On Wed, Jan 17, 2018 at 5:42 PM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> On Wed, Jan 17, 2018 at 12:18 PM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>> On Mon, Jan 15, 2018 at 4:56 PM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>>>
>> [...]
>>> >>
>>> >> So, a working algorithm would be going up to the first connected
>>> >> parent or root, lock parent, lookup name and restart. Not guaranteed
>>> >> to finish, since not protected against always racing with renames.
>>> >> Can we take s_vfs_rename_sem on ovl to prevent that?
>>> >>
>>> >
>>> > Sounds like a simple and good enough solution.
>>> > Do we really need the locking of parent and restart connect if
>>> > we take s_vfs_rename_sem around ovl_lookup_real()?
>>>
>>> No, but s_vfs_rename_sem is a really heavyweight solution, we should
>>> do better than that for decoding a file handle.
>>>
>>> And we probably don't need anything else, since rename on ancestor
>>> means renamed dir is connected, and hopefully not evicted from the
>>> cache until we repeat the walk up.
>>>
>>> So need to lock parent, lookup ovl dentry, verify we got the same
>>> upper, if not retry icache lookup.
>>>
>>> Not sure we need to worry about that "hopefully". Hopefully not.
>>>
>>
>> Something like this??
>>
>> This is just the raw fix to patch 4/17 without the icache lookup
>> that is added by later patches.
>>
>> I added rename_lock seqlock around backwalk to connected ancestor
>> and take_dentry_name_snapshot() for the stability of real name
>> during overlay lookup.
>>
>> I considered also storing OVL_I(d_inode(connected))->version
>> inside seqlock and comparing it to version in case lookup of child
>> failed. This could help us distinguish between overlay rename and
>> underlying rename (overlay dir version did not change) and return
>> ESTALE instead of restarting lookup in the latter case.
>> Wasn't sure if that was a good idea and what we loose if we leave it out.
>>
>> I tested this code, but only with upper file handles of course
>> (xfstest generic/467).
>>
>> Please let me know what you think.
>>
>> Thanks,
>> Amir.
>>
>> ================================================================
>>
>> From 337543c3fcdf9323d3720d17ab6fc13e287bbec1 Mon Sep 17 00:00:00 2001
>> From: Amir Goldstein <amir73il@xxxxxxxxx>
>> Date: Thu, 28 Dec 2017 18:36:16 +0200
>> Subject: [PATCH v3 4/17] ovl: decode connected upper dir file handles
>>
>> Until this change, we decoded upper file handles by instantiating an
>> overlay dentry from the real upper dentry. This is sufficient to handle
>> pure upper files, but insufficient to handle merge/impure dirs.
>>
>> To that end, if decoded real upper dir is connected and hashed, we
>> lookup an overlay dentry with the same path as the real upper dir.
>> If decoded real upper is non-dir, we instantiate a disconnected overlay
>> dentry as before this change.
>>
>> Because ovl_fh_to_dentry() returns a connected overlay dir dentry,
>> exportfs never needs to call get_parent() and get_name() to reconnect an
>> upper overlay dir. Because connectable non-dir file handles are not
>> supported, exportfs will not be able to use fh_to_parent() and get_name()
>> methods to reconnect a disconnected non-dir to its parent. Therefore, the
>> methods get_parent() and get_name() are implemented just to print out a
>> sanity warning and the method fh_to_parent() is implemented to warn the
>> user that using the 'subtree_check' exportfs option is not supported.
>>
>> An alternative approach could have been to implement instantiating of
>> an overlay directory inode from origin/index and implement get_parent()
>> and get_name() by calling into underlying fs operations and them
>> instantiating the overlay parent dir.
>>
>> The reasons for not choosing the get_parent() approach were:
>> - Obtaining a disconnected overlay dir dentry would requires a
>> delicate re-factoring of ovl_lookup() to get a dentry with overlay
>> parent info. It was preferred to avoid doing that re-factoring unless
>> it was proven worthy.
>> - Going down the path of disconnected dir would mean that the (non
>> trivial) code path of d_splice_alias() could be traveled and that
>> meant writing more tests and introduces race cases that are very hard
>> to hit on purpose. Taking the path of connecting overlay dentry by
>> forward lookup is therefore the safe and boring way to avoid surprises.
>>
>> The culprit of the chosen "connected overlay dentry" approach:
>> - We need to take special care to rename of ancestors while connecting
>> the overlay dentry by real dentry path. These subtleties are usually
>> handled by generic exportfs and VFS code.
>>
>> Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
>> ---
>> fs/overlayfs/export.c | 215 +++++++++++++++++++++++++++++++++++++++++++++++++-
>> 1 file changed, 214 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/overlayfs/export.c b/fs/overlayfs/export.c
>> index 557c29928e98..35f37a72d55e 100644
>> --- a/fs/overlayfs/export.c
>> +++ b/fs/overlayfs/export.c
>> @@ -130,6 +130,188 @@ static struct dentry *ovl_obtain_alias(struct
>> super_block *sb,
>> return dentry;
>> }
>>
>> +/*
>> + * Lookup a child overlay dentry whose real dentry is @real.
