[ELF][X86] Don't create IBT .plt if there is no PLT entry

[joaomoreira]

Summary

https://github.com/ClangBuiltLinux/linux/issues/1606
When GNU_PROPERTY_X86_FEATURE_1_IBT is enabled, ld.lld will create .plt output
section even if there is no PLT entry. Fix this by implementing
IBTPltSection::isNeeded instead of using the default code path (which always
returns true).

[MaskRay]

This is correct. I can add a test for you. I'll use a much conciser description.

[joaomoreira]

Hm. I got tests almost ready (fixed the broken one and am working on finishing a new one). I'll update the diff in a few minutes. I can also update the description, let me know if you have suggestions.

[MaskRay]

Fix description and add tests

[MaskRay]

In D120600#3346993, @joaomoreira wrote:

Hm. I got tests almost ready (fixed the broken one and am working on finishing a new one). I'll update the diff in a few minutes. I can also update the description, let me know if you have suggestions.

OK:) I usually have a preference on how the test should be written :)

[joaomoreira]

No problem! And thanks! The test looks better than the one I had half-way implemented. I'm also updating the test which got broken by this fix... let me know if you see a problem with it!

[MaskRay]

LGTM.

[xiangzhangllvm]

LGTM +1 , thanks a lot!

lld/ELF/SyntheticSections.h

public:
   static constexpr size_t footerSize = 64;
 };
 
 // This is x86-only.
 class IBTPltSection : public SyntheticSection {
 public:
   IBTPltSection();
   void writeTo(uint8_t *Buf) override;
+  bool isNeeded() const override;
   size_t getSize() const override;
 };
 
 class GdbIndexSection final : public SyntheticSection {
 public:
   struct AddressEntry {
     InputSection *section;
     uint64_t lowAddress;

lld/ELF/SyntheticSections.cpp

void IBTPltSection::writeTo(uint8_t *buf) {
   target->writeIBTPlt(buf, in.plt->getNumEntries());
 }
 
 size_t IBTPltSection::getSize() const {
   // 16 is the header size of .plt.
   return 16 + in.plt->getNumEntries() * target->pltEntrySize;
 }
 
+bool IBTPltSection::isNeeded() const { return in.plt->getNumEntries() > 0; }
+
 // The string hash function for .gdb_index.
 static uint32_t computeGdbHash(StringRef s) {
   uint32_t h = 0;
   for (uint8_t c : s)
     h = h * 67 + toLower(c) - 113;
   return h;
 }
 

lld/test/ELF/x86-64-feature-cet.s

 # DISASM-NEXT:               nopw    (%rax,%rax)
 
 # DISASM:      Disassembly of section .iplt:
 # DISASM:      0000000000201370 <.iplt>:
 # DISASM-NEXT: 201370:       endbr64
 # DISASM-NEXT:               jmpq    *0x2126(%rip)
 # DISASM-NEXT:               nopw    (%rax,%rax)
 
+## If there is no PLT entry, don't create .plt section.
+# RUN: ld.lld -e 0 %t1.o -o %t.noplt
+# RUN: llvm-readelf -S %t.noplt | FileCheck %s --check-prefix=NOPLT
+# RUN: ld.lld -r %t1.o -o %t.noplt
+# RUN: llvm-readelf -S %t.noplt | FileCheck %s --check-prefix=NOPLT
+
+# NOPLT:     [Nr] Name
+# NOPLT-NOT: .plt
+# NOPLT:     .note.gnu.property
+# NOPLT-NOT: .plt
+
 .section ".note.gnu.property", "a"
 .long 4
 .long 0x10
 .long 0x5
 .asciz "GNU"
 
 .long 0xc0000002 # GNU_PROPERTY_X86_FEATURE_1_AND
 .long 4