On Thu, Oct 28, 2021 at 1:46 AM Daniel Borkmann [off-list ref] wrote:

[ Adding Miklos & Greg to Cc for review given e0e0be8a8355 ("libfs: support RENAME_NOREPLACE in
   simple_rename()"). If you have a chance, would be great if you could take a look, thanks! ]

On 10/21/21 5:15 PM, Lorenz Bauer wrote:

quoted

Allow atomic exchange via RENAME_EXCHANGE when using simple_rename.
This affects binderfs, ramfs, hubetlbfs and bpffs. There isn't much
to do except update the various *time fields.

Signed-off-by: Lorenz Bauer <redacted>
---
  fs/libfs.c | 6 +++++-
  1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/libfs.c b/fs/libfs.c
index 51b4de3b3447..93c03d593749 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c

@@ -455,9 +455,12 @@ int simple_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
      struct inode *inode = d_inode(old_dentry);
      int they_are_dirs = d_is_dir(old_dentry);

-     if (flags & ~RENAME_NOREPLACE)
+     if (flags & ~(RENAME_NOREPLACE | RENAME_EXCHANGE))
              return -EINVAL;

+     if (flags & RENAME_EXCHANGE)
+             goto done;
+

This is not sufficient.   RENAME_EXCHANGE can swap a dir and a
non-dir, in which case the parent nlink counters need to be fixed up.

See shmem_exchange().   My suggestion is to move that function to
libfs.c:simple_rename_exchange().

Thanks,
Miklos