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,

+               return -EXDEV;
+
+       return 0;
+}
+
 /*
  * 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;
        }

        ret = do_copy_file_range(file_in, pos_in, file_out, pos_out, len,

@@ -1543,6 +1578,9 @@ SYSCALL_DEFINE6(copy_file_range, int, fd_in, loff_t __user *, off_in,
        struct fd f_out;
        ssize_t ret = -EBADF;

+       if (flags != 0)
+               return -EINVAL;
+
        f_in = fdget(fd_in);
        if (!f_in.file)
                goto out2;

diff --git a/include/linux/fs.h b/include/linux/fs.h
index fd47deea7c17..6f604926d955 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h

@@ -1815,6 +1815,13 @@ struct dir_context {
  */
 #define REMAP_FILE_ADVISORY            (REMAP_FILE_CAN_SHORTEN)

+/*
+ * This flag control the behavior of copy_file_range from internal (kernel)
+ * users.  It can be used to override the policy of forbidding copies when
+ * source and destination filesystems are different.
+ */
+#define COPY_FILE_SPLICE               (1 << 0)