cmd/ld: emit TLS relocations during external linking

This CL was written by rsc.  I just tweaked 8l.

This CL adds TLS relocation to the ELF .o file we write during external linking,
so that the host linker (gcc) can decide the final location of m and g.

Similar relocations are not necessary on OS X because we use an alternate
program start-time mechanism to acquire thread-local storage.

Similar relocations are not necessary on ARM or Plan 9 or Windows
because external linking mode is not yet supported on those systems.

On almost all ELF systems, the references we use are like %fs:-0x4 or %gs:-0x4,
which we write in 6a/8a as -0x4(FS) or -0x4(GS). On Linux/ELF, however,
Xen's lack of support for this mode forced us long ago to use a two-instruction
sequence: first we load %gs:0x0 into a register r, and then we use -0x4(r).
(The ELF program loader arranges that %gs:0x0 contains a regular pointer to
that same memory location.) In order to relocate those -0x4(r) references,
the linker must know where they are. This CL adds the equivalent notation
-0x4(r)(GS*1) for this purpose: it assembles to the same encoding as -0x4(r)
but the (GS*1) indicates to the linker that this is one of those thread-local
references that needs relocation.

Thanks to Elias Naur for reminding me about this missing piece and
also for writing the test.

R=r
CC=golang-dev
https://golang.org/cl/7891047

misc/cgo/testtls/tls.go

+// Copyright 2013 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cgotlstest
+
+// #include <pthread.h>
+// extern void setTLS(int);
+// extern int getTLS();
+import "C"
+
+import (
+	"runtime"
+	"testing"
+)
+
+func testTLS(t *testing.T) {
+	var keyVal C.int = 1234
+
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+	C.setTLS(C.int(keyVal))
+	storedVal := C.getTLS()
+
+	if storedVal != keyVal {
+		t.Fatalf("stored %d want %d", storedVal, keyVal)
+	}
+}

misc/cgo/testtls/tls_test.go

+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !windows
+
+package cgotlstest
+
+import "testing"
+
+func TestTLS(t *testing.T) {
+	testTLS(t)
+}

misc/cgo/testtls/tls_unix.c

+// Copyright 2013 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include <pthread.h>
+
+static __thread int tls;
+
+void
+setTLS(int v)
+{
+	tls = v;
+}
+
+int
+getTLS()
+{
+	return tls;
+}

src/cmd/5l/5.out.h

@@ -275,6 +275,7 @@ enum	as
 #define	D_PLT1		(D_NONE+44) // R_ARM_PLT32, 2nd inst: add ip, ip, #0xNN000
 #define	D_PLT2		(D_NONE+45) // R_ARM_PLT32, 3rd inst: ldr pc, [ip, #0xNNN]!
 #define	D_CALL		(D_NONE+46) // R_ARM_PLT32/R_ARM_CALL/R_ARM_JUMP24, bl xxxxx or b yyyyy
+#define	D_TLS		(D_NONE+47)
 
 /*
  * this is the ranlib header

src/cmd/6l/6.out.h

@@ -866,6 +866,7 @@ enum
 
 	D_SIZE = D_INDIR + D_INDIR,	/* 6l internal */
 	D_PCREL,
+	D_TLS,
 
 	T_TYPE		= 1<<0,
 	T_INDEX		= 1<<1,

src/cmd/6l/asm.c

@@ -310,6 +310,13 @@ elfreloc1(Reloc *r, vlong sectoff)
 		else
 			return -1;
 		break;
+	
+	case D_TLS:
+		if(r->siz == 4)
+			VPUT(R_X86_64_TPOFF32 | (uint64)elfsym<<32);
+		else
+			return -1;
+		break;		
 	}
 
 	VPUT(r->xadd);

src/cmd/6l/obj.c

@@ -83,7 +83,7 @@ main(int argc, char *argv[])
 	INITRND = -1;
 	INITENTRY = 0;
 	LIBINITENTRY = 0;
-	linkmode = LinkInternal; // TODO: LinkAuto once everything works.
+	linkmode = LinkAuto;
 	nuxiinit();
 
 	flagcount("1", "use alternate profiling code", &debug['1']);

src/cmd/6l/span.c

@@ -32,6 +32,7 @@
 
 #include	"l.h"
 #include	"../ld/lib.h"
+#include	"../ld/elf.h"
 
