On Sat, Jun 02, 2018 at 04:45:21PM +0100, David Howells wrote: > Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > > > TBH, I would probably prefer separate mount_setattr(2) for that kind > > of work, with something like > > int mount_setattr(int dirfd, const char *path, int flags, int attr) > > *not* opening any files. > > flags: > > AT_EMPTY_PATH, AT_NO_AUTOMOUNT, AT_RECURSIVE > > I would call these MOUNT_SETATTR_* rather than AT_*. Why? AT_EMPTY_PATH/AT_NO_AUTOMOUNT are common with other ...at() syscalls; AT_RECURSIVE - maybe, but it's still more like AT_... namespace fodder, IMO. > > attr: > > MOUNT_SETATTR_DEV (1<<0) > > MOUNT_SETATTR_NODEV (1<<0)|(1<<1) > > MOUNT_SETATTR_EXEC (1<<2) > > MOUNT_SETATTR_NOEXEC (1<<2)|(1<<3) > > MOUNT_SETATTR_SUID (1<<4) > > MOUNT_SETATTR_NOSUID (1<<4)|(1<<5) > > MOUNT_SETATTR_RW (1<<6) > > MOUNT_SETATTR_RO (1<<6)|(1<<7) > > MOUNT_SETATTR_RELATIME (1<<8) > > MOUNT_SETATTR_NOATIME (1<<8)|(1<<9) > > MOUNT_SETATTR_NODIRATIME (1<<8)|(2<<9) > > MOUNT_SETATTR_STRICTATIME (1<<8)|(3<<9) > > MOUNT_SETATTR_PRIVATE (1<<11) > > MOUNT_SETATTR_SHARED (1<<11)|(1<<12) > > MOUNT_SETATTR_SLAVE (1<<11)|(2<<12) > > MOUNT_SETATTR_UNBINDABLE (1<<11)|(3<<12) > > So, I like this generally, some notes though: > > I wonder if this should be two separate parameters, a mask and the settings? > I'm not sure that's worth it since some of the mask bits would cover multiple > settings. Nah, better put those bits in the same word, as in above. Here bits 0, 2, 4, 6, 8 and 11 tell which attributes are to be modified, with values to set living in bits 1, 3, 5, 7, 9--10 and 12--13. Look at the constants above.. > Also, should NODIRATIME be separate from the other *ATIME flags? I do also > like treating some of these settings as enumerations rather than a set of > bits. Huh? That's precisely what I'm doing there: bit 8 is "want to change atime settings", bits 9 and 10 hold a 4-element enumeration (rel/no/nodir/strict). Similar for propagation settings (bit 11 indicates that we want to set those, bits 12 and 13 - 4-element enum)... > I would make the prototype: > > int mount_setattr(int dirfd, const char *path, > unsigned int flags, unsigned int attr, > void *reserved5); > > Further, do we want to say you can either change the propagation type *or* > reconfigure the mountpoint restrictions, but not both at the same time? Why? MOUNT_SETATTR_PRIVATE | MOUNT_NOATIME | MOUNT_SUID, i.e. 00101100010000, i.e. 0xb10 for "turn nosuid off, switch atime polcy to noatime, change propagation to private, leave everything else as-is"... And for fsck sake, what's that "void *reserved5" for? > > With either openat() used as in this series, or explicit > > int open_tree(int dirfd, const char *path, int flags) > > Maybe open_mount(), grab_mount() or pick_mount()? > > I wonder if fsopen()/fspick() should be create_fs()/open_fs()... > > > returning a descriptor, with flags being > > AT_EMPTY_PATH, AT_NO_AUTOMOUNT, AT_RECURSIVE, AT_CLONE > > with AT_RECURSIVE without AT_CLONE being an error. > > You also need an O_CLOEXEC equivalent. Point. > I would make it: > > OPEN_TREE_CLOEXEC 0x00000001 Why not O_CLOEXEC, as with epoll_create()/timerfd_create()/etc.? > OPEN_TREE_EMPTY_PATH 0x00000002 > OPEN_TREE_FOLLOW_SYMLINK 0x00000004 > OPEN_TREE_NO_AUTOMOUNT 0x00000008 Why? How are those different from normal AT_EMPTY_PATH/AT_NO_AUTOMOUNT? > OPEN_TREE_CLONE 0x00000010 > OPEN_TREE_RECURSIVE 0x00000020 > > adding the follow-symlinks so that you don't grab a symlink target by > accident. (Can you actually mount on top of a symlink?) You can't - mount(2) uses LOOKUP_FOLLOW for mountpoint (well, user_path(), actually). > > Hell, might even add AT_UMOUNT for "open root and detach, to be dissolved on > > close", incompatible with AT_CLONE. > > Cute. Guess you could do: > > fd = open_mount(..., OPEN_TREE_DETACH); > mount_setattr(fd, "", > MOUNT_SETATTR_EMPTY_PATH, > MOUNT_SETATTR_NOSUID, NULL); > move_mount(fd, "", ...);
