On Sat, 2019-04-27 at 16:16 -0400, Amir Goldstein wrote: > [adding back samba/nfs and fsdevel] > cc'ing Pavel too -- he did a bunch of work in this area a few years ago. > On Fri, Apr 26, 2019 at 6:22 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > On Fri, 2019-04-26 at 10:50 -0400, J. Bruce Fields wrote: > > > On Fri, Apr 26, 2019 at 04:11:00PM +0200, Amir Goldstein wrote: > > > > On Fri, Apr 26, 2019, 4:00 PM J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: > > > > > > > > > On Fri, Apr 26, 2019 at 03:50:46PM +0200, Amir Goldstein wrote: > > > > > > On Fri, Feb 8, 2019, 5:03 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > > > > > > Share/deny open semantics are pretty similar across NFS and SMB (by > > > > > > > design, really). If you intend to solve that use-case, what you really > > > > > > > want is whole-file, shared/exclusive locks that are set atomically with > > > > > > > the open call. O_EXLOCK and O_SHLOCK seem like a reasonable fit there. > > > > > > > > > > > > > > Then you could have SMB and NFS servers set these flags when opening > > > > > > > files, and deal with the occasional denial at open time. Other > > > > > > > applications won't be aware of them of course, but that's probably fine > > > > > > > for most use-cases where you want this sort of protocol interop. > > > > > > > > > > > > Sorry for posting off list. Airport emails... > > > > > > I looked at implemeting O_EXLOCK and O_SHLOCK and it looks doable. > > > > > > > > > > > > I was wondering if there is an inherent reason not to allow an exclusive > > > > > > lock on a file that is open read-only. > > > > > > > > > > > > Samba seems to need it and currently flock and ofd locks won't allow it. > > > > > > Do you thing it will be ok to allow it with O_EXLOCK? > > > > > > > > > > Somebody could deny everyone access to a shared resource that everyone > > > > > needs to make progress, like /etc/passwd or a shared library. > > > > > > > > > > Have you looked at Pavel Shilovsky's O_DENY patches? He had the feature > > > > > off by default, with a mount option provided to turn it on. > > > > > > > > > > > > > O_EXLOCK is advisory. It only aquired flock or ofd lock atomically with > > > > open. > > > > > > Whoops, got it. > > > > > > Is that really adequate for open share locks, though? > > > > > > I assumed that Windows apps depend on the assumption that they're > > > mandatory. So e.g. if you can get a DENY_READ open on a shared library > > > then you know you can update it without the risk of making someone else > > > crash. > > > > > > > I think this is (slightly) better than doing it internally like we do > > today and would give you coherent locking between NFS and SMB. Other > > applications wouldn't see them, but for a NAS-style deployment, that's > > probably ok. > > > > We can do a little bit better. > We can make sure that O_DENY_WRITE (named for convenience) fails > if file is currently open for write by anyone and similarly for O_DENY_READ. > But if we cannot deny future non-cooperative opens what's the point?.... > As you said in another mail, the main interest here is in getting NFS+SMB semantics right. If the exported filesystem is _only_ available via NFS+SMB, then do we need to deny non-cooperative opens? > > Any open by samba or nfsd would need to start setting O_SHLOCK, and deny > > mode opens would have to set O_EXLOCK. We would actually need 2 per > > inode though (one for read and one for write). > > > > ...the point is that O_DENY_NONE does not need to be implemented with > a new type of lock object (O_WR_SHLOCK) its enough that it checks there > are no relevant exclusive locks and the then inode->i_writecount and > inode->i_readcount already provide enough context to cooperate with > O_DENY_WRITE and O_DENY_READ. > That would work, if the goal is to have deny modes affect all opens. We could also do this on the opt-in basis that I was suggesting with a new set of counters in struct file_lock_context. > I need to see if incrementing inode->i_readcount on O_RDWR opens is > possible (right now it only counts O_RDONLY opens). > > > I think these should probably be in their own "namespace" too. They > > could use the same semantics as flock, but should sit on their own list > > in file_lock_context. > > > > I would much rather that they didn't. The reason is that new open flags > are a backward compat problem. The way I want to solve it is this API: > > // On new kernel this will acquire OFD F_WRLCK atomically... > fd = open(..., O_RDWR | O_EXLOCK); > // ...check if it did acquire OFD lock > fcntl(fd, F_OFD_GETLK, ...); > > We'd need at least one new l_type F_EX_RDLCK and maybe also a new > semantic F_EX_RDWRLCK, although similar in conflicts to F_WRLCK it can be > acquired without FMODE_WRITE. Though I personally thing we can do without > it if the only way to acquire F_WRLCK on readonly file is via new open flag. > I don't think that will work at all. Share/deny modes are entirely orthogonal to byte-range locks in both NFS and SMB. Consider: Two clients open a file with O_RDWR | | O_SHARE_WRITE | O_SHARE_READ. One of them now wants to set byte-range write lock on the file. That should be allowed, but now it'll be denied, because the other client will effectively hold a whole-file readlock on it. There is also the problem that read and write deny modes are orthogonal to one other, so you have to have a way to deal with them independently. I'd suggest an API like this: // open read/write and deny read/write fd = open(..., O_RDWR | O_DENY_READ | O_DENY_WRITE); // test for flags with F_GETFL flags = fcntl(fd, F_GETFL); That would also allow you to use F_SETFL to change those flags on an existing fd. > > That said, we could also look at a vfs-level mount option that would > > make the kernel enforce these for any opener. That could also be useful, > > and shouldn't be too hard to implement. Maybe even make it a vfsmount- > > level option (like -o ro is). > > > > Yeh, I am humbly going to leave this struggle to someone else. > Not important enough IMO and completely independent effort to the > advisory atomic open&lock API. Having the kernel allow setting deny modes on any open call is a non- starter, for the reasons Bruce outlined earlier. This _must_ be restricted in some fashion or we'll be opening up a ginormous DoS mechanism. My proposal was to make this only be enforced by applications that explicitly opt-in by setting O_SH*/O_EX* flags. It wouldn't be too difficult to also allow them to be enforced on a per-fs basis via mount option or something. Maybe we could expand the meaning of '-o mand' ? How would you propose that we restrict this? > > If you're denied, what error should you get back when you try to open > > it? It should be something distinct. We may even want to add new error > > codes for this. > > IMO EBUSY does the job. Its distinct because open is not expected > to return EBUSY for regular files/dirs and when open is expected to > return EBUSY for blockdev its for the exact same use case (i.e. > exclusive write open is acquired by userspace tools). That works for me. We should probably have a close look at the work that Pavel did several years ago too. It has almost certainly bitrotted by now, but it may serve as a starting point (and he may he may have valuable input here). -- Jeff Layton <jlayton@xxxxxxxxxx>
