From: Cherry Zhang <cherryyz@google.com>
Date: Sat, 25 Apr 2020 00:22:39 +0000 (-0400)
Subject: [dev.link] cmd/link: stop setting ReadOnly attribute in late stage
X-Git-Tag: go1.15beta1~270^2^2~18
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=2a874562bf7e1831bca5bb2c89dd1a9d0beac6d2;p=gostls13.git

[dev.link] cmd/link: stop setting ReadOnly attribute in late stage

The ReadOnly attribute was used to do copy on write when applying
relocations to symbols with read-only backing stores. Now that we
always apply relocations in the output buffer (mmap or heap), it
is always writeable. No need to tamper with the ReadOnly
attribute anymore.

Wasm is an exception, where we don't copy symbol contents to the
output buffer first. Do copy-on-write there.

This is in preparation of converting reloc to using the loader.

Change-Id: I15e53b7c162b9124e6689dfd8eb45cbe2ffd7153
Reviewed-on: https://go-review.googlesource.com/c/go/+/229991
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
---

diff --git a/src/cmd/link/internal/ld/data.go b/src/cmd/link/internal/ld/data.go
index 5ab7661d60..0937540433 100644
--- a/src/cmd/link/internal/ld/data.go
+++ b/src/cmd/link/internal/ld/data.go
@@ -140,11 +140,14 @@ func relocsym(target *Target, ldr *loader.Loader, err *ErrorReporter, syms *Arch
 	if len(s.R) == 0 {
 		return
 	}
-	if s.Attr.ReadOnly() {
+	if target.IsWasm() && s.Attr.ReadOnly() {
 		// The symbol's content is backed by read-only memory.
 		// Copy it to writable memory to apply relocations.
+		// Only need to do this on Wasm. On other platforms we
+		// apply relocations to the output buffer, which is
+		// always writeable.
 		s.P = append([]byte(nil), s.P...)
-		s.Attr.Set(sym.AttrReadOnly, false)
+		// No need to unset AttrReadOnly because it will not be used.
 	}
 	for ri := int32(0); ri < int32(len(s.R)); ri++ {
 		r := &s.R[ri]
@@ -579,8 +582,10 @@ func (ctxt *Link) reloc() {
 	syms := &ctxt.ArchSyms
 	wg.Add(3)
 	go func() {
-		for _, s := range ctxt.Textp {
-			relocsym(target, ldr, reporter, syms, s)
+		if !ctxt.IsWasm() { // On Wasm, text relocations are applied in Asmb2.
+			for _, s := range ctxt.Textp {
+				relocsym(target, ldr, reporter, syms, s)
+			}
 		}
 		wg.Done()
 	}()
@@ -2503,8 +2508,6 @@ func compressSyms(ctxt *Link, syms []*sym.Symbol) []byte {
 		if len(s.R) != 0 && wasReadOnly {
 			relocbuf = append(relocbuf[:0], s.P...)
 			s.P = relocbuf
-			// TODO: This function call needs to be parallelized when the loader wavefront gets here.
-			s.Attr.Set(sym.AttrReadOnly, false)
 		}
 		relocsym(target, ldr, reporter, archSyms, s)
 		if _, err := z.Write(s.P); err != nil {
@@ -2526,7 +2529,6 @@ func compressSyms(ctxt *Link, syms []*sym.Symbol) []byte {
 		// contents, in which case we still need s.P.
 		if len(s.R) != 0 && wasReadOnly {
 			s.P = oldP
-			s.Attr.Set(sym.AttrReadOnly, wasReadOnly)
 			for i := range s.R {
 				s.R[i].Done = false
 			}
diff --git a/src/cmd/link/internal/ld/outbuf.go b/src/cmd/link/internal/ld/outbuf.go
index cee589fc05..40e02cb7cf 100644
--- a/src/cmd/link/internal/ld/outbuf.go
+++ b/src/cmd/link/internal/ld/outbuf.go
@@ -291,5 +291,4 @@ func (out *OutBuf) WriteSym(s *sym.Symbol) {
 	copy(buf[pos:], s.P)
 	out.off += n
 	s.P = buf[pos : pos+n]
-	s.Attr.Set(sym.AttrReadOnly, false)
 }