Re: [PATCH 1/5] debugfs: Prevent NULL dereference reading from string property

[Richard Fitzgerald]

On 16/5/23 17:33, Greg KH wrote:
> On Tue, May 16, 2023 at 05:07:49PM +0100, Richard Fitzgerald wrote:
> > Check in debugfs_read_file_str() if the string pointer is NULL.
> >
> > It is perfectly reasonable that a driver may wish to export a string
> > to debugfs that can have the value NULL to indicate empty/unused/ignore.
>
> Does any in-kernel driver do this today?

I don't know. The history here is that I was using debugfs_create_str()
to add a debugfs to a driver and made these improvements along the way.
Ultimately I had a reason to use a custom reader implementation.
But as I'd already written these patches I thought I'd send them.

> If not, why not fix up the driver instead?

Well... could do. Though it seems a bit odd to me that a driver
design should be forced by the debugfs API, instead of the debugfs API
fitting normal code design. It's pretty standard and idiomatic for code
to use if (!str) { /* bail */ } type logic, so why shouldn't the debugfs
API handle that?

> > Signed-off-by: Richard Fitzgerald <rf@xxxxxxxxxxxxxxxxxxxxx>
> > ---
> >   fs/debugfs/file.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
> > index 1f971c880dde..2c085ab4e800 100644
> > --- a/fs/debugfs/file.c
> > +++ b/fs/debugfs/file.c
> > @@ -878,6 +878,9 @@ ssize_t debugfs_read_file_str(struct file *file, char __user *user_buf,
> >   		return ret;
> >   
> >   	str = *(char **)file->private_data;
> > +	if (!str)
> > +		return simple_read_from_buffer(user_buf, count, ppos, "\n", 1);
>
> Why not print "(NULL)"?

Again, could do. My thought here is that a debugfs can be piped into
tools and having to insert a catch for "(NULL)" in the pipeline is a
nuisance. This is a bit different from a dmesg print, which is less
likely to be used this way or to guarantee machine-parsing.
However, I don't mind changing to "(NULL)" if you prefer.

> thanks,
>
> greg k-h