[PATCH 3.5 34/64] fuse: hotfix truncate_pagecache() issue

From: Luis Henriques
Date: Mon Oct 28 2013 - 11:00:27 EST -stable review patch. If anyone has any objections, please let me know.


From: Maxim Patlasov <MPatlasov@xxxxxxxxxxxxx>

commit 06a7c3c2781409af95000c60a5df743fd4e2f8b4 upstream.

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The typical outcome of doing truncate_pagecache() with outdated arguments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
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

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@xxxxxxxxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx>
[ luis: backported to 3.5, based on bwh's backport to 3.2:
- added state field to struct fuse_inode ]
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
fs/fuse/dir.c | 7 ++++++-
fs/fuse/file.c | 8 +++++++-
fs/fuse/fuse_i.h | 9 +++++++++
fs/fuse/inode.c | 4 +++-
4 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index b60cc5f..ac5b0b9 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -1354,6 +1354,7 @@ static int fuse_do_setattr(struct dentry *entry, struct iattr *attr,
struct inode *inode = entry->d_inode;
struct fuse_conn *fc = get_fuse_conn(inode);
+ struct fuse_inode *fi = get_fuse_inode(inode);
struct fuse_req *req;
struct fuse_setattr_in inarg;
struct fuse_attr_out outarg;
@@ -1384,8 +1385,10 @@ static int fuse_do_setattr(struct dentry *entry, struct iattr *attr,
if (IS_ERR(req))
return PTR_ERR(req);

- if (is_truncate)
+ if (is_truncate) {
+ set_bit(FUSE_I_SIZE_UNSTABLE, &fi->state);
+ }

memset(&inarg, 0, sizeof(inarg));
memset(&outarg, 0, sizeof(outarg));
@@ -1447,12 +1450,14 @@ static int fuse_do_setattr(struct dentry *entry, struct iattr *attr,

+ clear_bit(FUSE_I_SIZE_UNSTABLE, &fi->state);
return 0;

if (is_truncate)

+ clear_bit(FUSE_I_SIZE_UNSTABLE, &fi->state);
return err;

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index e7785e4..9cc2f91 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -515,7 +515,8 @@ static void fuse_read_update_size(struct inode *inode, loff_t size,
struct fuse_inode *fi = get_fuse_inode(inode);

- if (attr_ver == fi->attr_version && size < inode->i_size) {
+ if (attr_ver == fi->attr_version && size < inode->i_size &&
+ !test_bit(FUSE_I_SIZE_UNSTABLE, &fi->state)) {
fi->attr_version = ++fc->attr_version;
i_size_write(inode, size);
@@ -877,12 +878,16 @@ static ssize_t fuse_perform_write(struct file *file,
struct inode *inode = mapping->host;
struct fuse_conn *fc = get_fuse_conn(inode);
+ struct fuse_inode *fi = get_fuse_inode(inode);
int err = 0;
ssize_t res = 0;

if (is_bad_inode(inode))
return -EIO;

+ if (inode->i_size < pos + iov_iter_count(ii))
+ set_bit(FUSE_I_SIZE_UNSTABLE, &fi->state);
do {
struct fuse_req *req;
ssize_t count;
@@ -917,6 +922,7 @@ static ssize_t fuse_perform_write(struct file *file,
if (res > 0)
fuse_write_update_size(inode, pos);

+ clear_bit(FUSE_I_SIZE_UNSTABLE, &fi->state);

return res > 0 ? res : err;
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 771fb63..1f63d49 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -103,6 +103,15 @@ struct fuse_inode {

/** List of writepage requestst (pending or sent) */
struct list_head writepages;
+ /** Miscellaneous bits describing inode state */
+ unsigned long state;
+/** FUSE inode state bits */
+enum {
+ /** An operation changing file size is in progress */

struct fuse_conn;
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 1cd6165..bca4e62 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -92,6 +92,7 @@ static struct inode *fuse_alloc_inode(struct super_block *sb)
fi->attr_version = 0;
fi->writectr = 0;
fi->orig_ino = 0;
+ fi->state = 0;
@@ -199,7 +200,8 @@ void fuse_change_attributes(struct inode *inode, struct fuse_attr *attr,
loff_t oldsize;

- if (attr_version != 0 && fi->attr_version > attr_version) {
+ if ((attr_version != 0 && fi->attr_version > attr_version) ||
+ test_bit(FUSE_I_SIZE_UNSTABLE, &fi->state)) {

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/