2013/2/5 J. Bruce Fields <bfie...@fieldses.org>: > On Tue, Feb 05, 2013 at 03:33:21PM +0400, Pavel Shilovsky wrote: >> 2013/1/31 J. Bruce Fields <bfie...@fieldses.org>: >> > On Thu, Jan 17, 2013 at 08:52:09PM +0400, Pavel Shilovsky wrote: >> >> This patchset adds support of O_DENY* flags for Linux fs layer. These >> >> flags can be used by any application that needs share reservations to >> >> organize a file access. VFS already has some sort of this capability - >> >> now it's done through flock/LOCK_MAND mechanis, but that approach is >> >> non-atomic. This patchset build new capabilities on top of the existing >> >> one but doesn't bring any changes into the flock call semantic. >> >> >> >> These flags can be used by NFS (built-in-kernel) and CIFS (Samba) servers >> >> and Wine applications through VFS (for local filesystems) or CIFS/NFS >> >> modules. This will help when e.g. Samba and NFS server share the same >> >> directory for Windows and Linux users or Wine applications use Samba/NFS >> >> share to access the same data from different clients. >> >> >> >> According to the previous discussions the most problematic question is >> >> how to prevent situations like DoS attacks where e.g /lib/liba.so file >> >> can be open with DENYREAD, or smth like this. That's why one extra flag >> >> O_DENYMAND is added. It indicates to underlying layer that an application >> >> want to use O_DENY* flags semantic. It allows us not affect native Linux >> >> applications (that don't use O_DENYMAND flag) - so, these flags (and the >> >> semantic of open syscall that they bring) are used only for those >> >> applications that really want it proccessed that way. >> > >> > Maybe that's good enough. A mount flag might be simpler and give >> > consistent enforcement for all users. >> > >> >> >> >> So, we have four new flags: >> >> O_DENYREAD - to prevent other opens with read access, >> >> O_DENYWRITE - to prevent other opens with write access, >> >> O_DENYDELETE - to prevent delete operations (this flag is not implemented >> >> in VFS and NFS part and only suitable for CIFS module), >> >> O_DENYMAND - to switch on/off three flags above. >> > >> > It would be useful to have some really careful documentation of how >> > these are meant to work. Maybe try updating the open man page? >> >> Yes, that's a good idea. Do you mean smth like this? >> >> O_DENYMAND - used to inforce a mandatory share reservation scheme of >> the file access. If this flag is passed, >> the open fails with -ETXTBSY in following cases: >> >> 1) if O_DENYREAD flag is specified and there is another open with >> O_DENYMAND flag and READ access to the file; >> 2) if O_DENYWRITE flag is specified and there is another open with >> O_DENYMAND flag and WRITE access to the file; >> 3) if READ access is requested and there is another open with >> O_DENYMAND and O_DENYREAD flags; >> 4) if WRITE access is requested and there is another open with >> O_DENYMAND and O_DENYWRITE flags; >> >> Also, if O_DENYDELETE flag is specified and the open succeded, any >> further unlink operation will fail with -ETXTBSY untill this open is >> closed. Now this flag is processed by CIFS filesystems only. > > Do you need to document interactions with flock() as well?
Yes, this needs to be documented, thanks. I am going to propose ver2 of this patchset (with regards to your suggestions) - will add this part to open man page patch too. -- Best regards, Pavel Shilovsky.