 static int	rexflag;
 static int	asmode;
@@ -880,7 +881,29 @@ putrelv:
 		r = addrel(cursym);
 		*r = rel;
 		r->off = curp->pc + andptr - and;
+	} else if(iself && linkmode == LinkExternal && a->type == D_INDIR+D_FS) {
+		Reloc *r;
+		Sym *s;
+		
+		r = addrel(cursym);
+		r->off = curp->pc + andptr - and;
+		r->add = 0;
+		r->xadd = 0;
+		r->siz = 4;
+		r->type = D_TLS;
+		if(a->offset == tlsoffset+0)
+			s = lookup("runtime.g", 0);
+		else
+			s = lookup("runtime.m", 0);
+		s->type = STLSBSS;
+		s->reachable = 1;
+		s->size = PtrSize;
+		s->hide = 1;
+		r->sym = s;
+		r->xsym = s;
+		v = 0;
 	}
+		
 	put4(v);
 	return;
 

src/cmd/8a/a.y

@@ -507,6 +507,15 @@ omem:
 		$$.scale = $8;
 		checkscale($$.scale);
 	}
+|	con '(' LLREG ')' '(' LSREG '*' con ')'
+	{
+		$$ = nullgen;
+		$$.type = D_INDIR+$3;
+		$$.offset = $1;
+		$$.index = $6;
+		$$.scale = $8;
+		checkscale($$.scale);
+	}
 |	'(' LLREG ')'
 	{
 		$$ = nullgen;

src/cmd/8a/y.tab.h

-/* A Bison parser, made by GNU Bison 2.5.  */
+/* A Bison parser, made by GNU Bison 2.3.  */
 
-/* Bison interface for Yacc-like parsers in C
-   
-      Copyright (C) 1984, 1989-1990, 2000-2011 Free Software Foundation, Inc.
-   
-   This program is free software: you can redistribute it and/or modify
+/* Skeleton interface for Bison's Yacc-like parsers in C
+
+   Copyright (C) 1984, 1989, 1990, 2000, 2001, 2002, 2003, 2004, 2005, 2006
+   Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
-   the Free Software Foundation, either version 3 of the License, or
-   (at your option) any later version.
-   
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.
-   
+
    You should have received a copy of the GNU General Public License
-   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 51 Franklin Street, Fifth Floor,
+   Boston, MA 02110-1301, USA.  */
 
 /* As a special exception, you may create a larger work that contains
    part or all of the Bison parser skeleton and distribute that work
@@ -26,11 +29,10 @@
    special exception, which will cause the skeleton and the resulting
    Bison output files to be licensed under the GNU General Public
    License without this special exception.
-   
+
    This special exception was added by the Free Software Foundation in
    version 2.2 of Bison.  */
 
-
 /* Tokens.  */
 #ifndef YYTOKENTYPE
 # define YYTOKENTYPE
@@ -108,11 +110,8 @@
 
 #if ! defined YYSTYPE && ! defined YYSTYPE_IS_DECLARED
 typedef union YYSTYPE
-{
-
-/* Line 2068 of yacc.c  */
 #line 37 "a.y"
-
+{
 	Sym	*sym;
 	int32	lval;
 	struct {
@@ -123,17 +122,14 @@ typedef union YYSTYPE
 	char	sval[8];
 	Gen	gen;
 	Gen2	gen2;
-
-
-
-/* Line 2068 of yacc.c  */
-#line 131 "y.tab.h"
-} YYSTYPE;
-# define YYSTYPE_IS_TRIVIAL 1
+}
+/* Line 1529 of yacc.c.  */
+#line 128 "y.tab.h"
+	YYSTYPE;
 # define yystype YYSTYPE /* obsolescent; will be withdrawn */
 # define YYSTYPE_IS_DECLARED 1
+# define YYSTYPE_IS_TRIVIAL 1
 #endif
 
 extern YYSTYPE yylval;
 
