On 23/08/17 10:40, Ian Kent wrote: > On 23/08/17 10:32, Ian Kent wrote: >> On 23/08/17 09:06, NeilBrown wrote: >>> On Mon, Aug 21 2017, Ian Kent wrote: >>> >>>>> >>>>> A mount isn't triggered by kern_path(pathname, 0, &path). >>>>> That '0' would need to include one of >>>>> LOOKUP_PARENT | LOOKUP_DIRECTORY | >>>>> LOOKUP_OPEN | LOOKUP_CREATE | LOOKUP_AUTOMOUNT >>>>> >>>>> to trigger an automount (otherwise you just get -EISDIR). >>>> >>>> It's perfectly sensible to think that but there is a case where a >>>> a mount is triggered when using kern_path(). >>>> >>>> The EISDIR return occurs for positive dentrys, negative dentrys >>>> will still trigger an automount (which is autofs specific, >>>> indirect mount map using nobrowse option, the install default). >>> >>> Ok, I understand this better now. This difference between direct and >>> indirect mounts is slightly awkward. It is visible from user-space, but >>> not elegant to document. >>> When you use O_PATH to open a direct automount that has not already been >>> triggered, the open returns the underlying directory (and fstatfs >>> confirms that it is AUTOFS_SUPER_MAGIC). When you use O_PATH on >>> an indirect automount, it *will* trigger the automount when "nobrowse" is >>> in effect, but it won't when "browse" is in effect. >> >> That inconsistency has bothered me for quite a while now. >> >> It was carried over from the autofs module behavior when automounting >> support was added to the VFS. What's worse is it prevents the use of >> the AT_NO_AUTOMOUNT flag from working properly with fstatat(2) and with >> statx(). >> >> There is some risk in changing that so it does work but it really does >> need to work to enable userspace to not trigger an automount by using >> this flag. >> >> So that's (hopefully) going to change soonish, see: >> http://ozlabs.org/~akpm/mmotm/broken-out/autofs-fix-at_no_automount-not-being-honored.patch >> >> The result should be that stat family calls don't trigger automounts except >> for fstatat(2) and statx() which will require the AT_NO_AUTOMOUNT flag. >> >>> >>> So we cannot just say "O_PATH doesn't trigger automounts", which is >>> essentially what I said in >>> >>> https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=97a45d02e6671482e8b2cdcce3951930bf6bdb94 >>> >>> It might be possible to modify automount so that it was more consistent >>> - i.e. if the point is triggered by a mkdir has been done, just to the >>> mkdir. If it is triggered after a mkdir has been done, do the mount. I >>> guess that might be racy, and in any case is hard to justify. >>> >>> Maybe I should change it to be about "direct automounts", and add a note >>> that indirect automounts aren't so predictable. >> >> Right and the semantics should be much more consistent in the near future. >> I hope (and expect) this semantic change won't cause problems. >> >>> >>> But back to my original issue of wanting to discard >>> kern_path_mountpoint, what would you think of the following approach - >>> slight revised from before. >>> >>> Thanks, >>> NeilBrown >>> >>> diff --git a/fs/autofs4/autofs_i.h b/fs/autofs4/autofs_i.h >>> index beef981aa54f..7663ea82e68d 100644 >>> --- a/fs/autofs4/autofs_i.h >>> +++ b/fs/autofs4/autofs_i.h >>> @@ -135,10 +135,13 @@ static inline struct autofs_info *autofs4_dentry_ino(struct dentry *dentry) >>> /* autofs4_oz_mode(): do we see the man behind the curtain? (The >>> * processes which do manipulations for us in user space sees the raw >>> * filesystem without "magic".) >>> + * A process performing certain ioctls can get temporary oz status. >>> */ >>> +extern struct task_struct *autofs_tmp_oz; >>> static inline int autofs4_oz_mode(struct autofs_sb_info *sbi) >>> { >>> - return sbi->catatonic || task_pgrp(current) == sbi->oz_pgrp; >>> + return sbi->catatonic || task_pgrp(current) == sbi->oz_pgrp || >>> + autofs_tmp_oz == current; >>> } >>> >>> struct inode *autofs4_get_inode(struct super_block *, umode_t); >>> diff --git a/fs/autofs4/dev-ioctl.c b/fs/autofs4/dev-ioctl.c >>> index dd9f1bebb5a3..d76401669a20 100644 >>> --- a/fs/autofs4/dev-ioctl.c >>> +++ b/fs/autofs4/dev-ioctl.c >>> @@ -200,6 +200,20 @@ static int autofs_dev_ioctl_protosubver(struct file *fp, >>> return 0; >>> } >>> >>> +struct task_struct *autofs_tmp_oz; >>> +int kern_path_oz(const char *pathname, int flags, struct path *path) >>> +{ >>> + static DEFINE_MUTEX(autofs_oz); >>> + int err; >>> + >>> + mutex_lock(&autofs_oz); >>> + autofs_tmp_oz = current; >>> + err = kern_path(pathname, flags, path); >>> + autofs_tmp_oz = NULL; >>> + mutex_unlock(&autofs_oz); >>> + return err; >>> +} >>> + >> >> It's simple enough but does look like it will attract criticism as being >> a hack! >> >> The kern_path_locked() function is very similar to what was originally >> done, along with code to look down the mount stack (rather than up the >> way it does now) to get the mount point. In this case, to be valid the >> dentry can't be a symlink so that fits kern_path_locked() too. > > Oh wait, that __lookup_hash() tries too hard to resolve the dentry, > that won't quite work, and maybe d_lookup() can't be used safely in > this context either .... Umm .. d_lookup() does look ok so maybe path_parentat() + d_lookup() would be ok. > >> >> So maybe it is worth going back to the way it was in the beginning and >> be done with it .... OTOH Al must have had a reason for changing the >> way it was done that I didn't get. >> >>> /* Find the topmost mount satisfying test() */ >>> static int find_autofs_mount(const char *pathname, >>> struct path *res, >>> @@ -209,7 +223,8 @@ static int find_autofs_mount(const char *pathname, >>> struct path path; >>> int err; >>> >>> - err = kern_path_mountpoint(AT_FDCWD, pathname, &path, 0); >>> + err = kern_path_oz(pathname, 0, &path); >>> + >>> if (err) >>> return err; >>> err = -ENOENT; >>> @@ -552,8 +567,7 @@ static int autofs_dev_ioctl_ismountpoint(struct file *fp, >>> >>> if (!fp || param->ioctlfd == -1) { >>> if (autofs_type_any(type)) >>> - err = kern_path_mountpoint(AT_FDCWD, >>> - name, &path, LOOKUP_FOLLOW); >>> + err = kern_path_oz(name, LOOKUP_FOLLOW, &path); >>> else >>> err = find_autofs_mount(name, &path, >>> test_by_type, &type); >>> >> >
