lld: add -z interpose support

[emaste]

Summary

-z interpose sets the DF_1_INTERPOSE flag, marking the object as an interposer.

FreeBSD PR 230604

[emaste]

See also https://reviews.freebsd.org/D17172 for a lld 6 version intended for commit to FreeBSD's in-tree lld.

test/ELF/dt_flags.s

10–13 ↗

(On Diff #165498)

I think it makes sense to validate flags individually to ensure we don't mix up bits or accidentally set flags we shouldn't, but I will just add it to the list in FLAGS: above if that's desired.

[grimar]

test/ELF/dt_flags.s

10–13 ↗

(On Diff #165498)

I would be consistent with the existent test and reuse FLAGS

11 ↗

(On Diff #165498)

This could be a single line:

ld.lld -z interpose -Bsymbolic %t %t.so -o %t1

[emaste]

test/ELF/dt_flags.s

10–13 ↗

(On Diff #165498)

I can make that change, although the test coverage is not as good; I made this part of the change first:

 # RUN: ld.lld -z global -z initfirst -z now -z nodelete -z nodlopen -z origin \
-# RUN:   -Bsymbolic %t %t.so -o %t1
+# RUN:   -z interpose -Bsymbolic %t %t.so -o %t1
 # RUN: llvm-readobj -dynamic-table %t1 | FileCheck -check-prefix=FLAGS %s

and the test still passed, because the new INTERPOSE flag is at the end and FLAGS: 0x000000006FFFFFFB FLAGS_1 NOW GLOBAL NODELETE INITFIRST NOOPEN ORIGIN still matches.

[emaste]

Reuse existing FLAGS: test

[grimar]

LGTM.

[emaste]

keep flags in order

[grimar]

Still LGTM :)

[ruiu]

LGTM

lld/trunk/ELF/Config.h

struct Configuration {
   bool WarnSymbolOrdering;
   bool WriteAddends;
   bool ZCombreloc;
   bool ZCopyreloc;
   bool ZExecstack;
   bool ZGlobal;
   bool ZHazardplt;
   bool ZInitfirst;
+  bool ZInterpose;
   bool ZKeepTextSectionPrefix;
   bool ZNodelete;
   bool ZNodlopen;
   bool ZNow;
   bool ZOrigin;
   bool ZRelro;
   bool ZRodynamic;
   bool ZText;

lld/trunk/ELF/Driver.cpp

if (K2 == Arg->getValue())
       return false;
   }
   return Default;
 }
 
 static bool isKnown(StringRef S) {
   return S == "combreloc" || S == "copyreloc" || S == "defs" ||
          S == "execstack" || S == "global" || S == "hazardplt" ||
-         S == "initfirst" || S == "keep-text-section-prefix" || S == "lazy" ||
-         S == "muldefs" || S == "nocombreloc" || S == "nocopyreloc" ||
-         S == "nodelete" || S == "nodlopen" || S == "noexecstack" ||
+         S == "initfirst" || S == "interpose" ||
+         S == "keep-text-section-prefix" || S == "lazy" || S == "muldefs" ||
+         S == "nocombreloc" || S == "nocopyreloc" || S == "nodelete" ||
+         S == "nodlopen" || S == "noexecstack" ||
          S == "nokeep-text-section-prefix" || S == "norelro" || S == "notext" ||
          S == "now" || S == "origin" || S == "relro" || S == "retpolineplt" ||
          S == "rodynamic" || S == "text" || S == "wxneeded" ||
          S.startswith("max-page-size=") || S.startswith("stack-size=");
 }
 
 // Report an error for an unknown -z option.
 static void checkZOptions(opt::InputArgList &Args) {
void LinkerDriver::readConfigs(opt::InputArgList &Args) {
   Config->WarnSymbolOrdering =
       Args.hasFlag(OPT_warn_symbol_ordering, OPT_no_warn_symbol_ordering, true);
   Config->ZCombreloc = getZFlag(Args, "combreloc", "nocombreloc", true);
   Config->ZCopyreloc = getZFlag(Args, "copyreloc", "nocopyreloc", true);
   Config->ZExecstack = getZFlag(Args, "execstack", "noexecstack", false);
   Config->ZGlobal = hasZOption(Args, "global");
   Config->ZHazardplt = hasZOption(Args, "hazardplt");
   Config->ZInitfirst = hasZOption(Args, "initfirst");
+  Config->ZInterpose = hasZOption(Args, "interpose");
   Config->ZKeepTextSectionPrefix = getZFlag(
       Args, "keep-text-section-prefix", "nokeep-text-section-prefix", false);
   Config->ZNodelete = hasZOption(Args, "nodelete");
   Config->ZNodlopen = hasZOption(Args, "nodlopen");
   Config->ZNow = getZFlag(Args, "now", "lazy", false);
   Config->ZOrigin = hasZOption(Args, "origin");
   Config->ZRelro = getZFlag(Args, "relro", "norelro", true);
   Config->ZRetpolineplt = hasZOption(Args, "retpolineplt");

