On 24/08/17 11:21, NeilBrown wrote: > On Wed, Aug 23 2017, 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 .... >> > > Why do you think that __look_hash() tries too hard? > It does call into the filesystem ->lookup() if the name isn't in the > cache, which probably isn't strictly needed, but that isn't harmful and > the current code does that. Only that the execution path can encounter the negative dentry case in follow_automount(). Also, my saying that the dentry can't be a symlink is wrong. For the covered fd case it's reasonable but for the mount point check ioctl the path could be anything, eg. a symlink to an automount dentry, and I want that ioctl to be callable by anyone. > > Some the following seems sensible to me (though I haven't tested it). > > Thanks, > NeilBrown > > > diff --git a/fs/autofs4/dev-ioctl.c b/fs/autofs4/dev-ioctl.c > index dd9f1bebb5a3..859c198d0163 100644 > --- a/fs/autofs4/dev-ioctl.c > +++ b/fs/autofs4/dev-ioctl.c > @@ -208,12 +208,16 @@ static int find_autofs_mount(const char *pathname, > { > struct path path; > int err; > + struct dentry *de, *parent; > + > + de = kern_path_locked(pathname, &path); > + if (IS_ERR(de)) > + return PTR_ERR(de); > + parent = path.dentry; > + path.dentry = de; > > - err = kern_path_mountpoint(AT_FDCWD, pathname, &path, 0); > - if (err) > - return err; > err = -ENOENT; > - while (path.dentry == path.mnt->mnt_root) { > + do { > if (path.dentry->d_sb->s_magic == AUTOFS_SUPER_MAGIC) { > if (test(&path, data)) { > path_get(&path); > @@ -222,10 +226,11 @@ static int find_autofs_mount(const char *pathname, > break; > } > } > - if (!follow_up(&path)) > - break; > - } > + } while (follow_down_one(&path)); > + > path_put(&path); > + inode_unlock(d_inode(parent)); > + dput(parent); > return err; > } > >
