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

ClangBuiltLinux/linux#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).

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D120600

(cherry picked from commit 9d7001e)

Author: Joao Moreira

lld/ELF/SyntheticSections.cpp

@@ -2699,6 +2699,8 @@ size_t IBTPltSection::getSize() const {
   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;

lld/ELF/SyntheticSections.h

@@ -762,6 +762,7 @@ class IBTPltSection : public SyntheticSection {
 public:
   IBTPltSection();
   void writeTo(uint8_t *Buf) override;
+  bool isNeeded() const override;
   size_t getSize() const override;
 };
 

lld/test/ELF/format-binary.test

@@ -31,10 +31,10 @@
 
 # EXE:      Machine: Advanced Micro Devices X86-64
 # EXE:      [Nr] Name         Type     Address          Off    Size   ES Flg Lk Inf Al
-# EXE:      [ 3] .data        PROGBITS {{.*}}                  00000c 00  WA  0   0  8
+# EXE:      [ 2] .data        PROGBITS {{.*}}                  00000c 00  WA  0   0  8
 # EXE:      Size Type    Bind   Vis      Ndx Name
-# EXE:         0 OBJECT  GLOBAL DEFAULT    3 _binary_d_t_txt_start
-# EXE-NEXT:    0 OBJECT  GLOBAL DEFAULT    3 _binary_d_t_txt_end
+# EXE:         0 OBJECT  GLOBAL DEFAULT    2 _binary_d_t_txt_start
+# EXE-NEXT:    0 OBJECT  GLOBAL DEFAULT    2 _binary_d_t_txt_end
 # EXE-NEXT:    0 OBJECT  GLOBAL DEFAULT  ABS _binary_d_t_txt_size
 
 # RUN: not ld.lld -b foo 2>&1 | FileCheck --check-prefix=ERR %s

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

@@ -91,6 +91,17 @@
 # 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