Rainer Weikusat <r...@doppelsaurus.mobileactivedefense.com> writes:
> Hannes Frederic Sowa <han...@stressinduktion.org> writes:
>> On 27.12.2015 21:13, Rainer Weikusat wrote:
>>> -static int unix_mknod(const char *sun_path, umode_t mode, struct path *res)
>>> +static int unix_mknod(struct dentry *dentry, struct path *path, umode_t
>>> mode,
>>> + struct path *res)
>>> {
>>> - struct dentry *dentry;
>>> - struct path path;
>>> - int err = 0;
>>> - /*
>>> - * Get the parent directory, calculate the hash for last
>>> - * component.
>>> - */
>>> - dentry = kern_path_create(AT_FDCWD, sun_path, &path, 0);
>>> - err = PTR_ERR(dentry);
>>> - if (IS_ERR(dentry))
>>> - return err;
>>> + int err;
>>>
>>> - /*
>>> - * All right, let's create it.
>>> - */
>>> - err = security_path_mknod(&path, dentry, mode, 0);
>>> + err = security_path_mknod(path, dentry, mode, 0);
>>> if (!err) {
>>> - err = vfs_mknod(d_inode(path.dentry), dentry, mode, 0);
>>> + err = vfs_mknod(d_inode(path->dentry), dentry, mode, 0);
>>> if (!err) {
>>> - res->mnt = mntget(path.mnt);
>>> + res->mnt = mntget(path->mnt);
>>> res->dentry = dget(dentry);
>>> }
>>> }
>>> - done_path_create(&path, dentry);
>>> +
>>
>> The reordered call to done_path_create will change the locking
>> ordering between the i_mutexes and the unix readlock. Can you comment
>> on this? On a first sight this looks like a much more dangerous change
>> than the original deadlock report. Can't this also conflict with
>> splice code deep down in vfs layer?
>
> Practical consideration
[...]
> A deadlock was possible here if the thread doing the bind then blocked
> when trying to acquire the readlock while the thread holding the
> readlock is blocked on another lock held by a thread trying to perform
> an operation on the same directory as the bind (possibly with some
> indirection).
Since this was probably pretty much a "write only" sentence, I think I
should try this again (with apologies in case a now err on the other
side and rather explain to much --- my abilities to express myself such
that people understand what I mean to express instead of just getting
mad at me are not great).
For a deadlock to happen here, there needs to be a cycle (circle?) of
threads each holding one lock and blocking while trying to acquire
another lock which ultimatively ends with a thread trying to acquire the
i_mutex of the directory where the socket name is to be created. The
binding thread would need to block when trying to acquire the
readlock. But (contrary to what I originally wrote[*]) this cannot happen
because the af_unix code doesn't lock anything non-socket related while
holding the readlock. The only instance of that was in _bind and caused
the deadlock.
[*] I misread
static ssize_t skb_unix_socket_splice(struct sock *sk,
struct pipe_inode_info *pipe,
struct splice_pipe_desc *spd)
{
int ret;
struct unix_sock *u = unix_sk(sk);
mutex_unlock(&u->readlock);
ret = splice_to_pipe(pipe, spd);
mutex_lock(&u->readlock);
return ret;
}
as 'lock followed by unlock' instead of 'unlock followed by lock'.
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