On Tue, Nov 7, 2017 at 10:58 PM, James Bottomley
<James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 2017-11-07 at 22:50 +0200, Amir Goldstein wrote:
>> tmpfs uses the helper d_find_alias() to find a dentry from a decoded
>> inode, but d_find_alias() skips unhashed dentries, so unlinked files
>> cannot be decoded from a file handle.
>>
>> This can be reproduced using xfstests test program open_by_handle:
>> $ open_by handle -c /tmp/testdir
>> $ open_by_handle -dk /tmp/testdir
>> open_by_handle(/tmp/testdir/file000000) returned 116 incorrectly on
>> an
>> unlinked open file!
>>
>> To fix this, use a variant of d_find_alias() that returns any alias,
>> even an unhashed one.
>>
>> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
>> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
>> Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
>> ---
>> mm/shmem.c | 22 +++++++++++++++++++++-
>> 1 file changed, 21 insertions(+), 1 deletion(-)
>>
>> Miklos,
>>
>> Please see if that patch looks correct.
>>
>> Bruce and Jeff indicated that the current tmpfs behavior is not
>> desirable
>> for nfsd. It may be uncommon to export a tmpfs, but it is going to
>> become
>> a lot more common when exporting an overlayfs with upper tmpfs.
>>
>> Thanks,
>> Amir.
>>
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 07a1d22807be..f7c555ebf0f2 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -3404,6 +3404,26 @@ static int shmem_match(struct inode *ino, void
>> *vfh)
>> return ino->i_ino == inum && fh[0] == ino->i_generation;
>> }
>>
>> +/* Find any alias of inode, even an unhashed one */
>> +static struct dentry *shmem_find_alias(struct inode *inode)
>> +{
>> + struct dentry *alias;
>> +
>> + spin_lock(&inode->i_lock);
>> + hlist_for_each_entry(alias, &inode->i_dentry, d_u.d_alias) {
>> + dget(alias);
>> + if (alias->d_inode == inode) {
>> + spin_unlock(&inode->i_lock);
>> + return alias;
>> + }
>> + dput(alias);
>> + }
>> + spin_unlock(&inode->i_lock);
>> +
>> + return NULL;
>> +}
>
> This doesn't look right in the case of a multiply linked inode for
> which you've removing some of the link names because it will return the
> first alias it finds, which may be unhashed. Isn't what you want for
> it to return the first hashed alias if one exists, or the first
> unhashed one if none do, so this code
>
>> @@ -3420,7 +3440,7 @@ static struct dentry *shmem_fh_to_dentry(struct
>> super_block *sb,
>> inode = ilookup5(sb, (unsigned long)(inum + fid->raw[0]),
>> shmem_match, fid->raw);
>> if (inode) {
>> - dentry = d_find_alias(inode);
>> + dentry = shmem_find_alias(inode);
>> iput(inode);
>> }
>
>
> Should actually be
>
> if (inode) {
> dentry = d_find_alias(inode);
> if (!dentry)
> dentry = shmem_find_alias(inode);
> iput(inode)
> }
>
> ?
>
Perhaps, but I don't think that matters to nfsd. ??
I think nfsd is going to use that alias only for accessing the inode anyway.
tmpfs doesn't implement get_parent() for reconnect of non-dir, and for
directory, there is only one alias and d_find_alias() returns it even if it
is unhashed.
Amir.
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