-

src/cmd/8l/8.out.h

@@ -654,6 +654,7 @@ enum
 	D_PCREL,
 	D_GOTOFF,
 	D_GOTREL,
+	D_TLS,
 
 	T_TYPE		= 1<<0,
 	T_INDEX		= 1<<1,

src/cmd/8l/asm.c

@@ -294,6 +294,12 @@ elfreloc1(Reloc *r, vlong sectoff)
 		else
 			return -1;
 		break;
+	
+	case D_TLS:
+		if(r->siz == 4)
+			LPUT(R_386_TLS_LE | elfsym<<8);
+		else
+			return -1;
 	}
 
 	return 0;

src/cmd/8l/obj.c

@@ -90,7 +90,7 @@ main(int argc, char *argv[])
 	INITRND = -1;
 	INITENTRY = 0;
 	LIBINITENTRY = 0;
-	linkmode = LinkInternal; // TODO: LinkAuto once everything works.
+	linkmode = LinkAuto;
 	nuxiinit();
 
 	flagcount("1", "use alternate profiling code", &debug['1']);

src/cmd/8l/pass.c

@@ -296,16 +296,23 @@ patch(void)
 				//	MOVL 0(GS), reg
 				// and then off(reg) instead of saying off(GS) directly
 				// when the offset is negative.
+				// In external mode we just produce a reloc.
 				if(p->from.type == D_INDIR+D_GS && p->from.offset < 0
 				&& p->to.type >= D_AX && p->to.type <= D_DI) {
-					q = appendp(p);
-					q->from = p->from;
-					q->from.type = D_INDIR + p->to.type;
-					q->to = p->to;
-					q->as = p->as;
-					p->as = AMOVL;
-					p->from.type = D_INDIR+D_GS;
-					p->from.offset = 0;
+					if(linkmode != LinkExternal) {
+						q = appendp(p);
+						q->from = p->from;
+						q->from.type = D_INDIR + p->to.type;
+						q->to = p->to;
+						q->as = p->as;
+						p->as = AMOVL;
+						p->from.type = D_INDIR+D_GS;
+						p->from.offset = 0;
+					} else {
+						// Add signals to relocate.
+						p->from.index = D_GS;
+						p->from.scale = 1;
+					}
 				}
 			}
 			if(HEADTYPE == Hplan9x32) {
@@ -450,16 +457,25 @@ dostkoff(void)
 				break;
 			
 			case Hlinux:
-				p->as = AMOVL;
-				p->from.type = D_INDIR+D_GS;
-				p->from.offset = 0;
-				p->to.type = D_CX;
+				if(linkmode != LinkExternal) {
+					p->as = AMOVL;
+					p->from.type = D_INDIR+D_GS;
+					p->from.offset = 0;
+					p->to.type = D_CX;
 
-				p = appendp(p);
-				p->as = AMOVL;
-				p->from.type = D_INDIR+D_CX;
-				p->from.offset = tlsoffset + 0;
-				p->to.type = D_CX;
+					p = appendp(p);
+					p->as = AMOVL;
+					p->from.type = D_INDIR+D_CX;
+					p->from.offset = tlsoffset + 0;
+					p->to.type = D_CX;
+				} else {
+					p->as = AMOVL;
+					p->from.type = D_INDIR+D_GS;
+					p->from.offset = tlsoffset + 0;
+					p->to.type = D_CX;
+					p->from.index = D_GS;
+					p->from.scale = 1;
+				}
 				break;
 			
 			case Hplan9x32:

src/cmd/8l/span.c

@@ -32,6 +32,7 @@
 
 #include	"l.h"
 #include	"../ld/lib.h"
+#include	"../ld/elf.h"
 
 static int32	vaddr(Adr*, Reloc*);
 
