Re: [PATCH v3 1/3] debugfs: Add write support to debugfs_create_str()

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Aug 18, 2023 at 01:05:57PM +0300, Georgi Djakov wrote:
> Hi Greg,
> 
> On 12.08.23 13:40, Greg KH wrote:
> > On Mon, Aug 07, 2023 at 07:29:12AM -0700, Mike Tipton wrote:
> > > Currently, debugfs_create_str() only supports reading strings from
> > > debugfs. Add support for writing them as well.
> > > 
> > > Based on original implementation by Peter Zijlstra [0]. Write support
> > > was present in the initial patch version, but dropped in v2 due to lack
> > > of users. We have a user now, so reintroduce it.
> > > 
> > > [0] https://lore.kernel.org/all/YF3Hv5zXb%2F6lauzs@hirez.programming.kicks-ass.net/
> > > 
> > > Signed-off-by: Mike Tipton <quic_mdtipton@quicinc.com>
> > > ---
> > >   fs/debugfs/file.c | 48 +++++++++++++++++++++++++++++++++++++++++++++--
> > >   1 file changed, 46 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
> > > index b7711888dd17..87b3753aa4b1 100644
> > > --- a/fs/debugfs/file.c
> > > +++ b/fs/debugfs/file.c
> > > @@ -904,8 +904,52 @@ EXPORT_SYMBOL_GPL(debugfs_create_str);
> > >   static ssize_t debugfs_write_file_str(struct file *file, const char __user *user_buf,
> > >   				      size_t count, loff_t *ppos)
> > >   {
> > > -	/* This is really only for read-only strings */
> > > -	return -EINVAL;
> > > +	struct dentry *dentry = F_DENTRY(file);
> > > +	char *old, *new = NULL;
> > > +	int pos = *ppos;
> > > +	int r;
> > > +
> > > +	r = debugfs_file_get(dentry);
> > > +	if (unlikely(r))
> > > +		return r;
> > > +
> > > +	old = *(char **)file->private_data;
> > > +
> > > +	/* only allow strict concatenation */
> > > +	r = -EINVAL;
> > > +	if (pos && pos != strlen(old))
> > > +		goto error;
> > > +
> > > +	r = -E2BIG;
> > > +	if (pos + count + 1 > PAGE_SIZE)
> > > +		goto error;
> > > +
> > > +	r = -ENOMEM;
> > > +	new = kmalloc(pos + count + 1, GFP_KERNEL);
> > > +	if (!new)
> > > +		goto error;
> > > +
> > > +	if (pos)
> > > +		memcpy(new, old, pos);
> > > +
> > > +	r = -EFAULT;
> > > +	if (copy_from_user(new + pos, user_buf, count))
> > > +		goto error;
> > > +
> > > +	new[pos + count] = '\0';
> > > +	strim(new);
> > > +
> > > +	rcu_assign_pointer(*(char **)file->private_data, new);
> > > +	synchronize_rcu();
> > > +	kfree(old);
> > > +
> > > +	debugfs_file_put(dentry);
> > > +	return count;
> > > +
> > > +error:
> > > +	kfree(new);
> > > +	debugfs_file_put(dentry);
> > > +	return r;
> > >   }
> > 
> > So you just added write support for ALL debugfs files that use the
> > string interface, what did you just allow to break?
> 
> Not true. Write support is added only for debugfs string files that are
> created with +w permissions. All existing files are created as read-only
> and use the fops_str_ro ops.
> 
> > I recommend just using your own debugfs file function instead, as this
> > could cause bad problems, right?  Are you sure that all string calls can
> > handle the variable be freed underneath it like this call will allow to
> > happen?
> > 
> > So I wouldn't recommend doing this, sorry.
> > 
> 
> Maybe you missed the fact that the different file ops are already there
> and are selected based on permissions:
> 
> > static const struct file_operations fops_str = {
> >         .read =         debugfs_read_file_str,
> >         .write =        debugfs_write_file_str,
> >         .open =         simple_open,
> >         .llseek =       default_llseek,
> > };
> > 
> > static const struct file_operations fops_str_ro = {
> >         .read =         debugfs_read_file_str,
> >         .open =         simple_open,
> >         .llseek =       default_llseek,
> > };
> > 
> > static const struct file_operations fops_str_wo = {
> >         .write =        debugfs_write_file_str,
> >         .open =         simple_open,
> >         .llseek =       default_llseek,
> > };
> 
> ...so this patch is doing exactly what you suggested? If you agree,
> could you ack it again please?

Yes, I did miss that, sorry, my appologies for dragging this out so
long:

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>