06a7c3c278
The way how fuse calls truncate_pagecache() from fuse_change_attributes() is completely wrong. Because, w/o i_mutex held, we never sure whether 'oldsize' and 'attr->size' are valid by the time of execution of truncate_pagecache(inode, oldsize, attr->size). In fact, as soon as we released fc->lock in the middle of fuse_change_attributes(), we completely loose control of actions which may happen with given inode until we reach truncate_pagecache. The list of potentially dangerous actions includes mmap-ed reads and writes, ftruncate(2) and write(2) extending file size. The typical outcome of doing truncate_pagecache() with outdated arguments is data corruption from user point of view. This is (in some sense) acceptable in cases when the issue is triggered by a change of the file on the server (i.e. externally wrt fuse operation), but it is absolutely intolerable in scenarios when a single fuse client modifies a file without any external intervention. A real life case I discovered by fsx-linux looked like this: 1. Shrinking ftruncate(2) comes to fuse_do_setattr(). The latter sends FUSE_SETATTR to the server synchronously, but before getting fc->lock ... 2. fuse_dentry_revalidate() is asynchronously called. It sends FUSE_LOOKUP to the server synchronously, then calls fuse_change_attributes(). The latter updates i_size, releases fc->lock, but before comparing oldsize vs attr->size.. 3. fuse_do_setattr() from the first step proceeds by acquiring fc->lock and updating attributes and i_size, but now oldsize is equal to outarg.attr.size because i_size has just been updated (step 2). Hence, fuse_do_setattr() returns w/o calling truncate_pagecache(). 4. As soon as ftruncate(2) completes, the user extends file size by write(2) making a hole in the middle of file, then reads data from the hole either by read(2) or mmap-ed read. The user expects to get zero data from the hole, but gets stale data because truncate_pagecache() is not executed yet. The scenario above illustrates one side of the problem: not truncating the page cache even though we should. Another side corresponds to truncating page cache too late, when the state of inode changed significantly. Theoretically, the following is possible: 1. As in the previous scenario fuse_dentry_revalidate() discovered that i_size changed (due to our own fuse_do_setattr()) and is going to call truncate_pagecache() for some 'new_size' it believes valid right now. But by the time that particular truncate_pagecache() is called ... 2. fuse_do_setattr() returns (either having called truncate_pagecache() or not -- it doesn't matter). 3. The file is extended either by write(2) or ftruncate(2) or fallocate(2). 4. mmap-ed write makes a page in the extended region dirty. The result will be the lost of data user wrote on the fourth step. The patch is a hotfix resolving the issue in a simplistic way: let's skip dangerous i_size update and truncate_pagecache if an operation changing file size is in progress. This simplistic approach looks correct for the cases w/o external changes. And to handle them properly, more sophisticated and intrusive techniques (e.g. NFS-like one) would be required. I'd like to postpone it until the issue is well discussed on the mailing list(s). Changed in v2: - improved patch description to cover both sides of the issue. Signed-off-by: Maxim Patlasov <mpatlasov@parallels.com> Signed-off-by: Miklos Szeredi <mszeredi@suse.cz> Cc: stable@vger.kernel.org |
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9p | ||
adfs | ||
affs | ||
afs | ||
autofs4 | ||
befs | ||
bfs | ||
btrfs | ||
cachefiles | ||
ceph | ||
cifs | ||
coda | ||
configfs | ||
cramfs | ||
debugfs | ||
devpts | ||
dlm | ||
ecryptfs | ||
efivarfs | ||
efs | ||
exofs | ||
exportfs | ||
ext2 | ||
ext3 | ||
ext4 | ||
f2fs | ||
fat | ||
freevxfs | ||
fscache | ||
fuse | ||
gfs2 | ||
hfs | ||
hfsplus | ||
hostfs | ||
hpfs | ||
hppfs | ||
hugetlbfs | ||
isofs | ||
jbd | ||
jbd2 | ||
jffs2 | ||
jfs | ||
lockd | ||
logfs | ||
minix | ||
ncpfs | ||
nfs | ||
nfs_common | ||
nfsd | ||
nilfs2 | ||
nls | ||
notify | ||
ntfs | ||
ocfs2 | ||
omfs | ||
openpromfs | ||
proc | ||
pstore | ||
qnx4 | ||
qnx6 | ||
quota | ||
ramfs | ||
reiserfs | ||
romfs | ||
squashfs | ||
sysfs | ||
sysv | ||
ubifs | ||
udf | ||
ufs | ||
xfs | ||
aio.c | ||
anon_inodes.c | ||
attr.c | ||
bad_inode.c | ||
binfmt_aout.c | ||
binfmt_elf.c | ||
binfmt_elf_fdpic.c | ||
binfmt_em86.c | ||
binfmt_flat.c | ||
binfmt_misc.c | ||
binfmt_script.c | ||
binfmt_som.c | ||
bio-integrity.c | ||
bio.c | ||
block_dev.c | ||
buffer.c | ||
char_dev.c | ||
compat.c | ||
compat_binfmt_elf.c | ||
compat_ioctl.c | ||
coredump.c | ||
coredump.h | ||
dcache.c | ||
dcookies.c | ||
direct-io.c | ||
drop_caches.c | ||
eventfd.c | ||
eventpoll.c | ||
exec.c | ||
fcntl.c | ||
fhandle.c | ||
file.c | ||
file_table.c | ||
filesystems.c | ||
fs-writeback.c | ||
fs_struct.c | ||
generic_acl.c | ||
inode.c | ||
internal.h | ||
ioctl.c | ||
ioprio.c | ||
Kconfig | ||
Kconfig.binfmt | ||
libfs.c | ||
locks.c | ||
Makefile | ||
mbcache.c | ||
mount.h | ||
mpage.c | ||
namei.c | ||
namespace.c | ||
no-block.c | ||
open.c | ||
pipe.c | ||
pnode.c | ||
pnode.h | ||
posix_acl.c | ||
proc_namespace.c | ||
read_write.c | ||
readdir.c | ||
select.c | ||
seq_file.c | ||
signalfd.c | ||
splice.c | ||
stack.c | ||
stat.c | ||
statfs.c | ||
super.c | ||
sync.c | ||
timerfd.c | ||
utimes.c | ||
xattr.c | ||
xattr_acl.c |