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

1 files changed, 25 insertions, 6 deletions

diff --git a/src/cmd/ld/data.c b/src/cmd/ld/data.c
index b70d4636cc..9f7ae0c409 100644
--- a/src/cmd/ld/data.c
+++ b/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;