From 693b6bdda9206a5910c552a3997b2df5480d6947 Mon Sep 17 00:00:00 2001 From: Dean Deng Date: Tue, 7 Apr 2020 16:16:31 -0700 Subject: Correctly distinguish between seekable and non-seekable host fds. Check whether an fd is seekable by calling the seek syscall and examining the return value, instead of checking the file type, which is inaccurate. PiperOrigin-RevId: 305361593 --- pkg/sentry/fsimpl/host/host.go | 42 ++++++++++++++++++++---------------------- 1 file changed, 20 insertions(+), 22 deletions(-) (limited to 'pkg') diff --git a/pkg/sentry/fsimpl/host/host.go b/pkg/sentry/fsimpl/host/host.go index 7d9dcd4c9..97fa7f7ab 100644 --- a/pkg/sentry/fsimpl/host/host.go +++ b/pkg/sentry/fsimpl/host/host.go @@ -74,31 +74,34 @@ func ImportFD(mnt *vfs.Mount, hostFD int, isTTY bool) (*vfs.FileDescription, err } // Retrieve metadata. - var s syscall.Stat_t - if err := syscall.Fstat(hostFD, &s); err != nil { + var s unix.Stat_t + if err := unix.Fstat(hostFD, &s); err != nil { return nil, err } fileMode := linux.FileMode(s.Mode) fileType := fileMode.FileType() - // Pipes, character devices, and sockets. - isStream := fileType == syscall.S_IFIFO || fileType == syscall.S_IFCHR || fileType == syscall.S_IFSOCK + + // Determine if hostFD is seekable. If not, this syscall will return ESPIPE + // (see fs/read_write.c:llseek), e.g. for pipes, sockets, and some character + // devices. + _, err := unix.Seek(hostFD, 0, linux.SEEK_CUR) + seekable := err != syserror.ESPIPE i := &inode{ hostFD: hostFD, - isStream: isStream, + seekable: seekable, isTTY: isTTY, canMap: canMap(uint32(fileType)), ino: fs.NextIno(), mode: fileMode, - // For simplicity, set offset to 0. Technically, we should - // only set to 0 on files that are not seekable (sockets, pipes, etc.), - // and use the offset from the host fd otherwise. + // For simplicity, set offset to 0. Technically, we should use the existing + // offset on the host if the file is seekable. offset: 0, } - // These files can't be memory mapped, assert this. - if i.isStream && i.canMap { + // Non-seekable files can't be memory mapped, assert this. + if !i.seekable && i.canMap { panic("files that can return EWOULDBLOCK (sockets, pipes, etc.) cannot be memory mapped") } @@ -124,12 +127,12 @@ type inode struct { // This field is initialized at creation time and is immutable. hostFD int - // isStream is true if the host fd points to a file representing a stream, + // seekable is false if the host fd points to a file representing a stream, // e.g. a socket or a pipe. Such files are not seekable and can return // EWOULDBLOCK for I/O operations. // // This field is initialized at creation time and is immutable. - isStream bool + seekable bool // isTTY is true if this file represents a TTY. // @@ -481,8 +484,7 @@ func (f *fileDescription) Release() { // PRead implements FileDescriptionImpl. func (f *fileDescription) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts vfs.ReadOptions) (int64, error) { i := f.inode - // TODO(b/34716638): Some char devices do support offsets, e.g. /dev/null. - if i.isStream { + if !i.seekable { return 0, syserror.ESPIPE } @@ -492,8 +494,7 @@ func (f *fileDescription) PRead(ctx context.Context, dst usermem.IOSequence, off // Read implements FileDescriptionImpl. func (f *fileDescription) Read(ctx context.Context, dst usermem.IOSequence, opts vfs.ReadOptions) (int64, error) { i := f.inode - // TODO(b/34716638): Some char devices do support offsets, e.g. /dev/null. - if i.isStream { + if !i.seekable { n, err := readFromHostFD(ctx, i.hostFD, dst, -1, opts.Flags) if isBlockError(err) { // If we got any data at all, return it as a "completed" partial read @@ -538,8 +539,7 @@ func readFromHostFD(ctx context.Context, hostFD int, dst usermem.IOSequence, off // PWrite implements FileDescriptionImpl. func (f *fileDescription) PWrite(ctx context.Context, src usermem.IOSequence, offset int64, opts vfs.WriteOptions) (int64, error) { i := f.inode - // TODO(b/34716638): Some char devices do support offsets, e.g. /dev/null. - if i.isStream { + if !i.seekable { return 0, syserror.ESPIPE } @@ -549,8 +549,7 @@ func (f *fileDescription) PWrite(ctx context.Context, src usermem.IOSequence, of // Write implements FileDescriptionImpl. func (f *fileDescription) Write(ctx context.Context, src usermem.IOSequence, opts vfs.WriteOptions) (int64, error) { i := f.inode - // TODO(b/34716638): Some char devices do support offsets, e.g. /dev/null. - if i.isStream { + if !i.seekable { n, err := writeToHostFD(ctx, i.hostFD, src, -1, opts.Flags) if isBlockError(err) { err = syserror.ErrWouldBlock @@ -593,8 +592,7 @@ func writeToHostFD(ctx context.Context, hostFD int, src usermem.IOSequence, offs // allow directory fds to be imported at all. func (f *fileDescription) Seek(_ context.Context, offset int64, whence int32) (int64, error) { i := f.inode - // TODO(b/34716638): Some char devices do support seeking, e.g. /dev/null. - if i.isStream { + if !i.seekable { return 0, syserror.ESPIPE } -- cgit v1.2.3