Method PRead

pkg/sentry/fsimpl/fuse/regular_file.go:114–193

PRead implements vfs.FileDescriptionImpl.PRead.

(ctx context.Context, dst usermem.IOSequence, offset int64, opts vfs.ReadOptions)

// PRead implements vfs.FileDescriptionImpl.PRead.
func (fd *regularFileFD) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts vfs.ReadOptions) (int64, error) {
	if offset < 0 {
		return 0, linuxerr.EINVAL
	}

	// Check that flags are supported.
	//
	// TODO(gvisor.dev/issue/2601): Support select preadv2 flags.
	if opts.Flags&^linux.RWF_HIPRI != 0 {
		return 0, linuxerr.EOPNOTSUPP
	}

	size := dst.NumBytes()
	if size == 0 {
		// Early return if count is 0.
		return 0, nil
	}
	if size > math.MaxUint32 {
		// FUSE only supports uint32 for size.
		// Overflow.
		return 0, linuxerr.EINVAL
	}

	// TODO(gvisor.dev/issue/3678): Add direct IO support.

	inode := fd.inode()
	inode.attrMu.Lock()
	defer inode.attrMu.Unlock()

	// Reading beyond EOF, update file size if outdated.
	if uint64(offset+size) > inode.size.Load() {
		if err := inode.reviseAttr(ctx, linux.FUSE_GETATTR_FH, fd.Fh); err != nil {
			return 0, err
		}
		// If the offset after update is still too large, return error.
		if uint64(offset) >= inode.size.Load() {
			return 0, io.EOF
		}
	}

	// Truncate the read with updated file size.
	fileSize := inode.size.Load()
	if uint64(offset+size) > fileSize {
		size = int64(fileSize) - offset
	}

	buffers, n, err := inode.fs.ReadInPages(ctx, fd, uint64(offset), uint32(size))
	if err != nil {
		return 0, err
	}

	// TODO(gvisor.dev/issue/3237): support indirect IO (e.g. caching),
	// store the bytes that were read ahead.

	// Update the number of bytes to copy for short read.
	if n < uint32(size) {
		size = int64(n)
	}