Add IBT support to the retpoline PLTs. Since we use retpoline PLTs by default on OpenBSD this will give us IBT support by default. Fixes indirect function calls for functions in shared libraries.

This doesn't fix retpoline+znow PLTs yet; a fix for that will follow.

ok miod@, guenther@

Author: kettenis

lld/ELF/Arch/X86_64.cpp

@@ -1094,7 +1094,7 @@ Retpoline::Retpoline() {
 }
 
 void Retpoline::writeGotPlt(uint8_t *buf, const Symbol &s) const {
-  write64le(buf, s.getPltVA() + 17);
+  write64le(buf, s.getPltVA() + 21);
 }
 
 void Retpoline::writePltHeader(uint8_t *buf) const {
@@ -1122,22 +1122,23 @@ void Retpoline::writePltHeader(uint8_t *buf) const {
 void Retpoline::writePlt(uint8_t *buf, const Symbol &sym,
                          uint64_t pltEntryAddr) const {
   const uint8_t insn[] = {
-      0x4c, 0x8b, 0x1d, 0, 0, 0, 0, // 0:  mov foo@GOTPLT(%rip), %r11
-      0xe8, 0,    0,    0,    0,    // 7:  callq plt+0x20
-      0xe9, 0,    0,    0,    0,    // c:  jmp plt+0x12
-      0x68, 0,    0,    0,    0,    // 11: pushq <relocation index>
-      0xe9, 0,    0,    0,    0,    // 16: jmp plt+0
-      0xcc, 0xcc, 0xcc, 0xcc, 0xcc, // 1b: int3; padding
+      0xf3, 0x0f, 0x1e, 0xfa,       // 0:  endbr64
+      0x4c, 0x8b, 0x1d, 0, 0, 0, 0, // 4:  mov foo@GOTPLT(%rip), %r11
+      0xe8, 0,    0,    0,    0,    // b:  callq plt+0x20
+      0xe9, 0,    0,    0,    0,    // 10:  jmp plt+0x12
+      0x68, 0,    0,    0,    0,    // 15: pushq <relocation index>
+      0xe9, 0,    0,    0,    0,    // 1a: jmp plt+0
+      0xcc,                         // 1f: int3; padding
   };
   memcpy(buf, insn, sizeof(insn));
 
   uint64_t off = pltEntryAddr - in.plt->getVA();
 
-  write32le(buf + 3, sym.getGotPltVA() - pltEntryAddr - 7);
-  write32le(buf + 8, -off - 12 + 32);
-  write32le(buf + 13, -off - 17 + 18);
-  write32le(buf + 18, sym.getPltIdx());
-  write32le(buf + 23, -off - 27);
+  write32le(buf + 7, sym.getGotPltVA() - pltEntryAddr - 11);
+  write32le(buf + 12, -off - 16 + 32);
+  write32le(buf + 17, -off - 21 + 18);
+  write32le(buf + 22, sym.getPltIdx());
+  write32le(buf + 27, -off - 31);
 }
 
 RetpolineZNow::RetpolineZNow() {