>> + * If @real is on upper layer, we lookup a child overlay dentry with the same
>> + * name as the real dentry. Otherwise, we need to consult index for lookup.
>> + */
>> +static struct dentry *ovl_lookup_real_one(struct dentry *parent,
>> + struct dentry *real,
>> + struct ovl_layer *layer)
>> +{
>> + struct dentry *this;
>> + struct name_snapshot name;
>> + int err;
>> +
>> + /* TODO: use index when looking up by lower real dentry */
>> + if (layer->idx)
>> + return ERR_PTR(-EACCES);
>> +
>> + /*
>> + * Lookup overlay dentry by real name. The parent mutex protects us
>> + * from racing with overlay rename. If the overlay dentry that is
>> + * above real has already been moved to a different parent, then this
>> + * lookup will fail to find a child dentry whose real dentry is @real
>> + * and we will have to restart the lookup of real path from the top.
>> + *
>> + * We also need to take a snapshot of real dentry name to protect us
>> + * from racing with underlying layer rename. In this case, we don't
>> + * care about returning ESTALE, only from referencing a free name
>> + * pointer.
>> + *
>> + * TODO: try to lookup the renamed overlay dentry in inode cache by
>> + * real inode.
>> + */
>> + inode_lock_nested(d_inode(parent), I_MUTEX_PARENT);
>> + take_dentry_name_snapshot(&name, real);
>
> No need to snapshot, just check if parent hasn't changed after
> locking. If parent is same, then name is guaranteed to be stable.
>
I don't understand.
We are not holding a lock on real parent, only on overlay parent.
What makes the real name stable?
The snapshot is not to protect from racing with overlay rename.
The snapshot is for protecting from race with real rename, just to
make sure we don't dereference a stale name pointer.
> This also means, that only ESTALE is possible after this. And ESTALE
> is fatal, no need to retry after that.
OK. I will return ECHILD for parent that has changed
and will restart only on ECHILD.
>
>> + this = lookup_one_len(name.name, parent, strlen(name.name));
>> + err = PTR_ERR(this);
>> + if (IS_ERR(this)) {
>> + goto fail;
>> + } else if (!this || !this->d_inode) {
>> + dput(this);
>> + err = -ENOENT;
>> + goto fail;
>> + } else if (ovl_dentry_upper(this) != real) {
>> + dput(this);
>> + err = -ESTALE;
>> + goto fail;
>> + }
>> +
>> +out:
>> + release_dentry_name_snapshot(&name);
>> + inode_unlock(d_inode(parent));
>> + return this;
>> +
>> +fail:
>> + pr_warn_ratelimited("overlayfs: failed to lookup one by real
>> (%pd2, layer=%d, parent=%pd2, err=%i)\n",
>> + real, layer->idx, parent, err);
>> + this = ERR_PTR(err);
>> + goto out;
>> +}
>> +
>> +/*
>> + * Lookup an overlay dentry whose real dentry is @real.
>> + * If @real is on upper layer, we lookup a child overlay dentry with the same
>> + * path the real dentry. Otherwise, we need to consult index for lookup.
>> + */
>> +static struct dentry *ovl_lookup_real(struct super_block *sb,
>> + struct dentry *real,
>> + struct ovl_layer *layer)
>> +{
>> + struct dentry *connected;
>> + unsigned int seq;
>> + int err = 0;
>> +
>> + /* TODO: use index when looking up by lower real dentry */
>> + if (layer->idx)
>> + return ERR_PTR(-EACCES);
>> +
>> + connected = dget(sb->s_root);
>> + while (!err) {
>> + struct dentry *next, *this;
>> + struct dentry *parent = NULL;
>> + struct dentry *real_connected = layer->mnt->mnt_root;
>> +
>> + if (real_connected == real)
>> + break;
>
> This loop will never finish, since real_connected is mnt_root now.
> Would be nice if there was a guaranteed way to finish this without
> icache lookup, but I don't see how.
>
That's a bug.
The correct code is:
struct dentry *real_connected = ovl_dentry_upper(connected);
>> +
>> + /*
>> + * Find the topmost dentry not yet connected. Taking rename_lock
>> + * so at least we don't race with rename when walking back to
>> + * 'real_connected'.
>> + */
>> + seq = read_seqbegin(&rename_lock);
>
> I don't see what we gain with this.
>
I can't say that I do see it, but perhaps there is something yet
to be gained by adding this later for lower layers lookup.
Perhaps when looking on lower real layer, we can store the
overlay dir cache version of 'connected' (connected in this case
may be an indexed merge dir).
After we take 'connected' dir mutex, we cannot check that
real parent hasn't changes as an indication to no overlay rename
because overlay rename happens on upper, but we can compare
the dir cache version of 'connected' dir to the version we stored
under rename_lock.
Then we can tell if lower lookup has failed because of some
permanent error (e.g. middle layer redirect) or because of an
indexed rename, so we need to restart.
Maybe that gains us something?
Thanks,
Amir.
