[Virtio-fs] [PATCH 1/3] virtiofsd: Find original inode ID of mount points
Vivek Goyal
vgoyal at redhat.com
Wed May 26 18:50:17 UTC 2021
On Wed, May 26, 2021 at 02:13:24PM -0400, Vivek Goyal wrote:
> On Wed, May 12, 2021 at 02:55:42PM +0200, Max Reitz wrote:
> > Mount point directories represent two inodes: On one hand, they are a
> > normal directory on their parent filesystem. On the other, they are the
> > root node of the filesystem mounted there. Thus, they have two inode
> > IDs.
> >
> > Right now, we only report the latter inode ID (i.e. the inode ID of the
> > mounted filesystem's root node). This is fine once the guest has
> > auto-mounted a submount there (so this inode ID goes with a device ID
> > that is distinct from the parent filesystem), but before the auto-mount,
> > they have the device ID of the parent and the inode ID for the submount.
> > This is problematic because this is likely exactly the same
> > st_dev/st_ino combination as the parent filesystem's root node. This
> > leads to problems for example with `find`, which will thus complain
> > about a filesystem loop if it has visited the parent filesystem's root
> > node before, and then refuse to descend into the submount.
> >
> > There is a way to find the mount directory's original inode ID, and that
> > is to readdir(3) the parent directory, look for the mount directory, and
> > read the dirent.d_ino field. Using this, we can let lookup and
> > readdirplus return that original inode ID, which the guest will thus
> > show until the submount is auto-mounted.
>
> > (Then, it will invoke getattr
> > and that stat(2) call will return the inode ID for the submount.)
>
> Hi Max,
>
> How are we sure that GETATTR() will always be called and that will
> allow us to return inode number in mounted filesystem (instead of
> parent filesystem). I thought GETATTR will be called only if cached
> attrs have expired. (1 second default for cache=auto). Otherwise
> stat() will fill inode->i_no from cache and return. And I am afraid
> that in that case we will return inode number from parent fs,
> instead of mounted fs.
>
> Say following sequence of events happens pretty fast one after the
> other. Say /mnt/virtiofs/foo is a mount point in server but client
> has not created submount yet.
>
> A. stat(/mnt/virtiofs/foo, AT_NO_AUTOMOUNT)
> -> This should get inode number in parent filesystem on host and
> store in guest inode->i_no and return to user space. Say inode
> in guest is called a_ino.
> B. stat(/mnt/virtiofs/foo)
> -> This should create submount and create new inode (say b_ino), using
> properties from a_ino. IOW, this should copy a_ino->i_no to
> b_ino->b_ino given current code, IIUC.
>
> -> Assume timeout has not happened and cached attrs have not expired.
>
> -> And now b_ino->i_no (or ->orig_ino) will be returned to user space.
>
> Am I missing something. Do we need to always expire inode attrs when
> we create submount so that client is forced to issue GETATTR.
Looks like while initialzing b_ino, we are setting attr_valid=0, which
should set fi->i_time=0 and force issuing GETATTR later.
fuse_fill_super_submount()
root = fuse_iget(sb, parent_fi->nodeid, 0, &root_attr, 0, 0);
^
fuse_iget(attr_valid=0)
fuse_change_attributes(attr_valid=0)
fuse_change_attributes_common(attr_valid=0)
fi->i_time = attr_valid;
So may be this will force GETATTR and fetch new inode id when second
stat() is called.
Thanks
Vivek
> > Signed-off-by: Max Reitz <mreitz at redhat.com>
> > ---
> > tools/virtiofsd/passthrough_ll.c | 104 +++++++++++++++++++++++++++++--
> > 1 file changed, 99 insertions(+), 5 deletions(-)
> >
> > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
> > index 1553d2ef45..110b6e7e5b 100644
> > --- a/tools/virtiofsd/passthrough_ll.c
> > +++ b/tools/virtiofsd/passthrough_ll.c
> > @@ -968,14 +968,87 @@ static int do_statx(struct lo_data *lo, int dirfd, const char *pathname,
> > return 0;
> > }
> >
> > +/*
> > + * Use readdir() to find mp_name's inode ID on the parent's filesystem.
> > + * (For mount points, stat() will only return the inode ID on the
> > + * filesystem mounted there, i.e. the root directory's inode ID. The
> > + * mount point originally was a directory on the parent filesystem,
> > + * though, and so has a different inode ID there. When passing
> > + * submount information to the guest, we need to pass this other ID,
> > + * so the guest can use it as the inode ID until the submount is
> > + * auto-mounted. (At which point the guest will invoke getattr and
> > + * find the inode ID on the submount.))
> > + *
> > + * Return 0 on success, and -errno otherwise. *pino is set only in
> > + * case of success.
