[ELF] - Implemented -znotext

[grimar]

Summary

gold linker manual describes them as:

-z text Do not permit relocations in read-only segments
-z notext Permit relocations in read-only segments (default)

In LLD default is to not permit them. Patch implements -z notext.

[ruiu]

What is a program do you want to use this for?

[grimar]

In D30530#690725, @ruiu wrote:

What is a program do you want to use this for?

Linux kernel uses text relocations when KASLR (Kernel address space layout randomization) is enabled.

[ruiu]

Does it pass -z text?

[grimar]

In D30530#690735, @ruiu wrote:

Does it pass -z text?

No, we have no way not to fail with text relocations now.
ld.bfd accepts them by default. We - dont.
I think solution for them would be to pass -z notext.

[ruiu]

ELF/Driver.cpp

272 ↗

(On Diff #90305)

Use llvm::reverse and return early.

ELF/SyntheticSections.cpp

1109–1110 ↗

(On Diff #90305)

I don't think you need to check if there's actually text relocations. Set it unconditionally as long as Config->ZText is true.

[grimar]

• Addressed review comments.

[grimar]

ELF/Driver.cpp

272 ↗

(On Diff #90305)

That would not work out of box with filtered.
I prepared patch for that: D30570

[grimar]

• Reimplemented getZOption()

[grimar]

ELF/Driver.cpp

270 ↗

(On Diff #90666)

This can be reused, for example for "relro" vs "norelro", "execstack" vs "noexecstack" etc.

[ruiu]

ELF/Driver.cpp

614 ↗

(On Diff #90666)

Do you actually need -z notext? You can just write hasZOption(Args, "text") like other options.

[grimar]

ELF/Driver.cpp

614 ↗

(On Diff #90666)

The way other -z options implemented is incorrect I believe.
There are few options thatmake a pairs, like -z text/notext, -z execstack/noexecstack, -z relro/norelro, -z combreloc/nocombreloc.
Don't we want to be consistent and override previous -z options like we do for regular ones ?

[ruiu]

ELF/Driver.cpp

614 ↗

(On Diff #90666)

Strictly speaking that is true, but for now focusing on -z text is better.

[grimar]

• Addressed review comments.

[ruiu]

LGTM

lld/trunk/ELF/Config.h

struct Configuration {
   bool WarnMissingEntry;
   bool ZCombreloc;
   bool ZExecstack;
   bool ZNocopyreloc;
   bool ZNodelete;
   bool ZNow;
   bool ZOrigin;
   bool ZRelro;
+  bool ZText;
   bool ExitEarly;
   bool ZWxneeded;
   DiscardPolicy Discard;
   SortSectionPolicy SortSection;
   StripPolicy Strip;
   UnresolvedPolicy UnresolvedSymbols;
   Target2Policy Target2;
   BuildIdKind BuildId = BuildIdKind::None;

lld/trunk/ELF/Driver.cpp

void LinkerDriver::readConfigs(opt::InputArgList &Args) {
   Config->ZCombreloc = !hasZOption(Args, "nocombreloc");
   Config->ZExecstack = hasZOption(Args, "execstack");
   Config->ZNocopyreloc = hasZOption(Args, "nocopyreloc");
   Config->ZNodelete = hasZOption(Args, "nodelete");
   Config->ZNow = hasZOption(Args, "now");
   Config->ZOrigin = hasZOption(Args, "origin");
   Config->ZRelro = !hasZOption(Args, "norelro");
   Config->ZStackSize = getZOptionValue(Args, "stack-size", 0);
+  Config->ZText = !hasZOption(Args, "notext");
   Config->ZWxneeded = hasZOption(Args, "wxneeded");
 
   if (Config->LTOO > 3)
     error("invalid optimization level for LTO: " + getString(Args, OPT_lto_O));
   if (Config->LTOPartitions == 0)
     error("--lto-partitions: number of threads must be > 0");
   if (Config->ThinLTOJobs == 0)
     error("--thinlto-jobs: number of threads must be > 0");

lld/trunk/ELF/Relocations.cpp

static RelExpr adjustExpr(const elf::ObjectFile<ELFT> &File, SymbolBody &Body,
 
   if (IsWrite || isStaticLinkTimeConstant<ELFT>(Expr, Type, Body, S, RelOff))
     return Expr;
 
   // This relocation would require the dynamic linker to write a value to read
   // only memory. We can hack around it if we are producing an executable and
   // the refered symbol can be preemepted to refer to the executable.
   if (Config->Shared || (Config->pic() && !isRelExpr(Expr))) {
+    if (Config->ZText)
       error(S.getLocation<ELFT>(RelOff) + ": can't create dynamic relocation " +
             toString(Type) + " against " +
             (Body.getName().empty() ? "local symbol in readonly segment"
                                     : "symbol '" + toString(Body) + "'") +
             " defined in " + toString(Body.File));
     return Expr;
   }
   if (Body.getVisibility() != STV_DEFAULT) {
     error(S.getLocation<ELFT>(RelOff) + ": cannot preempt symbol '" +
           toString(Body) + "' defined in " + toString(Body.File));
     return Expr;
   }
   if (Body.isObject()) {

lld/trunk/ELF/SyntheticSections.cpp

add({Config->EMachine == EM_MIPS ? DT_MIPS_PLTGOT : DT_PLTGOT,
          In<ELFT>::GotPlt});
     add({DT_PLTREL, uint64_t(Config->isRela() ? DT_RELA : DT_REL)});
   }
 
   add({DT_SYMTAB, In<ELFT>::DynSymTab});
   add({DT_SYMENT, sizeof(Elf_Sym)});
   add({DT_STRTAB, In<ELFT>::DynStrTab});
   add({DT_STRSZ, In<ELFT>::DynStrTab->getSize()});
+  if (!Config->ZText)
+    add({DT_TEXTREL, (uint64_t)0});
   if (In<ELFT>::GnuHashTab)
     add({DT_GNU_HASH, In<ELFT>::GnuHashTab});
   if (In<ELFT>::HashTab)
     add({DT_HASH, In<ELFT>::HashTab});
 
   if (Out::PreinitArray) {
     add({DT_PREINIT_ARRAY, Out::PreinitArray});
     add({DT_PREINIT_ARRAYSZ, Out::PreinitArray, Entry::SecSize});

lld/trunk/test/ELF/ztext-text-notext.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: ld.lld -z notext %t.o -o %t -shared
+# RUN: llvm-readobj  -dynamic-table -r %t | FileCheck %s
+
+# CHECK:      Relocations [
+# CHECK-NEXT:    Section {{.*}} .rela.dyn {
+# CHECK-NEXT:      0x1000 R_X86_64_RELATIVE - 0x1000
+# CHECK-NEXT:    }
+# CHECK-NEXT:  ]
+# CHECK: DynamicSection [
+# CHECK: 0x0000000000000016 TEXTREL 0x0
+
+foo:
+.quad foo