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Subject[PATCH AUTOSEL 4.4 25/56] fuse: retrieve: cap requested size to negotiated max_write
From: Kirill Smelkov <>

[ Upstream commit 7640682e67b33cab8628729afec8ca92b851394f ]

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

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.


Signed-off-by: Kirill Smelkov <>
Cc: Han-Wen Nienhuys <>
Cc: Jakob Unterwurzacher <>
Signed-off-by: Miklos Szeredi <>
Signed-off-by: Sasha Levin <>
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 fbb978e75c6be..217ceca3eb063 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1734,7 +1734,7 @@ static int fuse_retrieve(struct fuse_conn *fc, struct inode *inode,
offset = outarg->offset & ~PAGE_CACHE_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)
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