linux-kernel - [RESEND4, PATCH 1/2] fuse: retrieve: cap requested size to negotiated max

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Date:   Wed, 27 Mar 2019 10:15:18 +0000
From:   Kirill Smelkov <kirr@...edi.com>
To:     Miklos Szeredi <miklos@...redi.hu>,
        Miklos Szeredi <mszeredi@...hat.com>
Cc:     Han-Wen Nienhuys <hanwen@...gle.com>,
        Jakob Unterwurzacher <jakobunt@...il.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <fuse-devel@...ts.sourceforge.net>,
        Kirill Smelkov <kirr@...edi.com>, <stable@...r.kernel.org>
Subject: [RESEND4, PATCH 1/2] fuse: retrieve: cap requested size to negotiated max_write

FUSE filesystem server and kernel client negotiate during initialization
phase, what should be the maximum write size the client will ever issue.
Correspondingly the filesystem server then queues sys_read calls to read
requests with buffer capacity large enough to carry request header
+ that max_write bytes. A filesystem server is free to set its max_write
in anywhere in the range between [1·page, fc->max_pages·page]. In
particular go-fuse[2] sets max_write by default as 64K, wheres default
fc->max_pages corresponds to 128K. Libfuse also allows users to
configure max_write, but by default presets it to possible maximum.

If max_write is < fc->max_pages·page, and in NOTIFY_RETRIEVE handler we
allow to retrieve more than max_write bytes, corresponding prepared
NOTIFY_REPLY will be thrown away by fuse_dev_do_read, because the
filesystem server, in full correspondence with server/client contract,
will be only queuing sys_read with ~max_write buffer capacity, and
fuse_dev_do_read throws away requests that cannot fit into server
request buffer. In turn the filesystem server could get stuck waiting
indefinitely for NOTIFY_REPLY since NOTIFY_RETRIEVE handler returned OK
which is understood by clients as that NOTIFY_REPLY was queued and will
be sent back.

-> Cap requested size to negotiate max_write to avoid the problem.
This aligns with the way NOTIFY_RETRIEVE handler works, which already
unconditionally caps requested retrieve size to fuse_conn->max_pages.
This way it should not hurt NOTIFY_RETRIEVE semantic if we return less
data than was originally requested.

Please see [1] for context where the problem of stuck filesystem was hit
for real, 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
[2] https://github.com/hanwen/go-fuse

Signed-off-by: Kirill Smelkov <kirr@...edi.com>
Cc: Han-Wen Nienhuys <hanwen@...gle.com>
Cc: Jakob Unterwurzacher <jakobunt@...il.com>
Cc: <stable@...r.kernel.org> # v2.6.36+
---
 fs/fuse/dev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 8a63e52785e9..38e94bc43053 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1749,7 +1749,7 @@ static int fuse_retrieve(struct fuse_conn *fc, struct inode *inode,
 	offset = outarg->offset & ~PAGE_MASK;
 	file_size = i_size_read(inode);
 
-	num = outarg->size;
+	num = min(outarg->size, fc->max_write);
 	if (outarg->offset > file_size)
 		num = 0;
 	else if (outarg->offset + num > file_size)
-- 
2.21.0.392.gf8f6787159