Forward named pipe creation to the gofer

The backing 9p server must allow named pipe creation, which the runsc
fsgofer currently does not.

There are small changes to the overlay here. GetFile may block when
opening a named pipe, which can cause a deadlock:

1. open(O_RDONLY) -> copyMu.Lock() -> GetFile()
2. open(O_WRONLY) -> copyMu.Lock() -> Deadlock

A named pipe usable for writing must already be on the upper filesystem,
but we are still taking copyMu for write when checking for upper. That
can be changed to a read lock to fix the common case.

However, a named pipe on the lower filesystem would still deadlock in
open(O_WRONLY) when it tries to actually perform copy up (which would
simply return EINVAL). Move the copy up type check before taking copyMu
for write to avoid this.

p9 must be modified, as it was incorrectly removing the file mode when
sending messages on the wire.

PiperOrigin-RevId: 249154033
Change-Id: Id6637130e567b03758130eb6c7cdbc976384b7d6

1 files changed, 16 insertions, 4 deletions

diff --git a/pkg/sentry/fs/gofer/path.go b/pkg/sentry/fs/gofer/path.go
index 6ed50a77f..092f8b586 100644
--- a/pkg/sentry/fs/gofer/path.go
+++ b/pkg/sentry/fs/gofer/path.go
@@ -282,10 +282,22 @@ func (i *inodeOperations) Bind(ctx context.Context, dir *fs.Inode, name string,
 	return childDir, nil
 }
 
-// CreateFifo implements fs.InodeOperations.CreateFifo. Gofer nodes do not support the
-// creation of fifos and always returns EPERM.
-func (*inodeOperations) CreateFifo(context.Context, *fs.Inode, string, fs.FilePermissions) error {
-	return syscall.EPERM
+// CreateFifo implements fs.InodeOperations.CreateFifo.
+func (i *inodeOperations) CreateFifo(ctx context.Context, dir *fs.Inode, name string, perm fs.FilePermissions) error {
+	if len(name) > maxFilenameLen {
+		return syserror.ENAMETOOLONG
+	}
+
+	owner := fs.FileOwnerFromContext(ctx)
+	mode := p9.FileMode(perm.LinuxMode()) | p9.ModeNamedPipe
+
+	// N.B. FIFOs use major/minor numbers 0.
+	if _, err := i.fileState.file.mknod(ctx, name, mode, 0, 0, p9.UID(owner.UID), p9.GID(owner.GID)); err != nil {
+		return err
+	}
+
+	i.touchModificationAndStatusChangeTime(ctx, dir)
+	return nil
 }
 
 // Remove implements InodeOperations.Remove.