cmd/ld: emit relocs for DWARF info when doing an external link

I would like opinions on whether this is a good idea for 1.1.
On the one hand it's a moderately important issue.  On the
other hand this introduces at least the possibility of
external linker errors due to the additional relocations and
it may be better to wait.

I'm fairly confident that the behaviour is unchanged when not
using an external linker.

Update #5221

This CL is tested lightly on 386 and amd64 and fixes the cases
I tested.  I have not tested it on Darwin or Windows.

R=golang-dev, dave, daniel.morsing, rsc
CC=golang-dev
https://golang.org/cl/8858047

src/cmd/ld/elf.c

@@ -1421,9 +1421,7 @@ elfobj:
 		sh->size = elfstrsize;
 		sh->addralign = 1;
 
-		// TODO(rsc): Enable for linkmode == LinkExternal too, once we know it works.
-		if(linkmode != LinkExternal)
-			dwarfaddelfheaders();
+		dwarfaddelfheaders();
 	}
 
 	/* Main header */

src/cmd/ld/elf.h

@@ -855,7 +855,8 @@ struct Elf64_Shdr {
 	Elf64_Xword	addralign;	/* Alignment in bytes. */
 	Elf64_Xword	entsize;	/* Size of each entry in section. */
 	
-	int	shnum; /* section number, not stored on disk */
+	int	shnum;  /* section number, not stored on disk */
+	Sym*	secsym; /* section symbol, if needed; not on disk */
 };
 
 /*
@@ -998,6 +999,7 @@ void	phsh(ElfPhdr*, ElfShdr*);
 void	doelf(void);
 void	elfsetupplt(void);
 void	dwarfaddshstrings(Sym*);
+void	dwarfaddelfsectionsyms(void);
 void	dwarfaddelfheaders(void);
 void	asmbelf(vlong symo);
 void	asmbelfsetup(void);
@@ -1006,6 +1008,7 @@ extern char freebsddynld[];
 extern char netbsddynld[];
 extern char openbsddynld[];
 int	elfreloc1(Reloc*, vlong sectoff);
+void	putelfsectionsyms(void);
 
 EXTERN	int	elfstrsize;
 EXTERN	char*	elfstrdat;

src/cmd/ld/lib.h

@@ -249,6 +249,8 @@ void	setuint64(Sym*, vlong, uint64);
 void	asmsym(void);
 void	asmelfsym(void);
 void	asmplan9sym(void);
+void	putelfsectionsym(Sym*, int);
+void	putelfsymshndx(vlong, int);
 void	strnput(char*, int);
 void	dodata(void);
 void	dosymtype(void);

src/cmd/ld/symtab.c

@@ -142,6 +142,31 @@ putelfsym(Sym *x, char *s, int t, vlong addr, vlong size, int ver, Sym *go)
 	x->elfsym = numelfsym++;
 }
 
+void
+putelfsectionsym(Sym* s, int shndx)
+{
+	putelfsyment(0, 0, 0, (STB_LOCAL<<4)|STT_SECTION, shndx, 0);
+	s->elfsym = numelfsym++;
+}
+
+void
+putelfsymshndx(vlong sympos, int shndx)
+{
+	vlong here;
+
+	here = cpos();
+	switch(thechar) {
+	case '6':
+		cseek(sympos+6);
+		break;
+	default:
+		cseek(sympos+14);
+		break;
+	}
+	WPUT(shndx);
+	cseek(here);
+}
+
 void
 asmelfsym(void)
 {
@@ -150,6 +175,8 @@ asmelfsym(void)
 	// the first symbol entry is reserved
 	putelfsyment(0, 0, 0, (STB_LOCAL<<4)|STT_NOTYPE, 0, 0);
 
+	dwarfaddelfsectionsyms();
+
 	elfbind = STB_LOCAL;
 	genasmsym(putelfsym);