Date:   Thu, 5 Aug 2021 14:15:16 +0200
From:   Jan Kara <jack@...e.cz>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     jack@...e.com, amir73il@...il.com, djwong@...nel.org,
        tytso@....edu, david@...morbit.com, dhowells@...hat.com,
        khazhy@...gle.com, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-api@...r.kernel.org,
        kernel@...labora.com
Subject: Re: [PATCH v5 18/23] fanotify: Handle FAN_FS_ERROR events

On Wed 04-08-21 12:06:07, Gabriel Krisman Bertazi wrote:
> Wire up FAN_FS_ERROR in the fanotify_mark syscall.  The event can only
> be requested for the entire filesystem, thus it requires the
> FAN_MARK_FILESYSTEM.
> 
> FAN_FS_ERROR has to be handled slightly differently from other events
> because it needs to be submitted in an atomic context, using
> preallocated memory.  This patch implements the submission path by only
> storing the first error event that happened in the slot (userspace
> resets the slot by reading the event).
> 
> Extra error events happening when the slot is occupied are merged to the
> original report, and the only information keep for these extra errors is
> an accumulator counting the number of events, which is part of the
> record reported back to userspace.
> 
> Reporting only the first event should be fine, since when a FS error
> happens, a cascade of error usually follows, but the most meaningful
> information is (usually) on the first erro.
> 
> The event dequeueing is also a bit special to avoid losing events. Since
> event merging only happens while the event is queued, there is a window
> between when an error event is dequeued (notification_lock is dropped)
> until it is reset (.free_event()) where the slot is full, but no merges
> can happen.
> 
> The proposed solution is to replace the event in the slot with a new
> structure, prior to dropping the lock.  This way, if a new event arrives
> in the time between the event was dequeued and the time it resets, the
> new errors will still be logged and merged in the new slot.
> 
> Signed-off-by: Gabriel Krisman Bertazi <krisman@...labora.com>

The splitting of the patches really helped. Now I think I can grok much
more details than before :) Thanks! Some comments below.

> diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
> index 0678d35432a7..4e9e271a4394 100644
> --- a/fs/notify/fanotify/fanotify.c
> +++ b/fs/notify/fanotify/fanotify.c
> @@ -681,6 +681,42 @@ static __kernel_fsid_t fanotify_get_fsid(struct fsnotify_iter_info *iter_info)
>  	return fsid;
>  }
>  
> +static int fanotify_merge_error_event(struct fsnotify_group *group,
> +				      struct fsnotify_event *event)
> +{
> +	struct fanotify_event *fae = FANOTIFY_E(event);
> +	struct fanotify_error_event *fee = FANOTIFY_EE(fae);
> +
> +	/*
> +	 * When err_count > 0, the reporting slot is full.  Just account
> +	 * the additional error and abort the insertion.
> +	 */
> +	if (fee->err_count) {
> +		fee->err_count++;
> +		return 1;
> +	}
> +
> +	return 0;
> +}
> +
> +static void fanotify_insert_error_event(struct fsnotify_group *group,
> +					struct fsnotify_event *event,
> +					const void *data)
> +{
> +	const struct fs_error_report *report = (struct fs_error_report *) data;
> +	struct fanotify_event *fae = FANOTIFY_E(event);
> +	struct fanotify_error_event *fee;
> +
> +	/* This might be an unexpected type of event (i.e. overflow). */
> +	if (!fanotify_is_error_event(fae->mask))
> +		return;
> +
> +	fee = FANOTIFY_EE(fae);
> +	fee->fae.type = FANOTIFY_EVENT_TYPE_FS_ERROR;
> +	fee->error = report->error;
> +	fee->err_count = 1;
> +}
> +
>  /*
>   * Add an event to hash table for faster merge.
>   */
> @@ -735,7 +771,7 @@ static int fanotify_handle_event(struct fsnotify_group *group, u32 mask,
>  	BUILD_BUG_ON(FAN_OPEN_EXEC_PERM != FS_OPEN_EXEC_PERM);
>  	BUILD_BUG_ON(FAN_FS_ERROR != FS_ERROR);
>  
> -	BUILD_BUG_ON(HWEIGHT32(ALL_FANOTIFY_EVENT_BITS) != 19);
> +	BUILD_BUG_ON(HWEIGHT32(ALL_FANOTIFY_EVENT_BITS) != 20);
>  
>  	mask = fanotify_group_event_mask(group, iter_info, mask, data,
>  					 data_type, dir);
> @@ -760,6 +796,18 @@ static int fanotify_handle_event(struct fsnotify_group *group, u32 mask,
>  			return 0;
>  	}
>  
> +	if (fanotify_is_error_event(mask)) {
> +		struct fanotify_sb_mark *sb_mark =
> +			FANOTIFY_SB_MARK(fsnotify_iter_sb_mark(iter_info));
> +
> +		ret = fsnotify_insert_event(group,
> +					    &sb_mark->fee_slot->fae.fse,
> +					    fanotify_merge_error_event,
> +					    fanotify_insert_error_event,
> +					    data);
> +		goto finish;
> +	}

