cmd/5l: generate FreeBSD compatible ELF

1. correctly initialize .plt.got entries (point to the 1st entry)
2. add section .rel.plt (FreeBSD insists PLT relocs to be there)
3. put relocs of .got.plt into .rel.plt
4. set ELFOSABI_FREEBSD in ELF header

R=rsc
CC=golang-dev
https://golang.org/cl/6643050

src/cmd/5l/asm.c

@@ -38,6 +38,7 @@
 static Prog *PP;
 
 char linuxdynld[] = "/lib/ld-linux.so.3"; // 2 for OABI, 3 for EABI
+char freebsddynld[] = "/usr/libexec/ld-elf.so.1";
 
 int32
 entryvalue(void)
@@ -344,13 +345,16 @@ addpltsym(Sym *s)
 	if(iself) {
 		plt = lookup(".plt", 0);
 		got = lookup(".got.plt", 0);
-		rel = lookup(".rel", 0);
+		rel = lookup(".rel.plt", 0);
 		if(plt->size == 0)
 			elfsetupplt();
 		
 		// .got entry
 		s->got = got->size;
-		adduint32(got, 0);
+		// In theory, all GOT should point to the first PLT entry,
+		// Linux/ARM's dynamic linker will do that for us, but FreeBSD/ARM's
+		// dynamic linker won't, so we'd better do it ourselves.
+		addaddrplus(got, plt, 0);
 
 		// .plt entry, this depends on the .got entry
 		s->plt = plt->size;
@@ -696,7 +700,7 @@ asmb(void)
 		/* !debug['d'] causes extra sections before the .text section */
 		elftextsh = 2;
 		if(!debug['d']) {
-			elftextsh += 9;
+			elftextsh += 10;
 			if(elfverneed)
 				elftextsh += 2;
 		}
@@ -755,6 +759,9 @@ asmb(void)
 	Bflush(&bso);
 	cseek(0L);
 	switch(HEADTYPE) {
+	default:
+		if(iself)
+			goto Elfput;
 	case Hnoheader:	/* no header */
 		break;
 	case Hrisc:	/* aif for risc os */
@@ -815,7 +822,7 @@ asmb(void)
 		lputl(0xe3300000);		/* nop */
 		lputl(0xe3300000);		/* nop */
 		break;
-	case Hlinux:
+	Elfput:
 		/* elf arm */
 		eh = getElfEhdr();
 		fo = HEADR;
@@ -851,8 +858,16 @@ asmb(void)
 			sh->type = SHT_PROGBITS;
 			sh->flags = SHF_ALLOC;
 			sh->addralign = 1;
-			if(interpreter == nil)
+			if(interpreter == nil) {
-				interpreter = linuxdynld;
+				switch(HEADTYPE) {
+				case Hlinux:
+					interpreter = linuxdynld;
+					break;
+				case Hfreebsd:
+					interpreter = freebsddynld;
+					break;
+				}
+			}
 			resoff -= elfinterp(sh, startva, resoff, interpreter);
 
 			ph = newElfPhdr();
@@ -886,6 +901,31 @@ asmb(void)
 		/* Dynamic linking sections */
 		if(!debug['d']) {	/* -d suppresses dynamic loader format */
 			/* S headers for dynamic linking */
+			dynsym = eh->shnum;
+			sh = newElfShdr(elfstr[ElfStrDynsym]);
+			sh->type = SHT_DYNSYM;
+			sh->flags = SHF_ALLOC;
+			sh->entsize = ELF32SYMSIZE;
+			sh->addralign = 4;
+			sh->link = dynsym+1;	// dynstr
+			// sh->info = index of first non-local symbol (number of local symbols)
+			shsym(sh, lookup(".dynsym", 0));
+
+			sh = newElfShdr(elfstr[ElfStrDynstr]);
+			sh->type = SHT_STRTAB;
+			sh->flags = SHF_ALLOC;
+			sh->addralign = 1;
+			shsym(sh, lookup(".dynstr", 0));
+
+			sh = newElfShdr(elfstr[ElfStrRelPlt]);
+			sh->type = SHT_REL;
+			sh->flags = SHF_ALLOC;
+			sh->entsize = ELF32RELSIZE;
+			sh->addralign = 4;
+			sh->link = dynsym;
+			sh->info = eh->shnum;	// .plt
+			shsym(sh, lookup(".rel.plt", 0));
+
 			// ARM ELF needs .plt to be placed before .got
 			sh = newElfShdr(elfstr[ElfStrPlt]);
 			sh->type = SHT_PROGBITS;
@@ -908,22 +948,6 @@ asmb(void)
 			sh->addralign = 4;
 			shsym(sh, lookup(".got", 0));
 
-			dynsym = eh->shnum;
-			sh = newElfShdr(elfstr[ElfStrDynsym]);
-			sh->type = SHT_DYNSYM;
-			sh->flags = SHF_ALLOC;
-			sh->entsize = ELF32SYMSIZE;
-			sh->addralign = 4;
-			sh->link = dynsym+1;	// dynstr
-			// sh->info = index of first non-local symbol (number of local symbols)
-			shsym(sh, lookup(".dynsym", 0));
-
-			sh = newElfShdr(elfstr[ElfStrDynstr]);
-			sh->type = SHT_STRTAB;
-			sh->flags = SHF_ALLOC;
-			sh->addralign = 1;
-			shsym(sh, lookup(".dynstr", 0));
-
 			if(elfverneed) {
 				sh = newElfShdr(elfstr[ElfStrGnuVersion]);
 				sh->type = SHT_GNU_VERSYM;
@@ -1029,6 +1053,11 @@ asmb(void)
 		eh->ident[EI_CLASS] = ELFCLASS32;
 		eh->ident[EI_DATA] = ELFDATA2LSB;
 		eh->ident[EI_VERSION] = EV_CURRENT;
+		switch(HEADTYPE) {
+		case Hfreebsd:
+			eh->ident[EI_OSABI] = ELFOSABI_FREEBSD;
+			break;
+		}
 
 		eh->type = ET_EXEC;
 		eh->machine = EM_ARM;

src/cmd/5l/obj.c

@@ -52,6 +52,7 @@ Header headers[] = {
    "ixp1200", Hixp1200,
    "ipaq", Hipaq,
    "linux", Hlinux,
+   "freebsd", Hfreebsd,
    0, 0
 };
 
@@ -152,7 +153,7 @@ main(int argc, char *argv[])
 	libinit();
 
 	if(HEADTYPE == -1)
-		HEADTYPE = Hlinux;
+		HEADTYPE = headtype(goos);
 	switch(HEADTYPE) {
 	default:
 		diag("unknown -H option");
@@ -212,6 +213,7 @@ main(int argc, char *argv[])
 			INITRND = 1024;
 		break;
 	case Hlinux:	/* arm elf */
+	case Hfreebsd:
 		debug['d'] = 0;	// with dynamic linking
 		tlsoffset = -8; // hardcoded number, first 4-byte word for g, and then 4-byte word for m
 		                // this number is known to ../../pkg/runtime/cgo/gcc_linux_arm.c