// Copyright 2018 Google Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package linux

import (
	"crypto/rand"
	"io"
	"math"

	"gvisor.googlesource.com/gvisor/pkg/sentry/arch"
	"gvisor.googlesource.com/gvisor/pkg/sentry/kernel"
	"gvisor.googlesource.com/gvisor/pkg/sentry/safemem"
	"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
	"gvisor.googlesource.com/gvisor/pkg/syserror"
)

const (
	_GRND_NONBLOCK = 0x1
	_GRND_RANDOM   = 0x2
)

// GetRandom implements the linux syscall getrandom(2).
//
// In a multi-tenant/shared environment, the only valid implementation is to
// fetch data from the urandom pool, otherwise starvation attacks become
// possible. The urandom pool is also expected to have plenty of entropy, thus
// the GRND_RANDOM flag is ignored. The GRND_NONBLOCK flag does not apply, as
// the pool will already be initialized.
func GetRandom(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	addr := args[0].Pointer()
	length := args[1].SizeT()
	flags := args[2].Int()

	// Flags are checked for validity but otherwise ignored. See above.
	if flags & ^(_GRND_NONBLOCK|_GRND_RANDOM) != 0 {
		return 0, nil, syserror.EINVAL
	}

	if length > math.MaxInt32 {
		length = math.MaxInt32
	}
	ar, ok := addr.ToRange(uint64(length))
	if !ok {
		return 0, nil, syserror.EFAULT
	}

	// "If the urandom source has been initialized, reads of up to 256 bytes
	// will always return as many bytes as requested and will not be
	// interrupted by signals. No such guarantees apply for larger buffer
	// sizes." - getrandom(2)
	min := int(length)
	if min > 256 {
		min = 256
	}
	n, err := t.MemoryManager().CopyOutFrom(t, usermem.AddrRangeSeqOf(ar), safemem.FromIOReader{&randReader{-1, min}}, usermem.IOOpts{
		AddressSpaceActive: true,
	})
	if n >= int64(min) {
		return uintptr(n), nil, nil
	}
	return 0, nil, err
}

// randReader is a io.Reader that handles partial reads from rand.Reader.
type randReader struct {
	done int
	min  int
}

// Read implements io.Reader.Read.
func (r *randReader) Read(dst []byte) (int, error) {
	if r.done >= r.min {
		return rand.Reader.Read(dst)
	}
	min := r.min - r.done
	if min > len(dst) {
		min = len(dst)
	}
	return io.ReadAtLeast(rand.Reader, dst, min)
}