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SubjectRe: [PATCH 7/7] autofs4 - fix pending mount race.
On Tue, 17 Jun 2008 20:24:12 +0800 Ian Kent <raven@themaw.net> wrote:

> Close a race between a pending mount that is about to finish and a new
> lookup for the same directory.
>
> Process P1 triggers a mount of directory foo.
> It sets DCACHE_AUTOFS_PENDING in the ->lookup routine, creates a waitq
> entry for 'foo', and calls out to the daemon to perform the mount.
> The autofs daemon will then create the directory 'foo', using a new dentry
> that will be hashed in the dcache.
>
> Before the mount completes, another process, P2, tries to walk into the
> 'foo' directory. The vfs path walking code finds an entry for 'foo' and
> calls the revalidate method. Revalidate finds that the entry is not
> PENDING (because PENDING was never set on the dentry created by the mkdir),
> but it does find the directory is empty. Revalidate calls try_to_fill_dentry,
> which sets the PENDING flag and then calls into the autofs4 wait code to
> trigger or wait for a mount of 'foo'. The wait code finds the entry for
> 'foo' and goes to sleep waiting for the completion of the mount.
>
> Yet another process, P3, tries to walk into the 'foo' directory. This
> process again finds a dentry in the dcache for 'foo', and calls into
> the autofs revalidate code.
>
> The revalidate code finds that the PENDING flag is set, and so calls
> try_to_fill_dentry.
>
> a) try_to_fill_dentry sets the PENDING flag redundantly for this dentry,
> then calls into the autofs4 wait code.
> b) the autofs4 wait code takes the waitq mutex and searches for an entry
> for 'foo'
>
> Between a and b, P1 is woken up because the mount completed.
> P1 takes the wait queue mutex, clears the PENDING flag from the dentry,
> and removes the waitqueue entry for 'foo' from the list.
>
> When it releases the waitq mutex, P3 (eventually) acquires it. At this
> time, it looks for an existing waitq for 'foo', finds none, and so
> creates a new one and calls out to the daemon to mount the 'foo' directory.
>
> Now, the reason that three processes are required to trigger this race
> is that, because the PENDING flag is not set on the dentry created by
> mkdir, the window for the race would be way to slim for it to ever occur.
> Basically, between the testing of d_mountpoint(dentry) and the taking of
> the waitq mutex, the mount would have to complete and the daemon would
> have to be woken up, and that in turn would have to wake up P1. This is
> simply impossible. Add the third process, though, and it becomes slightly
> more likely.
>
> ...
>
> diff --git a/fs/autofs4/waitq.c b/fs/autofs4/waitq.c
> index 5208cfb..cd21fd4 100644
> --- a/fs/autofs4/waitq.c
> +++ b/fs/autofs4/waitq.c
> @@ -207,19 +207,106 @@ autofs4_find_wait(struct autofs_sb_info *sbi, struct qstr *qstr)
> return wq;
> }
>
> +/*
> + * Check if we have a valid request.
> + * Returns
> + * 1 if the request should continue.
> + * In this case we can return an autofs_wait_queue entry if one is
> + * found or NULL to idicate a new wait needs to be created.
> + * 0 or a negative errno if the request shouldn't continue.
> + */
> +static int validate_request(struct autofs_wait_queue **wait,
> + struct autofs_sb_info *sbi,
> + struct qstr *qstr,
> + struct dentry*dentry, enum autofs_notify notify)
> +{
> + struct autofs_wait_queue *wq;
> + struct autofs_info *ino;
> +
> + /* Wait in progress, continue; */
> + wq = autofs4_find_wait(sbi, qstr);
> + if (wq) {
> + *wait = wq;
> + return 1;

Returns 1 with the mutex held.

> + }
> +
> + *wait = NULL;
> +
> + /* If we don't yet have any info this is a new request */
> + ino = autofs4_dentry_ino(dentry);
> + if (!ino)
> + return 1;
> +
> + /*
> + * If we've been asked to wait on an existing expire (NFY_NONE)
> + * but there is no wait in the queue ...
> + */
> + if (notify == NFY_NONE) {
> + /*
> + * Either we've betean the pending expire to post it's
> + * wait or it finished while we waited on the mutex.
> + * So we need to wait till either, the wait appears
> + * or the expire finishes.
> + */

Wanna have another go at that comment? The grammar and spelling should
cause an oops or something.

> + while (ino->flags & AUTOFS_INF_EXPIRING) {
> + mutex_unlock(&sbi->wq_mutex);
> + schedule_timeout_interruptible(HZ/10);
> + if (mutex_lock_interruptible(&sbi->wq_mutex))
> + return -EINTR;

Returns -EFOO with the mutex not held.

> +
> + wq = autofs4_find_wait(sbi, qstr);
> + if (wq) {
> + *wait = wq;
> + return 1;
> + }
> + }
> +
> + /*
> + * Not ideal but the status has already gone. Of the two
> + * cases where we wait on NFY_NONE neither depend on the
> + * return status of the wait.
> + */
> + return 0;

Returns zero with the mutex held.

> + }
> +
> + /*
> + * If we've been asked to trigger a mount and the request
> + * completed while we waited on the mutex ...
> + */
> + if (notify == NFY_MOUNT) {
> + /*
> + * If the dentry isn't hashed just go ahead and try the
> + * mount again with a new wait (not much else we can do).
> + */
> + if (!d_unhashed(dentry)) {
> + /*
> + * But if the dentry is hashed, that means that we
> + * got here through the revalidate path. Thus, we
> + * need to check if the dentry has been mounted
> + * while we waited on the wq_mutex. If it has,
> + * simply return success.
> + */
> + if (d_mountpoint(dentry))
> + return 0;
> + }
> + }
> +
> + return 1;
> +}
>
> ...
>
> + ret = validate_request(&wq, sbi, &qstr, dentry, notify);
> + if (ret <= 0) {
> + if (ret == 0)
> mutex_unlock(&sbi->wq_mutex);
> - return 0;
> - }
> + kfree(qstr.name);
> + return ret;
> }

Leave the mutex held if it returned 1. Doesn't unlock the mutex if it
returned -EFOO. Presumably callers of this function will unlock the
mutex if it returned zero.

Or something like that. My brain just exploded.

Please double-check the locking protocol here and then document the
sorry thing.



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