cmd/internal/obj, cmd/link: record ABIs and aliases in Go obj files

This repurposes the "version" field of a symbol reference in the Go
object file format to be an ABI field. Currently, this is just 0 or 1
depending on whether the symbol is static (the linker turns it into a
different internal version number), so it's already only tenuously a
symbol version. We change this to be -1 for static symbols and
otherwise by the ABI number.

This also adds a separate list of ABI alias symbols to be recorded in
the object file. The ABI aliases must be a separate list and not just
part of the symbol definitions because it's possible to have a symbol
defined in one package and the alias "defined" in a different package.
For example, this can happen if a symbol is defined in assembly in one
package and stubbed in a different package. The stub triggers the
generation of the ABI alias, but in a different package from the
definition.

For #27539.

Change-Id: I015c9fe54690c027de6ef77e22b5585976a01587
Reviewed-on: https://go-review.googlesource.com/c/147157
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

1 files changed, 21 insertions, 8 deletions

diff --git a/src/cmd/internal/goobj/read.go b/src/cmd/internal/goobj/read.go
index 2d618eefa5..2081098ca8 100644
--- a/src/cmd/internal/goobj/read.go
+++ b/src/cmd/internal/goobj/read.go
@@ -288,18 +288,31 @@ func (r *objReader) readSymID() SymID {
 }
 
 func (r *objReader) readRef() {
-	name, vers := r.readString(), r.readInt()
+	name, abiOrStatic := r.readString(), r.readInt()
 
 	// In a symbol name in an object file, "". denotes the
 	// prefix for the package in which the object file has been found.
 	// Expand it.
 	name = strings.ReplaceAll(name, `"".`, r.pkgprefix)
 
-	// An individual object file only records version 0 (extern) or 1 (static).
-	// To make static symbols unique across all files being read, we
-	// replace version 1 with the version corresponding to the current
-	// file number. The number is incremented on each call to parseObject.
-	if vers != 0 {
+	// The ABI field records either the ABI or -1 for static symbols.
+	//
+	// To distinguish different static symbols with the same name,
+	// we use the symbol "version". Version 0 corresponds to
+	// global symbols, and each file has a unique version > 0 for
+	// all of its static symbols. The version is incremented on
+	// each call to parseObject.
+	//
+	// For global symbols, we currently ignore the ABI.
+	//
+	// TODO(austin): Record the ABI in SymID. Since this is a
+	// public API, we'll have to keep Version as 0 and record the
+	// ABI in a new field (which differs from how the linker does
+	// this, but that's okay). Show the ABI in things like
+	// objdump.
+	var vers int64
+	if abiOrStatic == -1 {
+		// Static symbol
 		vers = r.p.MaxVersion
 	}
 	r.p.SymRefs = append(r.p.SymRefs, SymID{name, vers})
@@ -487,7 +500,7 @@ func (r *objReader) parseObject(prefix []byte) error {
 	// TODO: extract OS + build ID if/when we need it
 
 	r.readFull(r.tmp[:8])
-	if !bytes.Equal(r.tmp[:8], []byte("\x00\x00go19ld")) {
+	if !bytes.Equal(r.tmp[:8], []byte("\x00go112ld")) {
 		return r.error(errCorruptObject)
 	}
 
@@ -602,7 +615,7 @@ func (r *objReader) parseObject(prefix []byte) error {
 	}
 
 	r.readFull(r.tmp[:7])
-	if !bytes.Equal(r.tmp[:7], []byte("\xffgo19ld")) {
+	if !bytes.Equal(r.tmp[:7], []byte("go112ld")) {
 		return r.error(errCorruptObject)
 	}