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// Copyright 2019 The gVisor Authors.
//
// 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 flipcall
import (
"fmt"
"math/bits"
"os"
"syscall"
"gvisor.dev/gvisor/pkg/abi/linux"
"gvisor.dev/gvisor/pkg/memutil"
)
var (
pageSize = os.Getpagesize()
pageMask = pageSize - 1
)
func init() {
if bits.OnesCount(uint(pageSize)) != 1 {
// This is depended on by roundUpToPage().
panic(fmt.Sprintf("system page size (%d) is not a power of 2", pageSize))
}
if uintptr(pageSize) < PacketHeaderBytes {
// This is required since Endpoint.Init() imposes a minimum packet
// window size of 1 page.
panic(fmt.Sprintf("system page size (%d) is less than packet header size (%d)", pageSize, PacketHeaderBytes))
}
}
// PacketWindowDescriptor represents a packet window, a range of pages in a
// shared memory file that is used to exchange packets between partner
// Endpoints.
type PacketWindowDescriptor struct {
// FD is the file descriptor representing the shared memory file.
FD int
// Offset is the offset into the shared memory file at which the packet
// window begins.
Offset int64
// Length is the size of the packet window in bytes.
Length int
}
// PacketWindowLengthForDataCap returns the minimum packet window size required
// to accommodate datagrams of the given size in bytes.
func PacketWindowLengthForDataCap(dataCap uint32) int {
return roundUpToPage(int(dataCap) + int(PacketHeaderBytes))
}
func roundUpToPage(x int) int {
return (x + pageMask) &^ pageMask
}
// A PacketWindowAllocator owns a shared memory file, and allocates packet
// windows from it.
type PacketWindowAllocator struct {
fd int
nextAlloc int64
fileSize int64
}
// Init must be called on zero-value PacketWindowAllocators before first use.
// If it succeeds, Destroy() must be called once the PacketWindowAllocator is
// no longer in use.
func (pwa *PacketWindowAllocator) Init() error {
fd, err := memutil.CreateMemFD("flipcall_packet_windows", linux.MFD_CLOEXEC|linux.MFD_ALLOW_SEALING)
if err != nil {
return fmt.Errorf("failed to create memfd: %v", err)
}
// Apply F_SEAL_SHRINK to prevent either party from causing SIGBUS in the
// other by truncating the file, and F_SEAL_SEAL to prevent either party
// from applying F_SEAL_GROW or F_SEAL_WRITE.
if _, _, e := syscall.RawSyscall(syscall.SYS_FCNTL, uintptr(fd), linux.F_ADD_SEALS, linux.F_SEAL_SHRINK|linux.F_SEAL_SEAL); e != 0 {
syscall.Close(fd)
return fmt.Errorf("failed to apply memfd seals: %v", e)
}
pwa.fd = fd
return nil
}
// NewPacketWindowAllocator is a convenience function that returns an
// initialized PacketWindowAllocator allocated on the heap.
func NewPacketWindowAllocator() (*PacketWindowAllocator, error) {
var pwa PacketWindowAllocator
if err := pwa.Init(); err != nil {
return nil, err
}
return &pwa, nil
}
// Destroy releases resources owned by pwa. This invalidates file descriptors
// previously returned by pwa.FD() and pwd.Allocate().
func (pwa *PacketWindowAllocator) Destroy() {
syscall.Close(pwa.fd)
}
// FD represents the file descriptor of the shared memory file backing pwa.
func (pwa *PacketWindowAllocator) FD() int {
return pwa.fd
}
// Allocate allocates a new packet window of at least the given size and
// returns a PacketWindowDescriptor representing it.
//
// Preconditions: size > 0.
func (pwa *PacketWindowAllocator) Allocate(size int) (PacketWindowDescriptor, error) {
if size <= 0 {
return PacketWindowDescriptor{}, fmt.Errorf("invalid size: %d", size)
}
// Page-align size to ensure that pwa.nextAlloc remains page-aligned.
size = roundUpToPage(size)
if size <= 0 {
return PacketWindowDescriptor{}, fmt.Errorf("size %d overflows after rounding up to page size", size)
}
end := pwa.nextAlloc + int64(size) // overflow checked by ensureFileSize
if err := pwa.ensureFileSize(end); err != nil {
return PacketWindowDescriptor{}, err
}
start := pwa.nextAlloc
pwa.nextAlloc = end
return PacketWindowDescriptor{
FD: pwa.fd,
Offset: start,
Length: size,
}, nil
}
func (pwa *PacketWindowAllocator) ensureFileSize(min int64) error {
if min <= 0 {
return fmt.Errorf("file size would overflow")
}
if pwa.fileSize >= min {
return nil
}
newSize := 2 * pwa.fileSize
if newSize == 0 {
newSize = int64(pageSize)
}
for newSize < min {
newNewSize := newSize * 2
if newNewSize <= 0 {
return fmt.Errorf("file size would overflow")
}
newSize = newNewSize
}
if err := syscall.Ftruncate(pwa.fd, newSize); err != nil {
return fmt.Errorf("ftruncate failed: %v", err)
}
pwa.fileSize = newSize
return nil
}
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