Re: [RFC PATCH v3 2/3] fs: debugfs: Add write functionality to debugfs blobs

[Alexey Kardashevskiy]

On 13/6/23 17:59, Greg KH wrote:
> On Mon, Jun 12, 2023 at 09:51:38PM +0000, Avadhut Naik wrote:
> >   /**
> > - * debugfs_create_blob - create a debugfs file that is used to read a binary blob
> > + * debugfs_create_blob - create a debugfs file that is used to read and write
> > + * a binary blob
> >    * @name: a pointer to a string containing the name of the file to create.
> > - * @mode: the read permission that the file should have (other permissions are
> > - *	  masked out)
> > + * @mode: the permission that the file should have
> >    * @parent: a pointer to the parent dentry for this file.  This should be a
> >    *          directory dentry if set.  If this parameter is %NULL, then the
> >    *          file will be created in the root of the debugfs filesystem.
> > @@ -992,7 +1010,7 @@ static const struct file_operations fops_blob = {
> >    *
> >    * This function creates a file in debugfs with the given name that exports
> >    * @blob->data as a binary blob. If the @mode variable is so set it can be
> > - * read from. Writing is not supported.
> > + * read from and written to.
> >    *
> >    * This function will return a pointer to a dentry if it succeeds.  This
> >    * pointer must be passed to the debugfs_remove() function when the file is
> > @@ -1007,7 +1025,7 @@ struct dentry *debugfs_create_blob(const char *name, umode_t mode,
> >   				   struct dentry *parent,
> >   				   struct debugfs_blob_wrapper *blob)
> >   {
> > -	return debugfs_create_file_unsafe(name, mode & 0444, parent, blob, &fops_blob);
> > +	return debugfs_create_file_unsafe(name, mode, parent, blob, &fops_blob);
>
> Have you audited all calls to this function to verify that you haven't
> just turned on write access to some debugfs files?

I just did, it is one of S_IRUGO/S_IRUSR/0444/0400/(S_IFREG | 0444). So 
we are quite safe here. Except (S_IFREG | 0444) in 
drivers/platform/chrome/cros_ec_debugfs.c which seems wrong as debugfs 
files are not regular files.

> Why not rename this to debugfs_create_blob_wo() and then make a new
> debugfs_create_blob_rw() call to ensure that it all is ok?

It is already taking the mode for this purpose. imho just 
cros_ec_create_panicinfo()'s debugfs_create_blob("panicinfo", S_IFREG | 
0444,...) needs fixing.

> thanks,
>
> greg k-h

--
Alexey