Standalone repro for CL 21005 regression

repro.go

package syscall

import (
	"errors"
	"unsafe"
)

var	ErrBadStat   = errors.New("malformed stat buffer")

const (
	STATMAX    = 65535
	STATFIXLEN = 49
)

const (
	SYS_PREAD       = 50
)

// readdirnames returns the names of files inside the directory represented by dirfd.
func readdirnames(dirfd int) (names []string, err error) {
	names = make([]string, 0, 100)
	var buf [STATMAX]byte

	for {
		n, e := Read(dirfd, buf[:])
		if e != nil {
			return nil, e
		}
		if n == 0 {
			break
		}
		for i := 0; i < n; {
			m, _ := gbit16(buf[i:])
			m += 2

			if m < STATFIXLEN {
				return nil, ErrBadStat
			}

			s, _, ok := gstring(buf[i+41:])
			if !ok {
				return nil, ErrBadStat
			}
			names = append(names, s)
			i += int(m)
		}
	}
	return
}

// gbit16 reads a 16-bit number in little-endian order from b and returns it with the remaining slice of b.
func gbit16(b []byte) (uint16, []byte) {
	return uint16(b[0]) | uint16(b[1])<<8, b[2:]
}

// gstring reads a string from b, prefixed with a 16-bit length in little-endian order.
// It returns the string with the remaining slice of b and a boolean. If the length is
// greater than the number of bytes in b, the boolean will be false.
func gstring(b []byte) (string, []byte, bool) {
	n, b := gbit16(b)
	if int(n) > len(b) {
		return "", b, false
	}
	return string(b[:n]), b[n:], true
}

func Read(fd int, p []byte) (n int, err error) {
	return Pread(fd, p, -1)
}

func Pread(fd int, p []byte, offset int64) (n int, err error) {
	var _p0 unsafe.Pointer
	if len(p) > 0 {
		_p0 = unsafe.Pointer(&p[0])
	} else {
		_p0 = unsafe.Pointer(&_zero)
	}
	r0, _, e1 := Syscall6(SYS_PREAD, uintptr(fd), uintptr(_p0), uintptr(len(p)), uintptr(offset), uintptr(offset>>32), 0)
	n = int(r0)
	if int32(r0) == -1 {
		err = e1
	}
	return
}

func Syscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err ErrorString)

// Single-word zero for use when we need a valid pointer to 0 bytes.
// See mksyscall.pl.
var _zero uintptr 

// ErrorString implements Error's String method by returning itself.
type ErrorString string

func (e ErrorString) Error() string { return string(e) }