On Thu, Oct 17, 2019 at 01:23:57PM +0200, Miklos Szeredi wrote: > On Thu, Oct 17, 2019 at 12:05 PM Stefan Hajnoczi <stefa...@redhat.com> wrote: > > > > On Wed, Oct 16, 2019 at 07:37:52PM +0900, Misono Tomohiro wrote: > > > Hello, > > > > > > I test xattr operation on virtiofs using xfstest generic/062 > > > (with -o xattr option and XFS backend) and see some problems. > > > > > > These patches fixes the two of the problems. > > > > > > The remaining problems are: > > > 1. we cannot xattr to block device created by mknod > > > which does not have actual device (since open in virtiofsd fails) > > > 2. we cannot xattr to symbolic link > > > > > > I don't think 1 is a big problem but can we fix 2? > > > > Sorry, I don't know the answer. Maybe it would be necessary to add a > > new O_SYMLINK open flag to open(2) so that fgetxattr()/fsetxattr() > > operations can be performed. A kernel change like that would take some > > time to get accepted upstream and shipped by distros, but it might be > > the only way since the current syscall interface doesn't seem to offer a > > way to do this. > > The real problem is that open() on a non-regular, non-directory file > may have side effects (unless O_PATH is used). These patches try to > paper over that, but the fact is: opening special files from a file > server is forbidden. > > I see why this is being done, and it's not easy to fix properly > without the ..at() versions of these syscalls. One idea is to fork() > + fchdir(lo->proc_self_fd) + ..xattr(). Another related idea is to do > a unshare(CLONE_FS) after each thread's startup (will pthread library > balk? I don't know) so that it's safe to do fchdir(lo->proc_self_fd) > + ...xattr() + fchdir(lo->root_fd).
Looking at the f*xattr() code in the kernel, it doesn't really care about the file descriptor, it wants the inode instead. So the O_SYMLINK idea I mentioned could also be called O_XATTR and be similar to O_PATH, except that only f*xattr() calls are allowed on this file descriptor. Stefan
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