cmd/ld: support for linking with host linker

A step toward a fix for issue 4069.

To allow linking with arbitrary host object files, add a linker mode
that can generate a host object file instead of an executable.
Then the host linker can be invoked to generate the final executable.

This CL adds a new -hostobj flag that instructs the linker to write
a host object file instead of an executable.

That is, this works:

        go tool 6g x.go
        go tool 6l -hostobj -o x.o x.6
        ld -e _rt0_amd64_linux x.o
        ./a.out

as does:

        go tool 8g x.go
        go tool 8l -hostld ignored -o x.o x.8
        ld -m elf_i386 -e _rt0_386_linux x.o
        ./a.out

Because 5l was never updated to use the standard relocation scheme,
it will take more work to get this working on ARM.

This is a checkpoint of the basic functionality. It does not work
with cgo yet, and cgo is the main reason for the change.
The command-line interface will likely change too.
The gc linker has other information that needs to be returned to
the caller for use when invoking the host linker besides the single
object file.

R=iant, iant
CC=golang-dev
https://golang.org/cl/7060044

1 files changed, 180 insertions, 11 deletions

diff --git a/src/cmd/ld/elf.c b/src/cmd/ld/elf.c
index d0b5fa8304..9793feea03 100644
--- a/src/cmd/ld/elf.c
+++ b/src/cmd/ld/elf.c
@@ -758,7 +758,8 @@ elfshbits(Section *sect)
 		sh->flags |= SHF_EXECINSTR;
 	if(sect->rwx & 2)
 		sh->flags |= SHF_WRITE;
-	sh->addr = sect->vaddr;
+	if(!isobj)
+		sh->addr = sect->vaddr;
 	sh->addralign = PtrSize;
 	sh->size = sect->len;
 	sh->off = sect->seg->fileoff + sect->vaddr - sect->seg->vaddr;
@@ -766,6 +767,117 @@ elfshbits(Section *sect)
 	return sh;
 }
 
+ElfShdr*
+elfshreloc(Section *sect)
+{
+	int typ;
+	ElfShdr *sh;
+	char *prefix;
+	char buf[100];
+	
+	// If main section is SHT_NOBITS, nothing to relocate.
+	// Also nothing to relocate in .shstrtab.
+	if(sect->vaddr >= sect->seg->vaddr + sect->seg->filelen)
+		return nil;
+	if(strcmp(sect->name, ".shstrtab") == 0)
+		return nil;
+
+	if(thechar == '6') {
+		prefix = ".rela";
+		typ = SHT_RELA;
+	} else {
+		prefix = ".rel";
+		typ = SHT_REL;
+	}
+
+	snprint(buf, sizeof buf, "%s%s", prefix, sect->name);
+	sh = elfshname(buf);
+	sh->type = typ;
+	sh->entsize = PtrSize*(2+(typ==SHT_RELA));
+	sh->link = elfshname(".symtab")->shnum;
+	sh->info = sect->elfsect->shnum;
+	sh->off = sect->reloff;
+	sh->size = sect->rellen;
+	sh->addralign = PtrSize;
+	return sh;
+}
+
+void
+elfrelocsect(Section *sect, Sym *first)
+{
+	Sym *sym, *rs;
+	int32 eaddr;
+	Reloc *r;
+	int64 add;
+
+	// If main section is SHT_NOBITS, nothing to relocate.
+	// Also nothing to relocate in .shstrtab.
+	if(sect->vaddr >= sect->seg->vaddr + sect->seg->filelen)
+		return;
+	if(strcmp(sect->name, ".shstrtab") == 0)
+		return;
+
+	sect->reloff = cpos();
+	for(sym = first; sym != nil; sym = sym->next) {
+		if(!sym->reachable)
+			continue;
+		if(sym->value >= sect->vaddr)
+			break;
+	}
+	
+	eaddr = sect->vaddr + sect->len;
+	for(; sym != nil; sym = sym->next) {
+		if(!sym->reachable)
+			continue;
+		if(sym->value >= eaddr)
+			break;
+		cursym = sym;
+		
+		for(r = sym->r; r < sym->r+sym->nr; r++) {
+			// Ignore relocations handled by reloc already.
+			switch(r->type) {
+			case D_SIZE:
+				continue;
+			case D_ADDR:
+			case D_PCREL:
+				if(r->sym->type == SCONST)
+					continue;
+				break;
+			}
+
+			add = r->add;
+			rs = r->sym;
+			while(rs->outer != nil) {
+				add += rs->value - rs->outer->value;
+				rs = rs->outer;
+			}
+				
+			if(rs->elfsym == 0)
+				diag("reloc %d to non-elf symbol %s (rs=%s) %d", r->type, r->sym->name, rs->name, rs->type);
+
+			if(elfreloc1(r, sym->value - sect->vaddr + r->off, rs->elfsym, add) < 0)
+				diag("unsupported obj reloc %d/%d to %s", r->type, r->siz, r->sym->name);
+		}
+	}
+		
+	sect->rellen = cpos() - sect->reloff;
+}	
+	
+void
+elfemitreloc(void)
+{
+	Section *sect;
+
+	while(cpos()&7)
+		cput(0);
+
+	elfrelocsect(segtext.sect, textp);
+	for(sect=segtext.sect->next; sect!=nil; sect=sect->next)
+		elfrelocsect(sect, datap);	
+	for(sect=segdata.sect; sect!=nil; sect=sect->next)
+		elfrelocsect(sect, datap);	
+}
+
 void
 doelf(void)
 {
@@ -800,6 +912,33 @@ doelf(void)
 	addstring(shstrtab, ".gcbss");
 	addstring(shstrtab, ".gosymtab");
 	addstring(shstrtab, ".gopclntab");
+	
+	if(isobj) {
+		debug['s'] = 0;
+		debug['d'] = 1;
+
+		if(thechar == '6') {
+			addstring(shstrtab, ".rela.text");
+			addstring(shstrtab, ".rela.rodata");
+			addstring(shstrtab, ".rela.typelink");
+			addstring(shstrtab, ".rela.gcdata");
+			addstring(shstrtab, ".rela.gcbss");
+			addstring(shstrtab, ".rela.gosymtab");
+			addstring(shstrtab, ".rela.gopclntab");
+			addstring(shstrtab, ".rela.noptrdata");
+			addstring(shstrtab, ".rela.data");
+		} else {
+			addstring(shstrtab, ".rel.text");
+			addstring(shstrtab, ".rel.rodata");
+			addstring(shstrtab, ".rel.typelink");
+			addstring(shstrtab, ".rel.gcdata");
+			addstring(shstrtab, ".rel.gcbss");
+			addstring(shstrtab, ".rel.gosymtab");
+			addstring(shstrtab, ".rel.gopclntab");
+			addstring(shstrtab, ".rel.noptrdata");
+			addstring(shstrtab, ".rel.data");
+		}
+	}
 
