Re: [PATCH] seq_file: fix passing wrong private data

[Muchun Song <songmuchun@bytedance.com>]

On Fri, Oct 29, 2021 at 4:50 PM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
>
> On Fri, Oct 29, 2021 at 04:43:40PM +0800, Muchun Song wrote:
> > On Fri, Oct 29, 2021 at 4:26 PM Christian Brauner
> > <christian.brauner@ubuntu.com> wrote:
> > >
> > > On Fri, Oct 29, 2021 at 11:26:38AM +0800, Muchun Song wrote:
> > > > DEFINE_PROC_SHOW_ATTRIBUTE() is supposed to be used to define a series
> > > > of functions and variables to register proc file easily. And the users
> > > > can use proc_create_data() to pass their own private data and get it
> > > > via seq->private in the callback. Unfortunately, the proc file system
> > > > use PDE_DATA() to get private data instead of inode->i_private. So fix
> > > > it. Fortunately, there only one user of it which does not pass any
> > > > private data, so this bug does not break any in-tree codes.
> > > >
> > > > Fixes: 97a32539b956 ("proc: convert everything to "struct proc_ops"")
> > > > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > > > ---
> > > >  include/linux/seq_file.h | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h
> > > > index 103776e18555..72dbb44a4573 100644
> > > > --- a/include/linux/seq_file.h
> > > > +++ b/include/linux/seq_file.h
> > > > @@ -209,7 +209,7 @@ static const struct file_operations __name ## _fops = {                   \
> > > >  #define DEFINE_PROC_SHOW_ATTRIBUTE(__name)                           \
> > > >  static int __name ## _open(struct inode *inode, struct file *file)   \
> > > >  {                                                                    \
> > > > -     return single_open(file, __name ## _show, inode->i_private);    \
> > > > +     return single_open(file, __name ## _show, PDE_DATA(inode));     \
> > > >  }                                                                    \
> > > >                                                                       \
> > > >  static const struct proc_ops __name ## _proc_ops = {                 \
> > >
> > > Hm, after your change DEFINE_SHOW_ATTRIBUTE() and
> > > DEFINE_PROC_SHOW_ATTRIBUTE() macros do exactly the same things, right?:
> >
> > Unfortunately, they are not the same. The difference is the
> > operation structure, namely "struct file_operations" and
> > "struct proc_ops".
> >
> > DEFINE_SHOW_ATTRIBUTE() is usually used by
> > debugfs while DEFINE_SHOW_ATTRIBUTE() is
> > used by procfs.
>
> Ugh, right, thanks for pointing that out. I overlooked the _proc_ops
> appendix. Not sure what's right here. There seem to have been earlier
> callers to DEFINE_PROC_SHOW_ATTRIBUTE() that relied on PDE_DATA() but
> there's only one caller so that change wouldn't be too bad, I guess.

From my point of view, I think the macro is useful which can
simplify the code. However there is only one user of it. I suppose
few people know about it. For instance, the following ops can
become the user of it.

    cifs_mount_params_proc_ops
    nfsd_proc_ops
    rpc_proc_ops

Thanks.