> > + */
> > +static int get_mp_ino_on_parent(const struct lo_inode *dir, const char *mp_name,
> > + ino_t *pino)
> > +{
> > + int dirfd = -1;
> > + int ret;
> > + DIR *dp = NULL;
> > +
> > + dirfd = openat(dir->fd, ".", O_RDONLY);
> > + if (dirfd < 0) {
> > + ret = -errno;
> > + goto out;
> > + }
> > +
> > + dp = fdopendir(dirfd);
> > + if (!dp) {
> > + ret = -errno;
> > + goto out;
> > + }
> > + /* Owned by dp now */
> > + dirfd = -1;
> > +
> > + while (true) {
> > + struct dirent *de;
> > +
> > + errno = 0;
> > + de = readdir(dp);
> > + if (!de) {
> > + ret = errno ? -errno : -ENOENT;
> > + goto out;
> > + }
> > +
> > + if (!strcmp(de->d_name, mp_name)) {
> > + *pino = de->d_ino;
> > + ret = 0;
> > + goto out;
> > + }
> > + }
> > +
> > +out:
> > + if (dp) {
> > + closedir(dp);
> > + }
> > + if (dirfd >= 0) {
> > + close(dirfd);
> > + }
> > + return ret;
> > +}
> > +
> > /*
> > * Increments nlookup on the inode on success. unref_inode_lolocked() must be
> > * called eventually to decrement nlookup again. If inodep is non-NULL, the
> > * inode pointer is stored and the caller must call lo_inode_put().
> > + *
> > + * If parent_fs_st_ino is true, the entry is a mount point, and submounts are
> > + * announced to the guest, set e->attr.st_ino to the entry's inode ID on its
> > + * parent filesystem instead of its inode ID on the filesystem mounted on it.
> > + * (For mount points, the entry encompasses two inodes: One on the parent FS,
> > + * and one on the mounted FS (where it is the root node), so it has two inode
> > + * IDs. When looking up entries, we should show the guest the parent FS's inode
> > + * ID, because as long as the guest has not auto-mounted the submount, it should
> > + * see that original ID. Once it does perform the auto-mount, it will invoke
> > + * getattr and see the root node's inode ID.)
> > */
> > static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name,
> > struct fuse_entry_param *e,
> > - struct lo_inode **inodep)
> > + struct lo_inode **inodep,
> > + bool parent_fs_st_ino)
> > {
> > int newfd;
> > int res;
> > @@ -984,6 +1057,7 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name,
> > struct lo_data *lo = lo_data(req);
> > struct lo_inode *inode = NULL;
> > struct lo_inode *dir = lo_inode(req, parent);
> > + ino_t ino_id_for_guest;
> >
> > if (inodep) {
> > *inodep = NULL; /* in case there is an error */
> > @@ -1018,9 +1092,22 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name,
> > goto out_err;
> > }
> >
> > + ino_id_for_guest = e->attr.st_ino;
> > +
> > if (S_ISDIR(e->attr.st_mode) && lo->announce_submounts &&
> > (e->attr.st_dev != dir->key.dev || mnt_id != dir->key.mnt_id)) {
> > e->attr_flags |= FUSE_ATTR_SUBMOUNT;
> > +
> > + if (parent_fs_st_ino) {
> > + /*
> > + * Best effort, so ignore errors.
> > + * Also note that using readdir() means there may be races:
> > + * The directory entry we find (if any) may be different
> > + * from newfd. Again, this is a best effort. Reporting
> > + * the wrong inode ID to the guest is not catastrophic.
> > + */
> > + get_mp_ino_on_parent(dir, name, &ino_id_for_guest);
> > + }
> > }
> >
> > inode = lo_find(lo, &e->attr, mnt_id);
> > @@ -1043,6 +1130,10 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name,
> >
> > inode->nlookup = 1;
> > inode->fd = newfd;
> > + /*
> > + * For the inode key, use the dev/ino/mnt ID as reported by stat()
> > + * (i.e. not ino_id_for_guest)
> > + */
> > inode->key.ino = e->attr.st_ino;
> > inode->key.dev = e->attr.st_dev;
> > inode->key.mnt_id = mnt_id;
> > @@ -1058,6 +1149,9 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name,
> > }
> > e->ino = inode->fuse_ino;
> >
> > + /* Report ino_id_for_guest to the guest */
> > + e->attr.st_ino = ino_id_for_guest;
> > +
> > /* Transfer ownership of inode pointer to caller or drop it */
> > if (inodep) {
> > *inodep = inode;
> > @@ -1104,7 +1198,7 @@ static void lo_lookup(fuse_req_t req, fuse_ino_t parent, const char *name)
> > return;
> > }
> >
> > - err = lo_do_lookup(req, parent, name, &e, NULL);
> > + err = lo_do_lookup(req, parent, name, &e, NULL, true);
> > if (err) {
> > fuse_reply_err(req, err);
> > } else {
> > @@ -1217,7 +1311,7 @@ static void lo_mknod_symlink(fuse_req_t req, fuse_ino_t parent,
> > goto out;
> > }
> >
> > - saverr = lo_do_lookup(req, parent, name, &e, NULL);
> > + saverr = lo_do_lookup(req, parent, name, &e, NULL, false);
> > if (saverr) {
> > goto out;
> > }
> > @@ -1714,7 +1808,7 @@ static void lo_do_readdir(fuse_req_t req, fuse_ino_t ino, size_t size,
> >
> > if (plus) {
> > if (!is_dot_or_dotdot(name)) {
> > - err = lo_do_lookup(req, ino, name, &e, NULL);
> > + err = lo_do_lookup(req, ino, name, &e, NULL, true);
> > if (err) {
> > goto error;
> > }
> > @@ -1936,7 +2030,7 @@ static void lo_create(fuse_req_t req, fuse_ino_t parent, const char *name,
> > goto out;
> > }
> >
> > - err = lo_do_lookup(req, parent, name, &e, &inode);
> > + err = lo_do_lookup(req, parent, name, &e, &inode, false);
> > if (err) {
> > goto out;
> > }
> > --
> > 2.31.1
> >
> >
>
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