Add equivalents to FMODE_PREAD/PWRITE to VFS2.

This is mostly required for PipeTest_OffsetCalls.

The options are DenyPRead/PWrite rather than AllowPRead/PWrite since, in Linux
terms, fs/open.c:do_dentry_open sets FMODE_PREAD|FMODE_PWRITE unconditionally
(although it allows filesystem implementations of open to unset these flags),
so they're set for most FDs; it's usually FDs created outside of open(2) that
don't get them, e.g.:

- Syscall-created pipes (fs/pipe.c:create_pipe_files =>
  fs/file_table.c:alloc_file_pseudo)

- Epoll instances (fs/eventpoll.c:do_epoll_create =>
  fs/anon_inodes.c:anon_inode_getfile => alloc_file_pseudo)

- Sockets (net/socket.c:sock_alloc_file => alloc_file_pseudo)

This CL adds the flags to epoll instances; a subsequent CL reworks the VFS2
implementation of pipe FDs to be filesystem-independent and adds the flags
there, and sockets aren't implemented yet.

Updates #1035

PiperOrigin-RevId: 304506434

pkg/sentry/vfs/epoll.go

@@ -99,6 +99,8 @@ func (vfs *VirtualFilesystem) NewEpollInstanceFD() (*FileDescription, error) {
 		interest: make(map[epollInterestKey]*epollInterest),
 	}
 	if err := ep.vfsfd.Init(ep, linux.O_RDWR, vd.Mount(), vd.Dentry(), &FileDescriptionOptions{
+		DenyPRead:         true,
+		DenyPWrite:        true,
 		UseDentryMetadata: true,
 	}); err != nil {
 		return nil, err

pkg/sentry/vfs/file_description.go

@@ -84,6 +84,13 @@ type FileDescriptionOptions struct {
 	// usually only the case if O_DIRECT would actually have an effect.
 	AllowDirectIO bool
 
+	// If DenyPRead is true, calls to FileDescription.PRead() return ESPIPE.
+	DenyPRead bool
+
+	// If DenyPWrite is true, calls to FileDescription.PWrite() return
+	// ESPIPE.
+	DenyPWrite bool
+
 	// If UseDentryMetadata is true, calls to FileDescription methods that
 	// interact with file and filesystem metadata (Stat, SetStat, StatFS,
 	// Listxattr, Getxattr, Setxattr, Removexattr) are implemented by calling
@@ -306,6 +313,7 @@ type FileDescriptionImpl interface {
 	// - If opts.Flags specifies unsupported options, PRead returns EOPNOTSUPP.
 	//
 	// Preconditions: The FileDescription was opened for reading.
+	// FileDescriptionOptions.DenyPRead == false.
 	PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts ReadOptions) (int64, error)
 
 	// Read is similar to PRead, but does not specify an offset.
@@ -337,6 +345,7 @@ type FileDescriptionImpl interface {
 	// EOPNOTSUPP.
 	//
 	// Preconditions: The FileDescription was opened for writing.
+	// FileDescriptionOptions.DenyPWrite == false.
 	PWrite(ctx context.Context, src usermem.IOSequence, offset int64, opts WriteOptions) (int64, error)
 
 	// Write is similar to PWrite, but does not specify an offset, which is
@@ -515,6 +524,9 @@ func (fd *FileDescription) EventUnregister(e *waiter.Entry) {
 // offset, and returns the number of bytes read. PRead is permitted to return
 // partial reads with a nil error.
 func (fd *FileDescription) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts ReadOptions) (int64, error) {
+	if fd.opts.DenyPRead {
+		return 0, syserror.ESPIPE
+	}
 	if !fd.readable {
 		return 0, syserror.EBADF
 	}
@@ -533,6 +545,9 @@ func (fd *FileDescription) Read(ctx context.Context, dst usermem.IOSequence, opt
 // offset, and returns the number of bytes written. PWrite is permitted to
 // return partial writes with a nil error.
 func (fd *FileDescription) PWrite(ctx context.Context, src usermem.IOSequence, offset int64, opts WriteOptions) (int64, error) {
+	if fd.opts.DenyPWrite {
+		return 0, syserror.ESPIPE
+	}
 	if !fd.writable {
 		return 0, syserror.EBADF
 	}