 	if(!debug['s']) {
 		addstring(shstrtab, ".symtab");
@@ -1004,6 +1143,14 @@ asmbelf(vlong symo)
 
 	startva = INITTEXT - HEADR;
 	resoff = ELFRESERVE;
+	
+	pph = nil;
+	if(isobj) {
+		/* skip program headers */
+		eh->phoff = 0;
+		eh->phentsize = 0;
+		goto elfobj;
+	}
 
 	/* program header info */
 	pph = newElfPhdr();
@@ -1238,17 +1385,29 @@ asmbelf(vlong symo)
 	ph->flags = 0x2a00; // mprotect, randexec, emutramp disabled
 	ph->align = PtrSize;
 
+elfobj:
 	sh = elfshname(".shstrtab");
 	sh->type = SHT_STRTAB;
 	sh->addralign = 1;
 	shsym(sh, lookup(".shstrtab", 0));
 	eh->shstrndx = sh->shnum;
 
+	// put these sections early in the list
+	elfshname(".symtab");
+	elfshname(".strtab");
+
 	for(sect=segtext.sect; sect!=nil; sect=sect->next)
 		elfshbits(sect);
 	for(sect=segdata.sect; sect!=nil; sect=sect->next)
 		elfshbits(sect);
 
+	if(isobj) {
+		for(sect=segtext.sect; sect!=nil; sect=sect->next)
+			elfshreloc(sect);
+		for(sect=segdata.sect; sect!=nil; sect=sect->next)
+			elfshreloc(sect);
+	}
+		
 	if(!debug['s']) {
 		sh = elfshname(".symtab");
 		sh->type = SHT_SYMTAB;
@@ -1265,7 +1424,9 @@ asmbelf(vlong symo)
 		sh->size = elfstrsize;
 		sh->addralign = 1;
 
-		dwarfaddelfheaders();
+		// TODO(rsc): Enable for isobj too, once we know it works.
+		if(!isobj)
+			dwarfaddelfheaders();
 	}
 
 	/* Main header */
@@ -1288,14 +1449,20 @@ asmbelf(vlong symo)
 
 	if(flag_shared)
 		eh->type = ET_DYN;
+	else if(isobj)
+		eh->type = ET_REL;
 	else
 		eh->type = ET_EXEC;
 
+	if(!isobj)
+		eh->entry = entryvalue();
+
 	eh->version = EV_CURRENT;
-	eh->entry = entryvalue();
 
-	pph->filesz = eh->phnum * eh->phentsize;
-	pph->memsz = pph->filesz;
+	if(pph != nil) {
+		pph->filesz = eh->phnum * eh->phentsize;
+		pph->memsz = pph->filesz;
+	}
 
 	cseek(0);
 	a = 0;
@@ -1304,12 +1471,14 @@ asmbelf(vlong symo)
 	a += elfwriteshdrs();
 	if(!debug['d'])
 		a += elfwriteinterp();
-	if(HEADTYPE == Hnetbsd)
-		a += elfwritenetbsdsig();
-	if(HEADTYPE == Hopenbsd)
-		a += elfwriteopenbsdsig();
-	if(buildinfolen > 0)
-		a += elfwritebuildinfo();
+	if(!isobj) {
+		if(HEADTYPE == Hnetbsd)
+			a += elfwritenetbsdsig();
+		if(HEADTYPE == Hopenbsd)
+			a += elfwriteopenbsdsig();
+		if(buildinfolen > 0)
+			a += elfwritebuildinfo();
+	}
 	if(a > ELFRESERVE)	
 		diag("ELFRESERVE too small: %d > %d", a, ELFRESERVE);
 }