[Virtio-fs] [PATCH 1/3] virtiofsd: Find original inode ID of mount points
Max Reitz
mreitz at redhat.com
Mon May 17 17:26:29 UTC 2021
On 17.05.21 16:57, 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.)
>>
>> 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);
>
> 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.
Sure, why not.
> 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.
No, as you can see in the condition quoted above, we only set the
FUSE_ATTR_SUBMOUNT flag for directories. That seemed the most common
case for me, and I didn’t want to have to worry about weirdness that
might ensue for file mounts.
> - 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.
I guess at least before this patch this is no different from any other
filesystem change. Because st_dev+st_ino changed, it should basically
look like the old directory was removed and a different one was put in
its place.
Now, with this patch, we will return the old st_ino to the guest, but
internally virtiofsd will still use the submount’s st_dev/st_ino, so a
new lo_inode should be created, and so fuse_dentry_revalidate()’s lookup
should return a different node ID, resulting it to consider the entry
expired.
Besides, fuse_dentry_revalidate() has a condition on IS_AUTOMOUNT(inode)
!= flags & FUSE_ATTR_SUBMOUNT. Considering the previous paragraph, this
doesn’t seem necessary to me, but it can’t hurt.
Max
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