On Wed, Jun 12, 2019 at 09:44:49AM +0200, Miklos Szeredi wrote: > On Tue, Jun 11, 2019 at 10:28 PM Kirill Smelkov <kirr@xxxxxxxxxx> wrote: > > > Miklos, would 4K -> `sizeof(fuse_in_header) + sizeof(fuse_write_in)` for > > header room change be accepted? > > Yes, next cycle. For 4.2 I'll just push the revert. Thanks Miklos. Please consider queuing the following patch for 5.3. Sander, could you please confirm that glusterfs is not broken with this version of the check? Thanks beforehand, Kirill ---- 8< ---- >From 24a04e8be9bbf6e67de9e1908dcbe95d426d2521 Mon Sep 17 00:00:00 2001 From: Kirill Smelkov <kirr@xxxxxxxxxx> Date: Wed, 27 Mar 2019 10:15:15 +0000 Subject: [PATCH] fuse: require /dev/fuse reads to have enough buffer capacity (take 2) [ This retries commit d4b13963f217 which was reverted in 766741fcaa1f. In this version we require only `sizeof(fuse_in_header) + sizeof(fuse_write_in)` instead of 4K for FUSE request header room, because, contrary to libfuse and kernel client behaviour, GlusterFS actually provides only so much room for request header. ] A FUSE filesystem server queues /dev/fuse sys_read calls to get filesystem requests to handle. It does not know in advance what would be that request as it can be anything that client issues - LOOKUP, READ, WRITE, ... Many requests are short and retrieve data from the filesystem. However WRITE and NOTIFY_REPLY write data into filesystem. Before getting into operation phase, FUSE filesystem server and kernel client negotiate what should be the maximum write size the client will ever issue. After negotiation the contract in between server/client is that the filesystem server then should queue /dev/fuse sys_read calls with enough buffer capacity to receive any client request - WRITE in particular, while FUSE client should not, in particular, send WRITE requests with > negotiated max_write payload. FUSE client in kernel and libfuse historically reserve 4K for request header. However an existing filesystem server - GlusterFS - was found which reserves only 80 bytes for header room (= `sizeof(fuse_in_header) + sizeof(fuse_write_in)`). https://lore.kernel.org/linux-fsdevel/20190611202738.GA22556@xxxxxxxxxxxxxxxxxxx/ https://github.com/gluster/glusterfs/blob/v3.8.15-0-gd174f021a/xlators/mount/fuse/src/fuse-bridge.c#L4894 Since `sizeof(fuse_in_header) + sizeof(fuse_write_in)` == `sizeof(fuse_in_header) + sizeof(fuse_read_in)` == `sizeof(fuse_in_header) + sizeof(fuse_notify_retrieve_in)` is the absolute minimum any sane filesystem should be using for header room, the contract is that filesystem server should queue sys_reads with `sizeof(fuse_in_header) + sizeof(fuse_write_in)` + max_write buffer. If the filesystem server does not follow this contract, what can happen is that fuse_dev_do_read will see that request size is > buffer size, and then it will return EIO to client who issued the request but won't indicate in any way that there is a problem to filesystem server. This can be hard to diagnose because for some requests, e.g. for NOTIFY_REPLY which mimics WRITE, there is no client thread that is waiting for request completion and that EIO goes nowhere, while on filesystem server side things look like the kernel is not replying back after successful NOTIFY_RETRIEVE request made by the server. We can make the problem easy to diagnose if we indicate via error return to filesystem server when it is violating the contract. This should not practically cause problems because if a filesystem server is using shorter buffer, writes to it were already very likely to cause EIO, and if the filesystem is read-only it should be too following FUSE_MIN_READ_BUFFER minimum buffer size. Please see [1] for context where the problem of stuck filesystem was hit for real (because kernel client was incorrectly sending more than max_write data with NOTIFY_REPLY; see also previous patch), how the situation was traced and for more involving patch that did not make it into the tree. [1] https://marc.info/?l=linux-fsdevel&m=155057023600853&w=2 Signed-off-by: Kirill Smelkov <kirr@xxxxxxxxxx> Cc: Han-Wen Nienhuys <hanwen@xxxxxxxxxx> Cc: Jakob Unterwurzacher <jakobunt@xxxxxxxxx> Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx> --- fs/fuse/dev.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c index ea8237513dfa..15531ba560b5 100644 --- a/fs/fuse/dev.c +++ b/fs/fuse/dev.c @@ -1317,6 +1317,25 @@ static ssize_t fuse_dev_do_read(struct fuse_dev *fud, struct file *file, unsigned reqsize; unsigned int hash; + /* + * Require sane minimum read buffer - that has capacity for fixed part + * of any request header + negotiated max_write room for data. If the + * requirement is not satisfied return EINVAL to the filesystem server + * to indicate that it is not following FUSE server/client contract. + * Don't dequeue / abort any request. + * + * Historically libfuse reserves 4K for fixed header room, but e.g. + * GlusterFS reserves only 80 bytes + * + * = `sizeof(fuse_in_header) + sizeof(fuse_write_in)` + * + * which is the absolute minimum any sane filesystem should be using + * for header room. + */ + if (nbytes < max_t(size_t, FUSE_MIN_READ_BUFFER, + sizeof(fuse_in_header) + sizeof(fuse_write_in) + fc->max_write)) + return -EINVAL; + restart: spin_lock(&fiq->waitq.lock); err = -EAGAIN; -- 2.20.1
