cmd/internal/buildid: skip over GNU build ID from buildid computation

This is similar to CL 618597, but for GNU build ID on ELF. This
makes it possible to enable "-B gobuildid" by default on ELF.

Updates #41004.
For #63934.

Change-Id: I4e663a27a2f7824bce994c783fe6d9ce8d1a395a
Reviewed-on: https://go-review.googlesource.com/c/go/+/618600
Reviewed-by: Than McIntosh <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: cherrymui

src/cmd/internal/buildid/rewrite.go

@@ -9,6 +9,7 @@ import (
 	"cmd/internal/codesign"
 	imacho "cmd/internal/macho"
 	"crypto/sha256"
+	"debug/elf"
 	"debug/macho"
 	"fmt"
 	"io"
@@ -185,14 +186,26 @@ func findHostBuildID(r io.Reader) (offset int64, size int64, ok bool) {
 	if !ok {
 		return 0, 0, false
 	}
-	// TODO: handle ELF GNU build ID.
-	f, err := macho.NewFile(ra)
+
+	ef, err := elf.NewFile(ra)
+	if err == nil {
+		// ELF file. Find GNU build ID section.
+		sect := ef.Section(".note.gnu.build-id")
+		if sect == nil {
+			return 0, 0, false
+		}
+		// Skip over the 3-word note "header" and "GNU\x00".
+		return int64(sect.Offset + 16), int64(sect.Size - 16), true
+	}
+
+	mf, err := macho.NewFile(ra)
 	if err != nil {
 		return 0, 0, false
 	}
 
-	reader := imacho.NewLoadCmdReader(io.NewSectionReader(ra, 0, 1<<63-1), f.ByteOrder, imacho.FileHeaderSize(f))
-	for i := uint32(0); i < f.Ncmd; i++ {
+	// Mach-O file. Find LC_UUID load command.
+	reader := imacho.NewLoadCmdReader(io.NewSectionReader(ra, 0, 1<<63-1), mf.ByteOrder, imacho.FileHeaderSize(mf))
+	for i := uint32(0); i < mf.Ncmd; i++ {
 		cmd, err := reader.Next()
 		if err != nil {
 			break