[PATCH v5] vfs: fix copy_file_range regression in cross-fs copies

Luis Henriques
2021-02-18T17:09:23Z

A regression has been reported by Nicolas Boichat, found while using the
copy_file_range syscall to copy a tracefs file. Before commit
5dae222a5ff0 ("vfs: allow copy_file_range to copy across devices") the
kernel would return -EXDEV to userspace when trying to copy a file across
different filesystems. After this commit, the syscall doesn't fail anymore
and instead returns zero (zero bytes copied), as this file's content is
generated on-the-fly and thus reports a size of zero.

This patch restores some cross-filesystem copy restrictions that existed
prior to commit 5dae222a5ff0 ("vfs: allow copy_file_range to copy across
devices"). Filesystems are still allowed to fall-back to the VFS
generic_copy_file_range() implementation, but that has now to be done
explicitly.

nfsd is also modified to fall-back into generic_copy_file_range() in case
vfs_copy_file_range() fails with -EOPNOTSUPP or -EXDEV.

Fixes: 5dae222a5ff0 ("vfs: allow copy_file_range to copy across devices")
Link: https://lore.kernel.org/linux-fsdevel/20210212044405.4120619-1-drinkcat@chromium.org/
Link: https://lore.kernel.org/linux-fsdevel/CANMq1KDZuxir2LM5jOTm0xx+BnvW=ZmpsG47CyHFJwnw7zSX6Q@mail.gmail.com/
Link: https://lore.kernel.org/linux-fsdevel/20210126135012.1.If45b7cdc3ff707bc1efa17f5366057d60603c45f@changeid/
Reported-by: Nicolas Boichat <drinkcat@chromium.org>
Signed-off-by: Luis Henriques <lhenriques@suse.de>
---
And v5! Sorry. Sure, it makes sense to go through the all the vfs_cfr()
checks first.

Again, here's my request for testing.

Changes since v4
- nfsd falls-back to generic_copy_file_range() only *if* it gets -EOPNOTSUPP
or -EXDEV.
Changes since v3
- dropped the COPY_FILE_SPLICE flag
- kept the f_op's checks early in generic_copy_file_checks, implementing
Amir's suggestions
- modified nfsd to use generic_copy_file_range()
Changes since v2
- do all the required checks earlier, in generic_copy_file_checks(),
adding new checks for ->remap_file_range
- new COPY_FILE_SPLICE flag
- don't remove filesystem's fallback to generic_copy_file_range()
- updated commit changelog (and subject)
Changes since v1 (after Amir review)
- restored do_copy_file_range() helper
- return -EOPNOTSUPP if fs doesn't implement CFR
- updated commit description
fs/nfsd/vfs.c | 8 +++++++-
fs/read_write.c | 50 +++++++++++++++++++++++--------------------------
2 files changed, 30 insertions(+), 28 deletions(-)

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 04937e51de56..23dab0fa9087 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -568,6 +568,7 @@ __be32 nfsd4_clone_file_range(struct nfsd_file *nf_src, u64 src_pos,
ssize_t nfsd_copy_file_range(struct file *src, u64 src_pos, struct file *dst,
u64 dst_pos, u64 count)
{
+ ssize_t ret;

/*
* Limit copy to 4MB to prevent indefinitely blocking an nfsd
@@ -578,7 +579,12 @@ ssize_t nfsd_copy_file_range(struct file *src, u64 src_pos, struct file *dst,
* limit like this and pipeline multiple COPY requests.
*/
count = min_t(u64, count, 1 << 22);
- return vfs_copy_file_range(src, src_pos, dst, dst_pos, count, 0);
+ ret = vfs_copy_file_range(src, src_pos, dst, dst_pos, count, 0);
+
+ if (ret == -EOPNOTSUPP || ret == -EXDEV)
+ ret = generic_copy_file_range(src, src_pos, dst, dst_pos,
+ count, 0);
+ return ret;
}

__be32 nfsd4_vfs_fallocate(struct svc_rqst *rqstp, struct svc_fh *fhp,
diff --git a/fs/read_write.c b/fs/read_write.c
index 75f764b43418..214d44f7cbfa 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -1388,28 +1388,6 @@ ssize_t generic_copy_file_range(struct file *file_in, loff_t pos_in,
}
EXPORT_SYMBOL(generic_copy_file_range);

-static ssize_t do_copy_file_range(struct file *file_in, loff_t pos_in,
- struct file *file_out, loff_t pos_out,
- size_t len, unsigned int flags)
-{
- /*
- * Although we now allow filesystems to handle cross sb copy, passing
- * a file of the wrong filesystem type to filesystem driver can result
- * in an attempt to dereference the wrong type of ->private_data, so
- * avoid doing that until we really have a good reason. NFS defines
- * several different file_system_type structures, but they all end up
- * using the same ->copy_file_range() function pointer.
- */
- if (file_out->f_op->copy_file_range &&
- file_out->f_op->copy_file_range == file_in->f_op->copy_file_range)
- return file_out->f_op->copy_file_range(file_in, pos_in,
- file_out, pos_out,
- len, flags);
-
- return generic_copy_file_range(file_in, pos_in, file_out, pos_out, len,
- flags);
-}
-
/*
* Performs necessary checks before doing a file copy
*
@@ -1427,6 +1405,25 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
loff_t size_in;
int ret;

+ /*
+ * Although we now allow filesystems to handle cross sb copy, passing
+ * a file of the wrong filesystem type to filesystem driver can result
+ * in an attempt to dereference the wrong type of ->private_data, so
+ * avoid doing that until we really have a good reason. NFS defines
+ * several different file_system_type structures, but they all end up
+ * using the same ->copy_file_range() function pointer.
+ */
+ if (file_out->f_op->copy_file_range) {
+ if (file_in->f_op->copy_file_range !=
+ file_out->f_op->copy_file_range)
+ return -EXDEV;
+ } else if (file_in->f_op->remap_file_range) {
+ if (file_inode(file_in)->i_sb != file_inode(file_out)->i_sb)
+ return -EXDEV;
+ } else {
+ return -EOPNOTSUPP;
+ }
+
ret = generic_file_rw_checks(file_in, file_out);
if (ret)
return ret;
@@ -1499,8 +1496,7 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
* Try cloning first, this is supported by more file systems, and
* more efficient if both clone and copy are supported (e.g. NFS).
*/
- if (file_in->f_op->remap_file_range &&
- file_inode(file_in)->i_sb == file_inode(file_out)->i_sb) {
+ if (file_in->f_op->remap_file_range) {
loff_t cloned;

cloned = file_in->f_op->remap_file_range(file_in, pos_in,
@@ -1513,9 +1509,9 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
}
}

- ret = do_copy_file_range(file_in, pos_in, file_out, pos_out, len,
- flags);
- WARN_ON_ONCE(ret == -EOPNOTSUPP);
+ ret = file_out->f_op->copy_file_range(file_in, pos_in,
+ file_out, pos_out,
+ len, flags);
done:
if (ret > 0) {
fsnotify_access(file_in);