyn {
 # STATIC-NEXT:      0x201008 R_X86_64_64 bar 0x0
 # STATIC-NEXT:      0x201010 R_X86_64_PC64 zed 0x0
 # STATIC-NEXT:    }
 # STATIC-NEXT:  ]
+
 # STATIC: DynamicSection [
-# STATIC: 0x0000000000000016 TEXTREL 0x0
+# STATIC:   FLAGS TEXTREL
+# STATIC:   TEXTREL 0x0
 
 foo:
 .quad foo
 .quad bar
 .quad zed - .