Re: [PATCH 2/4] fuse: writepages: crop secondary requests

[Maxim Patlasov]

On 10/03/2013 01:57 PM, Miklos Szeredi wrote:
> On Wed, Oct 02, 2013 at 09:38:32PM +0400, Maxim Patlasov wrote:
> > If writeback happens while fuse is in FUSE_NOWRITE condition, the request
> > will be queued but not processed immediately (see fuse_flush_writepages()).
> > Until FUSE_NOWRITE becomes relaxed, more writebacks can happen. They will
> > be queued as "secondary" requests to that first ("primary") request.
> >
> > Existing implementation crops only primary request. This is not correct
> > because a subsequent extending write(2) may increase i_size and then secondary
> > requests won't be cropped properly. The result would be stale data written to
> > the server to a file offset where zeros must be.
> >
> > Similar problem may happen if secondary requests are attached to an in-flight
> > request that was already cropped.
> >
> > The patch solves the issue by cropping all secondary requests in
> > fuse_writepage_end(). Thanks to Miklos for idea.
> How about this, even simpler, one?

Very cute, but unfortunately it has a flaw. See please inline comment below.

> Thanks,
> Miklos
>
>
> Index: linux/fs/fuse/file.c
> ===================================================================
> --- linux.orig/fs/fuse/file.c	2013-10-03 11:27:00.597084704 +0200
> +++ linux/fs/fuse/file.c	2013-10-03 11:53:30.477208467 +0200
> @@ -1436,12 +1436,12 @@ static void fuse_writepage_finish(struct
>   }
>   
>   /* Called under fc->lock, may release and reacquire it */
> -static void fuse_send_writepage(struct fuse_conn *fc, struct fuse_req *req)
> +static void fuse_send_writepage(struct fuse_conn *fc, struct fuse_req *req,
> +				loff_t size)
>   __releases(fc->lock)
>   __acquires(fc->lock)
>   {
>   	struct fuse_inode *fi = get_fuse_inode(req->inode);
> -	loff_t size = i_size_read(req->inode);
>   	struct fuse_write_in *inarg = &req->misc.write.in;
>   	__u64 data_size = req->num_pages * PAGE_CACHE_SIZE;
>   
> @@ -1476,7 +1476,7 @@ __acquires(fc->lock)
>    *
>    * Called with fc->lock
>    */
> -void fuse_flush_writepages(struct inode *inode)
> +void __fuse_flush_writepages(struct inode *inode, loff_t crop)
>   __releases(fc->lock)
>   __acquires(fc->lock)
>   {
> @@ -1487,9 +1487,15 @@ __acquires(fc->lock)
>   	while (fi->writectr >= 0 && !list_empty(&fi->queued_writes)) {
>   		req = list_entry(fi->queued_writes.next, struct fuse_req, list);
>   		list_del_init(&req->list);
> -		fuse_send_writepage(fc, req);
> +		fuse_send_writepage(fc, req, crop);
>   	}
>   }
> +void fuse_flush_writepages(struct inode *inode)
> +__releases(fc->lock)
> +__acquires(fc->lock)
> +{
> +	__fuse_flush_writepages(inode, i_size_read(inode));
> +}
>   
>   static void fuse_writepage_end(struct fuse_conn *fc, struct fuse_req *req)
>   {
> @@ -1499,12 +1505,13 @@ static void fuse_writepage_end(struct fu
>   	mapping_set_error(inode->i_mapping, req->out.h.error);
>   	spin_lock(&fc->lock);
>   	while (req->misc.write.next) {
> +		struct fuse_write_in *inarg = &req->misc.write.in;
>   		struct fuse_req *next = req->misc.write.next;
>   		req->misc.write.next = next->misc.write.next;
>   		next->misc.write.next = NULL;
>   		list_add(&next->writepages_entry, &fi->writepages);
>   		list_add_tail(&next->list, &fi->queued_writes);
> -		fuse_flush_writepages(inode);
> +		__fuse_flush_writepages(inode, inarg->offset + inarg->size);

__fuse_flush_writepages() will ignore its 'crop' arg if fi->writectr is 
below zero. This can easily happen if a request is finalized after 
fuse_set_nowrite(). So in a scenario like this:

1. There is an in-flight primary request with a chain of secondary ones.
2. User calls ftruncate(2) to extend file; fuse_set_nowrite() makes 
fi->writectr negative and starts waiting for completion of that 
in-flight request
3. Userspace fuse daemon ACKs the request and fuse_writepage_end() is 
called; it calls __fuse_flush_writepages(), but the latter does nothing 
because fi->writectr < 0
4. fuse_do_setattr() proceeds extending i_size and calling 
__fuse_release_nowrite(). But now new (increased) i_size will be used as 
'crop' arg of __fuse_flush_writepages()

stale data can leak to the server.

Thanks,
Maxim
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