Hum, seeing this and how you had to extend fsnotify_add_event() to
accommodate this use, cannot we instead have something like:

	if (fanotify_is_error_event(mask)) {
		struct fanotify_sb_mark *sb_mark =
			FANOTIFY_SB_MARK(fsnotify_iter_sb_mark(iter_info));
		struct fanotify_error_event *event = &sb_mark->fee_slot;
		bool queue = false;

		spin_lock(&group->notification_lock);
		/* Not yet queued? */
		if (!event->err_count) {
			fee->error = report->error;
			queue = true;
		}
		event->err_count++;
		spin_unlock(&group->notification_lock);
		if (queue) {
			... fill in other error info in 'event' such as fhandle
			fsnotify_add_event(group, &event->fae.fse, NULL);
		}
	}

It would be IMHO simpler to follow what's going on and we don't have to
touch fsnotify_add_event(). I do recognize that due to races it may happen
that some racing fsnotify(FAN_FS_ERROR) call returns before the event is
actually visible in the event queue. It don't think it really matters but
if we wanted to be more careful, we would need to preformat fhandle into a
local buffer and only copy it into the event under notification_lock when
we see the event is unused.

> +/*
> + * Replace a mark's error event with a new structure in preparation for
> + * it to be dequeued.  This is a bit annoying since we need to drop the
> + * lock, so another thread might just steal the event from us.
> + */
> +static int fanotify_replace_fs_error_event(struct fsnotify_group *group,
> +					   struct fanotify_event *fae)
> +{
> +	struct fanotify_error_event *new, *fee = FANOTIFY_EE(fae);
> +	struct fanotify_sb_mark *sb_mark = fee->sb_mark;
> +	struct fsnotify_event *fse;
> +
> +	pr_debug("%s: event=%p\n", __func__, fae);
> +
> +	assert_spin_locked(&group->notification_lock);
> +
> +	spin_unlock(&group->notification_lock);
> +	new = fanotify_alloc_error_event(sb_mark);
> +	spin_lock(&group->notification_lock);
> +
> +	if (!new)
> +		return -ENOMEM;
> +
> +	/*
> +	 * Since we temporarily dropped the notification_lock, the event
> +	 * might have been taken from under us and reported by another
> +	 * reader.  If that is the case, don't play games, just retry.
> +	 */
> +	fse = fsnotify_peek_first_event(group);
> +	if (fse != &fae->fse) {
> +		kfree(new);
> +		return -EAGAIN;
> +	}
> +
> +	sb_mark->fee_slot = new;
> +
> +	return 0;
> +}
> +
>  /*
>   * Get an fanotify notification event if one exists and is small
>   * enough to fit in "count". Return an error pointer if the count
> @@ -212,9 +252,21 @@ static struct fanotify_event *get_one_event(struct fsnotify_group *group,
>  		goto out;
>  	}
>  
> +	if (fanotify_is_error_event(event->mask)) {
> +		/*
> +		 * Replace the error event ahead of dequeueing so we
> +		 * don't need to handle a incorrectly dequeued event.
> +		 */
> +		ret = fanotify_replace_fs_error_event(group, event);
> +		if (ret) {
> +			event = ERR_PTR(ret);
> +			goto out;
> +		}
> +	}
> +

The replacing, retry, and all is hairy. Cannot we just keep the same event
attached to the sb mark and copy-out to on-stack buffer under
notification_lock in get_one_event()? The event is big (due to fhandle) but
fanotify_read() is not called from a deep call chain so we should have
enough space on stack for that.

>  	/*
> -	 * Held the notification_lock the whole time, so this is the
> -	 * same event we peeked above.
> +	 * Even though we might have temporarily dropped the lock, this
> +	 * is guaranteed to be the same event we peeked above.
>  	 */
>  	fsnotify_remove_first_event(group);
>  	if (fanotify_is_perm_event(event->mask))
> @@ -596,6 +648,8 @@ static ssize_t fanotify_read(struct file *file, char __user *buf,
>  		event = get_one_event(group, count);
>  		if (IS_ERR(event)) {
>  			ret = PTR_ERR(event);
> +			if (ret == -EAGAIN)
> +				continue;
>  			break;
>  		}
>  

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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