@@ -559,6 +560,14 @@ vaddr(Adr *a, Reloc *r)
 	return v;
 }
 
+static int
+istls(Adr *a)
+{
+	if(HEADTYPE == Hlinux)
+		return a->index == D_GS;
+	return a->type == D_INDIR+D_GS;
+}
+
 void
 asmand(Adr *a, int r)
 {
@@ -569,7 +578,7 @@ asmand(Adr *a, int r)
 	v = a->offset;
 	t = a->type;
 	rel.siz = 0;
-	if(a->index != D_NONE) {
+	if(a->index != D_NONE && a->index != D_FS && a->index != D_GS) {
 		if(t < D_INDIR || t >= 2*D_INDIR) {
 			switch(t) {
 			default:
@@ -658,7 +667,7 @@ asmand(Adr *a, int r)
 			*andptr++ = (0 << 6) | (reg[t] << 0) | (r << 3);
 			return;
 		}
-		if(v >= -128 && v < 128 && rel.siz == 0) {
+		if(v >= -128 && v < 128 && rel.siz == 0 && a->index != D_FS && a->index != D_GS) {
 			andptr[0] = (1 << 6) | (reg[t] << 0) | (r << 3);
 			andptr[1] = v;
 			andptr += 2;
@@ -680,7 +689,29 @@ putrelv:
 		r = addrel(cursym);
 		*r = rel;
 		r->off = curp->pc + andptr - and;
+	} else if(iself && linkmode == LinkExternal && istls(a)) {
+		Reloc *r;
+		Sym *s;
+
+		r = addrel(cursym);
+		r->off = curp->pc + andptr - and;
+		r->add = 0;
+		r->xadd = 0;
+		r->siz = 4;
+		r->type = D_TLS;
+		if(a->offset == tlsoffset+0)
+			s = lookup("runtime.g", 0);
+		else
+			s = lookup("runtime.m", 0);
+		s->type = STLSBSS;
+		s->reachable = 1;
+		s->hide = 1;
+		s->size = PtrSize;
+		r->sym = s;
+		r->xsym = s;
+		v = 0;
 	}
+
 	put4(v);
 	return;
 

src/cmd/dist/buildruntime.c

@@ -133,12 +133,17 @@ static struct {
 		"// which is where these macros come into play.\n"
 		"// get_tls sets up the temporary and then g and r use it.\n"
 		"//\n"
-		"// The final wrinkle is that get_tls needs to read from %gs:0,\n"
+		"// Another wrinkle is that get_tls needs to read from %gs:0,\n"
 		"// but in 8l input it's called 8(GS), because 8l is going to\n"
 		"// subtract 8 from all the offsets, as described above.\n"
+		"//\n"
+		"// The final wrinkle is that when generating an ELF .o file for\n"
+		"// external linking mode, we need to be able to relocate the\n"
+		"// -8(r) and -4(r) instructions. Tag them with an extra (GS*1)\n"
+		"// that is ignored by the linker except for that identification.\n"
 		"#define	get_tls(r)	MOVL 8(GS), r\n"
-		"#define	g(r)	-8(r)\n"
-		"#define	m(r)	-4(r)\n"
+		"#define	g(r)	-8(r)(GS*1)\n"
+		"#define	m(r)	-4(r)(GS*1)\n"
 	},
 	{"386", "",
 		"#define	get_tls(r)\n"

src/cmd/ld/data.c

@@ -172,7 +172,7 @@ relocsym(Sym *s)
 		if(r->sym != S && r->sym->type == SDYNIMPORT)
 			diag("unhandled relocation for %s (type %d rtype %d)", r->sym->name, r->sym->type, r->type);
 
-		if(r->sym != S && !r->sym->reachable)
+		if(r->sym != S && r->sym->type != STLSBSS && !r->sym->reachable)
 			diag("unreachable sym in relocation: %s %s", s->name, r->sym->name);
 
 		switch(r->type) {
@@ -181,6 +181,10 @@ relocsym(Sym *s)
 			if(linkmode == LinkExternal || archreloc(r, s, &o) < 0)
 				diag("unknown reloc %d", r->type);
 			break;
+		case D_TLS:
+			r->done = 0;
+			o = 0;
+			break;
 		case D_ADDR:
 			if(linkmode == LinkExternal && r->sym->type != SCONST) {
 				r->done = 0;
@@ -1193,11 +1197,7 @@ dodata(void)
 	sect->vaddr = datsize;
 	lookup("noptrbss", 0)->sect = sect;
 	lookup("enoptrbss", 0)->sect = sect;
-	for(; s != nil; s = s->next) {
-		if(s->type > SNOPTRBSS) {
-			cursym = s;
-			diag("unexpected symbol type %d", s->type);
-		}
+	for(; s != nil && s->type == SNOPTRBSS; s = s->next) {
 		datsize = aligndatsize(datsize, s);
 		s->sect = sect;
 		s->value = datsize;
@@ -1205,6 +1205,25 @@ dodata(void)
 	}
 	sect->len = datsize - sect->vaddr;
 	lookup("end", 0)->sect = sect;
+	
+	if(iself && linkmode == LinkExternal && s != nil && s->type == STLSBSS) {
+		sect = addsection(&segdata, ".tbss", 06);
+		sect->align = PtrSize;
+		sect->vaddr = 0;
+		datsize = 0;
+		for(; s != nil && s->type == STLSBSS; s = s->next) {
+			datsize = aligndatsize(datsize, s);
+			s->sect = sect;
+			s->value = datsize;
+			datsize += s->size;
+		}
+		sect->len = datsize;
+	}
+	
+	if(s != nil) {
+		cursym = nil;
+		diag("unexpected symbol type %d for %s", s->type, s->name);
+	}
 
