cmd/link/internal/ld: don't panic on short buildid

When the linker was written in C, command line arguments were passed
around as null-terminated byte arrays which encouraged checking
characters one at a time. In Go, that can easily lead to
out-of-bounds panics.

Use the more idiomatic strings.HasPrefix when checking cmd/link's -B
argument to avoid the panic, and replace the manual hex decode with
use of the encoding/hex package.

Fixes #14636

Change-Id: I45f765bbd8cf796fee1a9a3496178bf76b117827
Reviewed-on: https://go-review.googlesource.com/20211
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -8,6 +8,7 @@ import (
 	"cmd/internal/obj"
 	"crypto/sha1"
 	"encoding/binary"
+	"encoding/hex"
 	"fmt"
 	"path/filepath"
 	"sort"
@@ -1196,45 +1197,30 @@ func elfwriteopenbsdsig() int {
 }
 
 func addbuildinfo(val string) {
-	var j int
-
-	if val[0] != '0' || val[1] != 'x' {
+	if !strings.HasPrefix(val, "0x") {
 		Exitf("-B argument must start with 0x: %s", val)
 	}
 
 	ov := val
 	val = val[2:]
-	i := 0
-	var b int
-	for val != "" {
-		if len(val) == 1 {
-			Exitf("-B argument must have even number of digits: %s", ov)
-		}
-
-		b = 0
-		for j = 0; j < 2; j, val = j+1, val[1:] {
-			b *= 16
-			if val[0] >= '0' && val[0] <= '9' {
-				b += int(val[0]) - '0'
-			} else if val[0] >= 'a' && val[0] <= 'f' {
-				b += int(val[0]) - 'a' + 10
-			} else if val[0] >= 'A' && val[0] <= 'F' {
-				b += int(val[0]) - 'A' + 10
-			} else {
-				Exitf("-B argument contains invalid hex digit %c: %s", val[0], ov)
-			}
-		}
 
 	const maxLen = 32
-		if i >= maxLen {
+	if hex.DecodedLen(len(val)) > maxLen {
 		Exitf("-B option too long (max %d digits): %s", maxLen, ov)
 	}
 
-		buildinfo = append(buildinfo, uint8(b))
-		i++
+	b, err := hex.DecodeString(val)
+	if err != nil {
+		if err == hex.ErrLength {
+			Exitf("-B argument must have even number of digits: %s", ov)
+		}
+		if inv, ok := err.(hex.InvalidByteError); ok {
+			Exitf("-B argument contains invalid hex digit %c: %s", byte(inv), ov)
+		}
+		Exitf("-B argument contains invalid hex: %s", ov)
 	}
 
-	buildinfo = buildinfo[:i]
+	buildinfo = b
 }
 
 // Build info note

test/fixedbugs/issue14636.go

+// +build !nacl,!android,!darwin darwin,!arm
+// run
+
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"bytes"
+	"log"
+	"os/exec"
+	"strings"
+)
+
+func main() {
+	checkLinkOutput("", "-B argument must start with 0x")
+	checkLinkOutput("0", "-B argument must start with 0x")
+	checkLinkOutput("0x", "usage")
+	checkLinkOutput("0x0", "-B argument must have even number of digits")
+	checkLinkOutput("0x00", "usage")
+	checkLinkOutput("0xYZ", "-B argument contains invalid hex digit")
+	checkLinkOutput("0x"+strings.Repeat("00", 32), "usage")
+	checkLinkOutput("0x"+strings.Repeat("00", 33), "-B option too long (max 32 digits)")
+}
+
+func checkLinkOutput(buildid string, message string) {
+	cmd := exec.Command("go", "tool", "link", "-B", buildid)
+	out, err := cmd.CombinedOutput()
+	if err == nil {
+		log.Fatalf("expected cmd/link to fail")
+	}
+
+	firstLine := string(bytes.SplitN(out, []byte("\n"), 2)[0])
+	if strings.HasPrefix(firstLine, "panic") {
+		log.Fatalf("cmd/link panicked:\n%s", out)
+	}
+
+	if !strings.Contains(firstLine, message) {
+		log.Fatalf("cmd/link output did not include expected message %q: %s", message, firstLine)
+	}
+}