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-filesystems copy restrictions that existed
prior to commit 5dae222a5ff0 ("vfs: allow copy_file_range to copy across
devices").  It also introduces a flag (COPY_FILE_SPLICE) that can be used
by filesystems calling directly into the vfs copy_file_range to override
these restrictions.  Right now, only NFS needs to set this flag.

Fixes: 5dae222a5ff0 ("vfs: allow copy_file_range to copy across devices")
Link: https://lore.kernel.org/linux-fsdevel/20210212044405.4120619-1-drinkcat@chromium.org/ (local)
Link: https://lore.kernel.org/linux-fsdevel/CANMq1KDZuxir2LM5jOTm0xx+BnvW=ZmpsG47CyHFJwnw7zSX6Q@mail.gmail.com/ (local)
Link: https://lore.kernel.org/linux-fsdevel/20210126135012.1.If45b7cdc3ff707bc1efa17f5366057d60603c45f@changeid/ (local)
Reported-by: Nicolas Boichat <redacted>
Signed-off-by: Luis Henriques <redacted>
---
Ok, I've tried to address all the issues and comments.  Hopefully this v3
is a bit closer to the final fix.

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      |  3 ++-
 fs/read_write.c    | 44 +++++++++++++++++++++++++++++++++++++++++---
 include/linux/fs.h |  7 +++++++
 3 files changed, 50 insertions(+), 4 deletions(-)

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 04937e51de56..14e55822c223 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c

@@ -578,7 +578,8 @@ 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);
+       return vfs_copy_file_range(src, src_pos, dst, dst_pos, count,
+                                  COPY_FILE_SPLICE);
 }

 __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..40a16003fb05 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c

@@ -1410,6 +1410,33 @@ static ssize_t do_copy_file_range(struct file *file_in, loff_t pos_in,
                                       flags);
 }

+/*
+ * This helper function checks whether copy_file_range can actually be used,
+ * depending on the source and destination filesystems being the same.
+ *
+ * In-kernel callers may set COPY_FILE_SPLICE to override these checks.
+ */
+static int fops_copy_file_checks(struct file *file_in, struct file *file_out,
+                                unsigned int flags)
+{
+       if (WARN_ON_ONCE(flags & ~COPY_FILE_SPLICE))
+               return -EINVAL;
+
+       if (flags & COPY_FILE_SPLICE)
+               return 0;
+       /*
+        * We got here from userspace, so forbid copies if copy_file_range isn't
+        * implemented or if we're doing a cross-fs copy.
+        */

+
 /*
  * Performs necessary checks before doing a file copy
  *

@@ -1427,6 +1454,14 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
        loff_t size_in;
        int ret;

+       /* Only check f_ops if we're not trying to clone */
+       if (!file_in->f_op->remap_file_range ||
+           (file_inode(file_in)->i_sb == file_inode(file_out)->i_sb)) {
+               ret = fops_copy_file_checks(file_in, file_out, flags);
+               if (ret)
+                       return ret;
+       }
+

        ret = generic_file_rw_checks(file_in, file_out);
        if (ret)
                return ret;

@@ -1474,9 +1509,6 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
 {
        ssize_t ret;

-       if (flags != 0)
-               return -EINVAL;
-
        ret = generic_copy_file_checks(file_in, pos_in, file_out, pos_out, &len,
                                       flags);
        if (unlikely(ret))

@@ -1511,6 +1543,9 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
                        ret = cloned;
                        goto done;
                }
+               ret = fops_copy_file_checks(file_in, file_out, flags);
+               if (ret)
+                       return ret;