cmd/internal/obj/x86, cmd/link/internal/x86: support IE model TLS on linux/386

This includes the first parts of the general approach to PIC: load PC into CX
whenever it is needed. This is going to lead to large binaries and poor
performance but it's a start and easy to get right.

Change-Id: Ic8bf1d0a74284cca0d94a68cf75024e8ab063b4e
Reviewed-on: https://go-review.googlesource.com/16383
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/obj/x86/asm6.go

@@ -2018,6 +2018,21 @@ func prefixof(ctxt *obj.Link, p *obj.Prog, a *obj.Addr) int {
 	}
 
 	if p.Mode == 32 {
+		if a.Index == REG_TLS && ctxt.Flag_shared != 0 {
+			// When building for inclusion into a shared library, an instruction of the form
+			//     MOVL 0(CX)(TLS*1), AX
+			// becomes
+			//     mov %gs:(%ecx), %eax
+			// which assumes that the correct TLS offset has been loaded into %ecx (today
+			// there is only one TLS variable -- g -- so this is OK). When not building for
+			// a shared library the instruction it becomes
+			//     mov 0x0(%ecx), $eax
+			// and a R_TLS_LE relocation, and so does not require a prefix.
+			if a.Offset != 0 {
+				ctxt.Diag("cannot handle non-0 offsets to TLS")
+			}
+			return 0x65 // GS
+		}
 		return 0
 	}
 
@@ -2518,10 +2533,12 @@ func vaddr(ctxt *obj.Link, p *obj.Prog, a *obj.Addr, r *obj.Reloc) int64 {
 			log.Fatalf("reloc")
 		}
 
+		if ctxt.Flag_shared == 0 {
 			r.Type = obj.R_TLS_LE
 			r.Siz = 4
 			r.Off = -1 // caller must fill in
 			r.Add = a.Offset
+		}
 		return 0
 	}
 
@@ -3922,6 +3939,38 @@ func doasm(ctxt *obj.Link, p *obj.Prog) {
 
 						case obj.Hlinux,
 							obj.Hnacl:
+							if ctxt.Flag_shared != 0 {
+								// Note that this is not generating the same insns as the other cases.
+								//     MOV TLS, R_to
+								// becomes
+								//     call __x86.get_pc_thunk.cx
+								//     movl (gotpc + g@gotntpoff)(%ecx),$R_To
+								// which is encoded as
+								//     call __x86.get_pc_thunk.cx
+								//     movq 0(%ecx), R_to
+								// and R_CALL & R_TLS_IE relocs. This all assumes the only tls variable we access
+								// is g, which we can't check here, but will when we assemble the second
+								// instruction.
+								ctxt.Andptr[0] = 0xe8
+								ctxt.Andptr = ctxt.Andptr[1:]
+								r = obj.Addrel(ctxt.Cursym)
+								r.Off = int32(p.Pc + int64(-cap(ctxt.Andptr)+cap(ctxt.And[:])))
+								r.Type = obj.R_CALL
+								r.Siz = 4
+								r.Sym = obj.Linklookup(ctxt, "__x86.get_pc_thunk.cx", 0)
+								put4(ctxt, 0)
+
+								ctxt.Andptr[0] = 0x8B
+								ctxt.Andptr = ctxt.Andptr[1:]
+								ctxt.Andptr[0] = byte(2<<6 | reg[REG_CX] | (reg[p.To.Reg] << 3))
+								ctxt.Andptr = ctxt.Andptr[1:]
+								r = obj.Addrel(ctxt.Cursym)
+								r.Off = int32(p.Pc + int64(-cap(ctxt.Andptr)+cap(ctxt.And[:])))
+								r.Type = obj.R_TLS_IE
+								r.Siz = 4
+								r.Add = 2
+								put4(ctxt, 0)
+							} else {
 								// ELF TLS base is 0(GS).
 								pp.From = p.From
 
@@ -3935,7 +3984,7 @@ func doasm(ctxt *obj.Link, p *obj.Prog) {
 								ctxt.Andptr[0] = 0x8B
 								ctxt.Andptr = ctxt.Andptr[1:]
 								asmand(ctxt, p, &pp.From, &p.To)
-
+							}
 						case obj.Hplan9:
 							if ctxt.Plan9privates == nil {
 								ctxt.Plan9privates = obj.Linklookup(ctxt, "_privates", 0)

src/cmd/link/internal/ld/lib.go

@@ -1669,7 +1669,7 @@ func stkcheck(up *Chain, depth int) int {
 		// should never be called directly.
 		// only diagnose the direct caller.
 		// TODO(mwhudson): actually think about this.
-		if depth == 1 && s.Type != obj.SXREF && !DynlinkingGo() {
+		if depth == 1 && s.Type != obj.SXREF && !DynlinkingGo() && Buildmode != BuildmodePIE {
 			Diag("call to external function %s", s.Name)
 		}
 		return -1

src/cmd/link/internal/x86/asm.go

@@ -38,6 +38,30 @@ import (
 )
 
 func gentext() {
+	if !ld.DynlinkingGo() && ld.Buildmode != ld.BuildmodePIE {
+		return
+	}
+
+	thunkfunc := ld.Linklookup(ld.Ctxt, "__x86.get_pc_thunk.cx", 0)
+	thunkfunc.Type = obj.STEXT
+	thunkfunc.Local = true
+	thunkfunc.Reachable = true
+	o := func(op ...uint8) {
+		for _, op1 := range op {
+			ld.Adduint8(ld.Ctxt, thunkfunc, op1)
+		}
+	}
+	// 8b 0c 24	mov    (%esp),%ecx
+	o(0x8b, 0x0c, 0x24)
+	// c3		ret
+	o(0xc3)
+
+	if ld.Ctxt.Etextp != nil {
+		ld.Ctxt.Etextp.Next = thunkfunc
+	} else {
+		ld.Ctxt.Textp = thunkfunc
+	}
+	ld.Ctxt.Etextp = thunkfunc
 }
 
 func adddynrela(rela *ld.LSym, s *ld.LSym, r *ld.Reloc) {
@@ -257,6 +281,15 @@ func elfreloc1(r *ld.Reloc, sectoff int64) int {
 		} else {
 			return -1
 		}
+
+	case obj.R_TLS_IE:
+		if r.Siz == 4 {
+			ld.Thearch.Lput(ld.R_386_GOTPC)
+			ld.Thearch.Lput(uint32(sectoff))
+			ld.Thearch.Lput(ld.R_386_TLS_GOTIE | uint32(elfsym)<<8)
+		} else {
+			return -1
+		}
 	}
 
 	return 0

src/cmd/link/internal/x86/obj.go

@@ -86,6 +86,13 @@ func archinit() {
 		ld.Linkmode = ld.LinkInternal
 	}
 
+	if ld.Buildmode == ld.BuildmodeCShared || ld.Buildmode == ld.BuildmodePIE || ld.DynlinkingGo() {
+		ld.Linkmode = ld.LinkExternal
+		got := ld.Linklookup(ld.Ctxt, "_GLOBAL_OFFSET_TABLE_", 0)
+		got.Type = obj.SDYNIMPORT
+		got.Reachable = true
+	}
+
 	switch ld.HEADTYPE {
 	default:
 		if ld.Linkmode == ld.LinkAuto {