On Thu, Jan 18, 2018 at 10:35 PM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
> On Thu, Jan 18, 2018 at 10:10 PM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>> On Thu, Jan 18, 2018 at 8:49 PM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>>> On Thu, Jan 18, 2018 at 4:39 PM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>>>> On Thu, Jan 18, 2018 at 3:34 PM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>>>>> On Thu, Jan 18, 2018 at 4:09 PM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>>>>>> [Added Al Viro]
>>>>>>
>>>>>> On Thu, Jan 4, 2018 at 6:20 PM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>>>>>>> Decoding an upper file handle is done by decoding the upper dentry from
>>>>>>> underlying upper fs, finding or allocating an overlay inode that is
>>>>>>> hashed by the real upper inode and instantiating an overlay dentry with
>>>>>>> that inode.
>>>>>>>
>>>>>>> Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
>>>>>>> ---
>>>>>>> fs/overlayfs/export.c | 91 ++++++++++++++++++++++++++++++++++++++++++++++++
>>>>>>> fs/overlayfs/namei.c | 4 +--
>>>>>>> fs/overlayfs/overlayfs.h | 2 ++
>>>>>>> 3 files changed, 95 insertions(+), 2 deletions(-)
>>>>>>>
>>>>>>> diff --git a/fs/overlayfs/export.c b/fs/overlayfs/export.c
>>>>>>> index 58c4f5e8a67e..5c72784a0b4d 100644
>>>>>>> --- a/fs/overlayfs/export.c
>>>>>>> +++ b/fs/overlayfs/export.c
>>>>>>> @@ -93,6 +93,97 @@ static int ovl_encode_inode_fh(struct inode *inode, u32 *fid, int *max_len,
>>>>>>> return type;
>>>>>>> }
>>>>>>>
>>>>>>> +/*
>>>>>>> + * Find or instantiate an overlay dentry from real dentries.
>>>>>>> + */
>>>>>>> +static struct dentry *ovl_obtain_alias(struct super_block *sb,
>>>>>>> + struct dentry *upper,
>>>>>>> + struct ovl_path *lowerpath)
>>>>>>> +{
>>>>>>> + struct inode *inode;
>>>>>>> + struct dentry *dentry;
>>>>>>> + struct ovl_entry *oe;
>>>>>>> +
>>>>>>> + /* TODO: obtain non pure-upper */
>>>>>>> + if (lowerpath)
>>>>>>> + return ERR_PTR(-EIO);
>>>>>>> +
>>>>>>> + inode = ovl_get_inode(sb, dget(upper), NULL, NULL, 0);
>>>>>>> + if (IS_ERR(inode)) {
>>>>>>> + dput(upper);
>>>>>>> + return ERR_CAST(inode);
>>>>>>> + }
>>>>>>> +
>>>>>>> + dentry = d_obtain_alias(inode);
>>>>>>> + if (IS_ERR(dentry) || dentry->d_fsdata)
>>>>>>
>>>>>> Racing two instances of this code, each thinking it got a new alias
>>>>>> and trying to fill it, results in a memory leak.
>>>>>>
>>>>>> Haven't checked in too much depth, but apparently other filesystems
>>>>>> are not affected, so we need something special here.
>>>>>>
>>>>>> One solution: split d_instantiate_anon(dentry, inode) out of
>>>>>> __d_obtain_alias() and supply that with the already initialized
>>>>>> dentry.
>>>>>>
>>>>>
>>>>> Can't we use &OVL_I(inode)->lock to avoid the race?
>>>>
>>>> We could. But then d_splice_alias() will find our half baked dentry
>>>> and return that from ovl_lookup().
>>>
>>> No it won't, because we do not obtain dir dentries this way.
>>> We actually do in this patch [3/17], but since patch [4/17] we don't,
>>> so I only need to fix this patch not to obtain dir dentry and to
>>> protect concurrent decode of non-dir with &OVL_I(inode)->lock.
>>>
>>>> So we do need to have the dentry
>>>> fully initialized by the time it's added into the inode's alias list.
>>>>
>>>
>>> The only problems I see with adding a non-dir disconnected alias
>>> that is not fully baked are:
>>> 1. We can get it in ovl_encode_inode_fh() from d_find_any_alias()
>>> 2. nfsd can get it in exportfs_decode_fh() from find_acceptable_alias()
>>> in a weird hypothetical case where the fully baked dentry we just
>>> returned from ovl_obtain_alias() in NOT acceptable by nfsd but
>>> the half baked dentry IS acceptable
>>> 3. Another kernel user that uses d_find_any_alias() or one of the use
>>> case that only Al can think of...
>>>
>>> Cases 2 and 3, I don't know if they are for real.
>>>
>>> Case 1 is only a problem due to lack of export_operations method
>>> 'dentry_to_fh'. exportfs_encode_fh() has the right dentry, but it does
>>> not pass it to the filesystem for encoding, so I think it should be
>>> solved by adding this method.
>>
>> I agree with your analysis.
>>
>> However, I don't see what's wrong with adding fully baked dentries to
>> the inode.
>
> I agree that adding half baked dentries is not a good practice and
> we should avoid it.
>
How is this for an option?
===========================================
+/*
+ * Find or instantiate an overlay dentry from real dentries.
+ */
+static struct dentry *ovl_obtain_alias(struct super_block *sb,
+ struct dentry *upper,
+ struct ovl_path *lowerpath)
+{
+ struct inode *inode;
+ struct dentry *dentry;
+ struct ovl_entry *oe;
+ void *fsdata = &oe;
+
+ /* TODO: obtain non pure-upper */
+ if (lowerpath)
+ return ERR_PTR(-EIO);
+
+ inode = ovl_get_inode(sb, dget(upper), NULL, NULL, 0);
+ if (IS_ERR(inode)) {
+ dput(upper);
+ return ERR_CAST(inode);
+ }
+
+ oe = ovl_alloc_entry(0);
+ if (!oe) {
+ iput(inode);
+ return ERR_PTR(-ENOMEM);
+ }
+ oe->has_upper = true;
+
+ dentry = d_obtain_alias_fsdata(inode, fsdata);
+ /* A new allocated dentry assigns *fsdata and sets it to NULL */
+ if (oe)
+ kfree(oe);
+
+ return dentry;
+}
+
...
-static struct dentry *__d_obtain_alias(struct inode *inode, int disconnected)
+static struct dentry *__d_obtain_alias(struct inode *inode, int disconnected,
+ void **fsdata)
{
struct dentry *tmp;
struct dentry *res;
@@ -1962,6 +1963,12 @@ static struct dentry *__d_obtain_alias(struct
inode *inode, int disconnected)
if (disconnected)
add_flags |= DCACHE_DISCONNECTED;
+ /* Take ownership of pre-allocated fs-specific data */
+ if (fsdata) {
+ tmp->d_fsdata = fsdata;
+ *fsdata = NULL;
+ }
+
spin_lock(&tmp->d_lock);
__d_set_inode_and_type(tmp, inode, add_flags);
hlist_add_head(&tmp->d_u.d_alias, &inode->i_dentry);
@@ -1998,7 +2005,13 @@ static struct dentry *__d_obtain_alias(struct
inode *inode, int disconnected)
*/
struct dentry *d_obtain_alias(struct inode *inode)
{
- return __d_obtain_alias(inode, 1);
+ return __d_obtain_alias(inode, 1, NULL);
+}
+EXPORT_SYMBOL(d_obtain_alias);
+
+struct dentry *d_obtain_alias_fsdata(struct inode *inode, void **fsdata)
+{
+ return __d_obtain_alias(inode, 1, fsdata);
}
EXPORT_SYMBOL(d_obtain_alias);
@@ -2019,7 +2032,7 @@ EXPORT_SYMBOL(d_obtain_alias);
*/
struct dentry *d_obtain_root(struct inode *inode)
{
- return __d_obtain_alias(inode, 0);
+ return __d_obtain_alias(inode, 0, NULL);
}
EXPORT_SYMBOL(d_obtain_root);
