Re: [PATCH v3 3/4] ovl: copy up sync/noatime fileattr flags

[Miklos Szeredi <miklos@szeredi.hu>]

On Sat, 19 Jun 2021 at 11:26, Amir Goldstein <amir73il@gmail.com> wrote:
>
> When a lower file has sync/noatime fileattr flags, the behavior of
> overlayfs post copy up is inconsistent.
>
> Immediattely after copy up, ovl inode still has the S_SYNC/S_NOATIME
> inode flags copied from lower inode, so vfs code still treats the ovl
> inode as sync/noatime.  After ovl inode evict or mount cycle,
> the ovl inode does not have these inode flags anymore.
>
> To fix this inconsitency, try to copy the fileattr flags on copy up
> if the upper fs supports the fileattr_set() method.
>
> This gives consistent behavior post copy up regardless of inode eviction
> from cache.
>
> We cannot copy up the immutable/append-only inode flags in a similar
> manner, because immutable/append-only inodes cannot be linked and because
> overlayfs will not be able to set overlay.* xattr on the upper inodes.
>
> Those flags will be addressed by a followup patch.
>
> Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> ---
>  fs/overlayfs/copy_up.c   | 49 ++++++++++++++++++++++++++++++++++------
>  fs/overlayfs/inode.c     | 36 ++++++++++++++++++-----------
>  fs/overlayfs/overlayfs.h | 14 +++++++++++-
>  3 files changed, 78 insertions(+), 21 deletions(-)
>
> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> index 3fa68a5cc16e..a06b423ca5d1 100644
> --- a/fs/overlayfs/copy_up.c
> +++ b/fs/overlayfs/copy_up.c
> @@ -8,6 +8,7 @@
>  #include <linux/fs.h>
>  #include <linux/slab.h>
>  #include <linux/file.h>
> +#include <linux/fileattr.h>
>  #include <linux/splice.h>
>  #include <linux/xattr.h>
>  #include <linux/security.h>
> @@ -130,6 +131,31 @@ int ovl_copy_xattr(struct super_block *sb, struct dentry *old,
>         return error;
>  }
>
> +static int ovl_copy_fileattr(struct path *old, struct path *new)
> +{
> +       struct fileattr oldfa = { .flags_valid = true };
> +       struct fileattr newfa = { .flags_valid = true };
> +       int err;
> +
> +       err = ovl_real_fileattr(old, &oldfa, false);
> +       if (err)
> +               return err;
> +
> +       err = ovl_real_fileattr(new, &newfa, false);
> +       if (err)
> +               return err;
> +
> +       BUILD_BUG_ON(OVL_COPY_FS_FLAGS_MASK & ~FS_COMMON_FL);
> +       newfa.flags &= ~OVL_COPY_FS_FLAGS_MASK;
> +       newfa.flags |= (oldfa.flags & OVL_COPY_FS_FLAGS_MASK);
> +
> +       BUILD_BUG_ON(OVL_COPY_FSX_FLAGS_MASK & ~FS_XFLAG_COMMON);
> +       newfa.fsx_xflags &= ~OVL_COPY_FSX_FLAGS_MASK;
> +       newfa.fsx_xflags |= (oldfa.fsx_xflags & OVL_COPY_FSX_FLAGS_MASK);
> +
> +       return ovl_real_fileattr(new, &newfa, true);
> +}
> +
>  static int ovl_copy_up_data(struct ovl_fs *ofs, struct path *old,
>                             struct path *new, loff_t len)
>  {
> @@ -493,20 +519,21 @@ static int ovl_link_up(struct ovl_copy_up_ctx *c)
>  static int ovl_copy_up_inode(struct ovl_copy_up_ctx *c, struct dentry *temp)
>  {
>         struct ovl_fs *ofs = OVL_FS(c->dentry->d_sb);
> +       struct inode *inode = d_inode(c->dentry);
> +       struct path upperpath, datapath;
>         int err;
>
> +       ovl_path_upper(c->dentry, &upperpath);
> +       if (WARN_ON(upperpath.dentry != NULL))
> +               return -EIO;
> +
> +       upperpath.dentry = temp;
> +
>         /*
>          * Copy up data first and then xattrs. Writing data after
>          * xattrs will remove security.capability xattr automatically.
>          */
>         if (S_ISREG(c->stat.mode) && !c->metacopy) {
> -               struct path upperpath, datapath;
> -
> -               ovl_path_upper(c->dentry, &upperpath);
> -               if (WARN_ON(upperpath.dentry != NULL))
> -                       return -EIO;
> -               upperpath.dentry = temp;
> -
>                 ovl_path_lowerdata(c->dentry, &datapath);
>                 err = ovl_copy_up_data(ofs, &datapath, &upperpath,
>                                        c->stat.size);
> @@ -518,6 +545,14 @@ static int ovl_copy_up_inode(struct ovl_copy_up_ctx *c, struct dentry *temp)
>         if (err)
>                 return err;
>
> +       if (inode->i_flags & OVL_COPY_I_FLAGS_MASK) {
> +               /*
> +                * Copy the fileattr inode flags that are the source of already
> +                * copied i_flags (best effort).
> +                */
> +               ovl_copy_fileattr(&c->lowerpath, &upperpath);

I'm not sure this should be ignoring errors.  Was this done to prevent
regressing cases where the upper fs cannot store the flags?  Do you
have a concrete example?

Thanks,
Miklos