linux-kernel - Re: [PATCH] seq_file: fix passing wrong private data

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Date:   Fri, 29 Oct 2021 16:43:40 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     andriy.shevchenko@...ux.intel.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>, revest@...omium.org,
        Alexey Dobriyan <adobriyan@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] seq_file: fix passing wrong private data

On Fri, Oct 29, 2021 at 4:26 PM Christian Brauner
<christian.brauner@...ntu.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@...edance.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.

Thanks.

>
> #define DEFINE_SHOW_ATTRIBUTE(__name)                                   \
> static int __name ## _open(struct inode *inode, struct file *file)      \
> {                                                                       \
>         return single_open(file, __name ## _show, inode->i_private);    \
> }                                                                       \
>                                                                         \
> static const struct file_operations __name ## _fops = {                 \
>         .owner          = THIS_MODULE,                                  \
>         .open           = __name ## _open,                              \
>         .read           = seq_read,                                     \
>         .llseek         = seq_lseek,                                    \
>         .release        = single_release,                               \
> }
>
> #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);    \
> }                                                                       \
>
> Can't you just replace the single instance where
> DEFINE_PROC_SHOW_ATTRIBUTE with DEFINE_SHOW_ATTRIBUTE() and remove
> DEFINE_PROC_SHOW_ATTRIBUTE completely?
>
> Christian