lld/trunk/ELF/SyntheticSections.cpp

template <class ELFT> void DynamicSection<ELFT>::finalizeContents() {
   uint32_t DtFlags = 0;
   uint32_t DtFlags1 = 0;
   if (Config->Bsymbolic)
     DtFlags |= DF_SYMBOLIC;
   if (Config->ZGlobal)
     DtFlags1 |= DF_1_GLOBAL;
   if (Config->ZInitfirst)
     DtFlags1 |= DF_1_INITFIRST;
+  if (Config->ZInterpose)
+    DtFlags1 |= DF_1_INTERPOSE;
   if (Config->ZNodelete)
     DtFlags1 |= DF_1_NODELETE;
   if (Config->ZNodlopen)
     DtFlags1 |= DF_1_NOOPEN;
   if (Config->ZNow) {
     DtFlags |= DF_BIND_NOW;
     DtFlags1 |= DF_1_NOW;
   }

lld/trunk/docs/ld.lld.1

 .\" This file is distributed under the University of Illinois Open Source
 .\" License. See LICENSE.TXT for details.
 .\"
 .\" This man page documents only lld's ELF linking support, obtained originally
 .\" from FreeBSD.
-.Dd July 30, 2018
+.Dd September 14, 2018
 .Dt LD.LLD 1
 .Os
 .Sh NAME
 .Nm ld.lld
 .Nd ELF linker from the LLVM project
 .Sh SYNOPSIS
 .Nm ld.lld
 .Op Ar options

 Make the main stack executable.
 Stack permissions are recorded in the
 .Dv PT_GNU_STACK
 segment.
 .It Cm initfirst
 Sets the
 .Dv DF_1_INITFIRST
 flag to indicate the module should be initialized first.
+.It Cm interpose
+Set the
+.Dv DF_1_INTERPOSE
+flag to indicate to the runtime linker that the object is an interposer.
+During symbol resolution interposers are searched after the application
+but before other dependencies.
 .It Cm muldefs
 Do not error if a symbol is defined multiple times.
 The first definition will be used.
 This is a synonym for
 .Fl -allow-multiple-definition.
 .It Cm nocombreloc
 Disable combining and sorting multiple relocation sections.
 .It Cm nocopyreloc

lld/trunk/test/ELF/dt_flags.s

 # REQUIRES: x86
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 # RUN: ld.lld -shared %t -o %t.so
 
-# RUN: ld.lld -z global -z initfirst -z now -z nodelete -z nodlopen -z origin \
-# RUN:   -Bsymbolic %t %t.so -o %t1
+# RUN: ld.lld -z global -z initfirst -z interpose -z now -z nodelete \
+# RUN:   -z nodlopen -z origin -Bsymbolic %t %t.so -o %t1
 # RUN: llvm-readobj -dynamic-table %t1 | FileCheck -check-prefix=FLAGS %s
 
 # RUN: ld.lld %t %t.so -o %t2
 # RUN: llvm-readobj -dynamic-table %t2 | FileCheck %s
 
 # RUN: ld.lld -z lazy %t %t.so -o %t2
 # RUN: llvm-readobj -dynamic-table %t2 | FileCheck %s
 
 # FLAGS: DynamicSection [
 # FLAGS:   0x000000000000001E FLAGS ORIGIN SYMBOLIC BIND_NOW
-# FLAGS:   0x000000006FFFFFFB FLAGS_1 NOW GLOBAL NODELETE INITFIRST NOOPEN ORIGIN
+# FLAGS:   0x000000006FFFFFFB FLAGS_1 NOW GLOBAL NODELETE INITFIRST NOOPEN ORIGIN INTERPOSE
 # FLAGS: ]
 
 # CHECK: DynamicSection [
 # CHECK-NOT:   FLAGS
 # CHECK-NOT:   FLAGS_1
 # CHECK: ]
 
 .globl _start
 _start: