cmd/go: fix Go buildid reading on Solaris

TestNoteReading fails on Solaris with linkmode=external due to some
assumptions made about how ELF .note sections are written by some
linkers.

On current versions of Solaris and older derivatives, SHF_ALLOC is
intentionally ignored for .note sections unless the .note section is
assigned to the text segment via a mapfile.  Also, if .note sections
are assigned to the text segment, no PT_NOTE program header will be
created thwarting Go's attempts at attempting to quickly find the
.note.

Furthermore, Go assumes that the relevant note segment will be placed
early in the file while the Solaris linker currently places the note
segment last in the file, additionally thwarting Go's optimisation
attempts that read only the first 16KB of the file to find the
buildid.

The fix is to detect when the note section is outside of the first
16KB of the file and then fallback to additionally reading that
section of the file.  This way, in future versions of Solaris when
this linking behaviour is changed, the fast path will always succeed
and we'll only be slower if it fails; likewise, any other linker that
does this will also just work.

Fixes #12178

Change-Id: I61c1dc3f744ae3ad63938386d2ace8a432c0efe1
Reviewed-on: https://go-review.googlesource.com/14210
Run-TryBot: Aram Hăvărneanu <aram@mgk.ro>
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>

src/cmd/go/note.go

@@ -69,11 +69,11 @@ func readELFNote(filename, name string, typ int32) ([]byte, error) {
 
 var elfGoNote = []byte("Go\x00\x00")
 
-// readELFGoBuildID the Go build ID string from an ELF binary.
-// The Go build ID is stored in a note described by an ELF PT_NOTE prog header.
-// The caller has already opened filename, to get f, and read the first 4 kB out, in data.
+// The Go build ID is stored in a note described by an ELF PT_NOTE prog
+// header.  The caller has already opened filename, to get f, and read
+// at least 4 kB out, in data.
 func readELFGoBuildID(filename string, f *os.File, data []byte) (buildid string, err error) {
-	// Assume the note content is in the first 4 kB, already read.
+	// Assume the note content is in the data, already read.
 	// Rewrite the ELF header to set shnum to 0, so that we can pass
 	// the data to elf.NewFile and it will decode the Prog list but not
 	// try to read the section headers and the string table from disk.
@@ -95,11 +95,31 @@ func readELFGoBuildID(filename string, f *os.File, data []byte) (buildid string,
 		return "", &os.PathError{Path: filename, Op: "parse", Err: err}
 	}
 	for _, p := range ef.Progs {
-		if p.Type != elf.PT_NOTE || p.Off >= uint64(len(data)) || p.Off+p.Filesz >= uint64(len(data)) || p.Filesz < 16 {
+		if p.Type != elf.PT_NOTE || p.Filesz < 16 {
 			continue
 		}
 
-		note := data[p.Off : p.Off+p.Filesz]
+		var note []byte
+		if p.Off+p.Filesz < uint64(len(data)) {
+			note = data[p.Off : p.Off+p.Filesz]
+		} else {
+			// For some linkers, such as the Solaris linker,
+			// the buildid may not be found in data (which
+			// likely contains the first 16kB of the file)
+			// or even the first few megabytes of the file
+			// due to differences in note segment placement;
+			// in that case, extract the note data manually.
+			_, err = f.Seek(int64(p.Off), 0)
+			if err != nil {
+				return "", err
+			}
+
+			note = make([]byte, p.Filesz)
+			_, err = io.ReadFull(f, note)
+			if err != nil {
+				return "", err
+			}
+		}
 		nameSize := ef.ByteOrder.Uint32(note)
 		valSize := ef.ByteOrder.Uint32(note[4:])
 		tag := ef.ByteOrder.Uint32(note[8:])

src/cmd/go/note_test.go

@@ -33,9 +33,6 @@ func TestNoteReading(t *testing.T) {
 		// no external linking
 		t.Logf("no external linking - skipping linkmode=external test")
 
-	case "solaris":
-		t.Logf("skipping - golang.org/issue/12178")
-
 	default:
 		tg.run("build", "-ldflags", "-buildid="+buildID+" -linkmode=external", "-o", tg.path("hello.exe"), tg.path("hello.go"))
 		id, err := main.ReadBuildIDFromBinary(tg.path("hello.exe"))

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

@@ -1725,10 +1725,6 @@ func doelf() {
 			Addstring(shstrtab, ".note.go.pkg-list")
 			Addstring(shstrtab, ".note.go.deps")
 		}
-
-		if buildid != "" {
-			Addstring(shstrtab, ".note.go.buildid")
-		}
 	}
 
 	hasinitarr := Linkshared