 	/* we finished segdata, begin segtext */
 	s = datap;

src/cmd/ld/elf.c

@@ -758,6 +758,10 @@ elfshbits(Section *sect)
 		sh->flags |= SHF_EXECINSTR;
 	if(sect->rwx & 2)
 		sh->flags |= SHF_WRITE;
+	if(strcmp(sect->name, ".tbss") == 0) {
+		sh->flags |= SHF_TLS;
+		sh->type = SHT_NOBITS;
+	}
 	if(linkmode != LinkExternal)
 		sh->addr = sect->vaddr;
 	sh->addralign = sect->align;
@@ -779,7 +783,7 @@ elfshreloc(Section *sect)
 	// Also nothing to relocate in .shstrtab.
 	if(sect->vaddr >= sect->seg->vaddr + sect->seg->filelen)
 		return nil;
-	if(strcmp(sect->name, ".shstrtab") == 0)
+	if(strcmp(sect->name, ".shstrtab") == 0 || strcmp(sect->name, ".tbss") == 0)
 		return nil;
 
 	if(thechar == '6') {
@@ -883,6 +887,8 @@ doelf(void)
 	addstring(shstrtab, ".data");
 	addstring(shstrtab, ".bss");
 	addstring(shstrtab, ".noptrbss");
+	if(linkmode == LinkExternal)
+		addstring(shstrtab, ".tbss");
 	if(HEADTYPE == Hnetbsd)
 		addstring(shstrtab, ".note.netbsd.ident");
 	if(HEADTYPE == Hopenbsd)

src/cmd/ld/lib.h

@@ -52,6 +52,7 @@ enum
 	SWINDOWS,
 	SBSS,
 	SNOPTRBSS,
+	STLSBSS,
 
 	SXREF,
 	SMACHOSYMSTR,

src/cmd/ld/symtab.c

@@ -152,6 +152,26 @@ asmelfsym(void)
 
 	elfbind = STB_LOCAL;
 	genasmsym(putelfsym);
+	
+	if(linkmode == LinkExternal) {
+		s = lookup("runtime.m", 0);
+		if(s->sect == nil) {
+			cursym = nil;
+			diag("missing section for %s", s->name);
+			errorexit();
+		}
+		putelfsyment(putelfstr(s->name), 0, PtrSize, (STB_LOCAL<<4)|STT_TLS, s->sect->elfsect->shnum, 0);
+		s->elfsym = numelfsym++;
+
+		s = lookup("runtime.g", 0);
+		if(s->sect == nil) {
+			cursym = nil;
+			diag("missing section for %s", s->name);
+			errorexit();
+		}
+		putelfsyment(putelfstr(s->name), PtrSize, PtrSize, (STB_LOCAL<<4)|STT_TLS, s->sect->elfsect->shnum, 0);
+		s->elfsym = numelfsym++;
+	}
 
 	elfbind = STB_GLOBAL;
 	elfglobalsymndx = numelfsym;

src/run.bash

@@ -83,8 +83,14 @@ set -e
 go test -ldflags '-linkmode=auto'
 go test -ldflags '-linkmode=internal'
 case "$GOHOSTOS-$GOARCH" in
-darwin-386 | darwin-amd64 | freebsd-386 | freebsd-amd64 | linux-386 | linux-amd64 | netbsd-386 | netbsd-amd64 | openbsd-386 | openbsd-amd64)
+darwin-386 | darwin-amd64 | openbsd-386 | openbsd-amd64)
+	# test linkmode=external, but __thread not supported, so skip testtls.
 	go test -ldflags '-linkmode=external'
+	;;
+freebsd-386 | freebsd-amd64 | linux-386 | linux-amd64 | netbsd-386 | netbsd-amd64)
+	go test -ldflags '-linkmode=external'
+	go test -ldflags '-linkmode=auto' ../testtls
+	go test -ldflags '-linkmode=external' ../testtls
 esac
 ) || exit $?