cmd/link: make it easy to find binary versions

It is useful to be able to dig the Go version out of a binary,
even a stripped binary. rsc.io/goversion does this for x86
binaries by disassembling the binary to find runtime/proc.go's
startup-time reference to runtime.buildVersion's address.
That approach is quite fragile: the implementation doesn't
work for non-x86 and must be updated as the generated
code changes.

rsc.io/goversion finds the module version string by looking
for random 16-byte framing around the actual string.
This is less fragile but fairly kludgy and requires scanning
the entire data segment.

cmd/buildid finds the build ID by looking for an ELF note
or else falling back to scanning the beginning of the text
segment for a magic string. This has proved quite reliable
and doesn't require scanning much of the binary.

This CL makes it possible to find the Go and module versions
using a scan more like the build ID scan: a symbol early in
the writable data segment starts with a magic number and
then has pointers to the two string variables.

Setting up for "go version <binary>".

For #31624.

Change-Id: I78ea8c52fe1686b5cc5a829ca5f198104d10ebf0
Reviewed-on: https://go-review.googlesource.com/c/go/+/173342
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/link/internal/ld/data.go

@@ -1302,6 +1302,7 @@ func (ctxt *Link) dodata() {
 
 	// Writable data sections that do not need any specialized handling.
 	writable := []sym.SymKind{
+		sym.SBUILDINFO,
 		sym.SELFSECT,
 		sym.SMACHO,
 		sym.SMACHOGOT,
@@ -1963,6 +1964,39 @@ func (ctxt *Link) textbuildid() {
 	ctxt.Textp[0] = s
 }
 
+func (ctxt *Link) buildinfo() {
+	if ctxt.linkShared || ctxt.BuildMode == BuildModePlugin {
+		// -linkshared and -buildmode=plugin get confused
+		// about the relocations in go.buildinfo
+		// pointing at the other data sections.
+		// The version information is only available in executables.
+		return
+	}
+
+	s := ctxt.Syms.Lookup(".go.buildinfo", 0)
+	s.Attr |= sym.AttrReachable
+	s.Type = sym.SBUILDINFO
+	s.Align = 16
+	// The \xff is invalid UTF-8, meant to make it less likely
+	// to find one of these accidentally.
+	const prefix = "\xff Go buildinf:" // 14 bytes, plus 2 data bytes filled in below
+	data := make([]byte, 32)
+	copy(data, prefix)
+	data[len(prefix)] = byte(ctxt.Arch.PtrSize)
+	data[len(prefix)+1] = 0
+	if ctxt.Arch.ByteOrder == binary.BigEndian {
+		data[len(prefix)+1] = 1
+	}
+	s.P = data
+	s.Size = int64(len(s.P))
+	s1 := ctxt.Syms.Lookup("runtime.buildVersion", 0)
+	s2 := ctxt.Syms.Lookup("runtime.modinfo", 0)
+	s.R = []sym.Reloc{
+		{Off: 16, Siz: uint8(ctxt.Arch.PtrSize), Type: objabi.R_ADDR, Sym: s1},
+		{Off: 16 + int32(ctxt.Arch.PtrSize), Siz: uint8(ctxt.Arch.PtrSize), Type: objabi.R_ADDR, Sym: s2},
+	}
+}
+
 // assign addresses to text
 func (ctxt *Link) textaddress() {
 	addsection(ctxt.Arch, &Segtext, ".text", 05)

src/cmd/link/internal/ld/elf.go

@@ -1439,6 +1439,7 @@ func (ctxt *Link) doelf() {
 	Addstring(shstrtab, ".data")
 	Addstring(shstrtab, ".bss")
 	Addstring(shstrtab, ".noptrbss")
+	Addstring(shstrtab, ".go.buildinfo")
 
 	// generate .tbss section for dynamic internal linker or external
 	// linking, so that various binutils could correctly calculate
@@ -1485,6 +1486,7 @@ func (ctxt *Link) doelf() {
 		if ctxt.UseRelro() {
 			Addstring(shstrtab, elfRelType+".data.rel.ro")
 		}
+		Addstring(shstrtab, elfRelType+".go.buildinfo")
 
 		// add a .note.GNU-stack section to mark the stack as non-executable
 		Addstring(shstrtab, ".note.GNU-stack")

src/cmd/link/internal/ld/macho.go

@@ -828,7 +828,7 @@ func machoShouldExport(ctxt *Link, s *sym.Symbol) bool {
 	if strings.HasPrefix(s.Name, "go.link.pkghash") {
 		return true
 	}
-	return s.Type >= sym.SELFSECT // only writable sections
+	return s.Type >= sym.SFirstWritable // only writable sections
 }
 
 func machosymtab(ctxt *Link) {

src/cmd/link/internal/ld/main.go

@@ -237,6 +237,7 @@ func Main(arch *sys.Arch, theArch Arch) {
 	ctxt.findfunctab()
 	ctxt.typelink()
 	ctxt.symtab()
+	ctxt.buildinfo()
 	ctxt.dodata()
 	order := ctxt.address()
 	dwarfcompress(ctxt)

src/cmd/link/internal/sym/symkind.go

@@ -81,6 +81,8 @@ const (
 	SPCLNTAB
 
 	// Writable sections.
+	SFirstWritable
+	SBUILDINFO
 	SELFSECT
 	SMACHO
 	SMACHOGOT