[ELF] - Add missing dynamic tags when producing output with IRelative relocations only.

[grimar]

Summary

This is "Bug 35751 - .dynamic relocation entries omitted if output
contains only IFUNC relocations"

We have InX::RelaPlt and InX::RelaIPlt synthetic sections for PLT relocations.
They are usually live in rela.plt section. Problem appears when InX::RelaPlt
section is empty. In that case we did not produce normal set of dynamic tags
required, because logic was written in the way assuming we always have
non-IRelative relocations in rela.plt.

Patch fixes the issue.

[grimar]

• Reimplemented in according to review suggestions.

[emaste]

ELF/SyntheticSections.cpp

1090 ↗

(On Diff #128278)

The previous code checked that getParent() was not null; why is it no longer necessary?

[grimar]

ELF/SyntheticSections.cpp

1090 ↗

(On Diff #128278)

I believe it was not necessary.
We always add this section to output for non-scripted case (and then remove as it is empty synthetic in removeUnusedSyntheticSections).
For scripted case we disallow discarding rel[a] sections (https://github.com/llvm-mirror/lld/blob/master/ELF/LinkerScript.cpp#L293) and so
I believe always add it as orphan and then, again, remove it in removeUnusedSyntheticSections.
So I believe it is always have parent set.

I am not sure we have testcase showing we ignore discarding .rela.plt. I can double check above (just in case) and add this testcase,
if it is missing, tomorrow.

[emaste]

With this patch applied Kostik's original testcase passes.

[grimar]

ELF/SyntheticSections.cpp

1090 ↗

(On Diff #128278)

I committed NFC change removing checking for null:
https://reviews.llvm.org/rL321581

Also can confirm we silently ignore scripts like:

/DISCARD/ : { *(.rela.plt)
/DISCARD/ : { *(.rela.dyn)

Patch for that is D41640

lld/trunk/ELF/SyntheticSections.cpp

if (!InX::RelaDyn->empty()) {
     // The problem is in the tight relation between dynamic
     // relocations and GOT. So do not emit this tag on MIPS.
     if (Config->EMachine != EM_MIPS) {
       size_t NumRelativeRels = InX::RelaDyn->getRelativeRelocCount();
       if (Config->ZCombreloc && NumRelativeRels)
         addInt(IsRela ? DT_RELACOUNT : DT_RELCOUNT, NumRelativeRels);
     }
   }
-  if (!InX::RelaPlt->empty()) {
+  // .rel[a].plt section usually consists of two parts, containing plt and
+  // iplt relocations. It is possible to have only iplt relocations in the
+  // output. In that case RelaPlt is empty and have zero offset, the same offset
+  // as RelaIplt have. And we still want to emit proper dynamic tags for that
+  // case, so here we always use RelaPlt as marker for the begining of
+  // .rel[a].plt section.
+  if (InX::RelaPlt->getParent()->Live) {
     addInSec(DT_JMPREL, InX::RelaPlt);
     addSize(DT_PLTRELSZ, InX::RelaPlt->getParent());
     switch (Config->EMachine) {
     case EM_MIPS:
       addInSec(DT_MIPS_PLTGOT, InX::GotPlt);
       break;
     case EM_SPARCV9:
       addInSec(DT_PLTGOT, InX::Plt);

lld/trunk/test/ELF/gnu-ifunc-dyntags.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: ld.lld -pie %t.o -o %tout
+# RUN: llvm-objdump -section-headers %tout | FileCheck %s
+# RUN: llvm-readobj -dynamic-table -r %tout | FileCheck %s --check-prefix=TAGS
+
+## Check we produce DT_PLTREL/DT_JMPREL/DT_PLTGOT and DT_PLTRELSZ tags
+## when there are no other relocations except R_*_IRELATIVE.
+
+# CHECK:  Name          Size      Address
+# CHECK:  .rela.plt   00000030 0000000000000210
+# CHECK:  .got.plt    00000010 0000000000002000
+
+# TAGS:      Relocations [
+# TAGS-NEXT:   Section {{.*}} .rela.plt {
+# TAGS-NEXT:     R_X86_64_IRELATIVE
+# TAGS-NEXT:     R_X86_64_IRELATIVE
+# TAGS-NEXT:   }
+# TAGS-NEXT: ]
+
+# TAGS:   Tag                Type                 Name/Value
+# TAGS:   0x0000000000000017 JMPREL               0x210
+# TAGS:   0x0000000000000002 PLTRELSZ             48
+# TAGS:   0x0000000000000003 PLTGOT               0x2000
+# TAGS:   0x0000000000000014 PLTREL               RELA
+
+.text
+.type foo STT_GNU_IFUNC
+.globl foo
+foo:
+ ret
+
+.type bar STT_GNU_IFUNC
+.globl bar
+bar:
+ ret
+
+.globl _start
+_start:
+ call foo
+ call bar