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.)
>
> Signed-off-by: Max Reitz <mre...@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);
Hi Max,
[CC virtio-fs list ]
In general patch looks good to me. A minor nit. get_mp_ino_on_parent()
is retruning error. It might be better to capture error and print a
message and continue.
I have couple of general questions about submounts.
- What happens in case of single file mounted on top of another file.
mount --bind foo.txt bar.txt
Do submounts work when mount point is not a directory.
- Say a directory is not a mount point yet and lookup instantiates an
inode. Later user mounts something on that directory. When does
client/server notice this change. I am assuming this is probably
part of revalidation path.
Thanks
Vivek
> + }
> }
>
> 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
>
>
