linux-kernel - Re: [PATCH v4 2/2] pinctrl: pinmux: Add pinmux-select debugfs file

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Date:   Thu, 11 Feb 2021 19:37:56 -0800
From:   Drew Fustini <drew@...gleboard.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Tony Lindgren <tony@...mide.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Jason Kridner <jkridner@...gleboard.org>,
        Robert Nelson <robertcnelson@...gleboard.org>,
        Joe Perches <joe@...ches.com>,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [PATCH v4 2/2] pinctrl: pinmux: Add pinmux-select debugfs file

On Thu, Feb 11, 2021 at 09:09:03AM +0100, Geert Uytterhoeven wrote:
> Hi Drew,
> 
> On Wed, Feb 10, 2021 at 11:33 PM Drew Fustini <drew@...gleboard.org> wrote:
> > Add "pinmux-select" to debugfs which will activate a function and group
> > when "<function-name group-name>" are written to the file. The write
> > operation pinmux_select() handles this by checking that the names map to
> > valid selectors and then calling ops->set_mux().
> >
> > The existing "pinmux-functions" debugfs file lists the pin functions
> > registered for the pin controller. For example:
> >
> > function: pinmux-uart0, groups = [ pinmux-uart0-pins ]
> > function: pinmux-mmc0, groups = [ pinmux-mmc0-pins ]
> > function: pinmux-mmc1, groups = [ pinmux-mmc1-pins ]
> > function: pinmux-i2c0, groups = [ pinmux-i2c0-pins ]
> > function: pinmux-i2c1, groups = [ pinmux-i2c1-pins ]
> > function: pinmux-spi1, groups = [ pinmux-spi1-pins ]
> >
> > To activate function pinmux-i2c1 and group pinmux-i2c1-pins:
> >
> > echo "pinmux-i2c1 pinmux-i2c1-pins" > pinmux-select
> >
> > Signed-off-by: Drew Fustini <drew@...gleboard.org>
> 
> Thanks for your patch!
> 
> > --- a/drivers/pinctrl/pinmux.c
> > +++ b/drivers/pinctrl/pinmux.c
> > @@ -673,6 +673,111 @@ void pinmux_show_setting(struct seq_file *s,
> >  DEFINE_SHOW_ATTRIBUTE(pinmux_functions);
> >  DEFINE_SHOW_ATTRIBUTE(pinmux_pins);
> >
> > +#define PINMUX_MAX_NAME 64
> > +static ssize_t pinmux_select(struct file *file, const char __user *user_buf,
> > +                                  size_t len, loff_t *ppos)
> > +{
> > +       struct seq_file *sfile = file->private_data;
> > +       struct pinctrl_dev *pctldev = sfile->private;
> > +       const struct pinmux_ops *pmxops = pctldev->desc->pmxops;
> > +       const char *const *groups;
> > +       char *buf, *fname, *gname;
> > +       unsigned int num_groups;
> > +       int fsel, gsel, ret;
> > +
> > +       if (len > (PINMUX_MAX_NAME * 2)) {
> > +               dev_err(pctldev->dev, "write too big for buffer");
> > +               return -EINVAL;
> > +       }
> > +
> > +       buf = kzalloc(PINMUX_MAX_NAME * 2, GFP_KERNEL);
> > +       if (!buf)
> > +               return -ENOMEM;
> > +
> > +       fname = kzalloc(PINMUX_MAX_NAME, GFP_KERNEL);
> > +       if (!fname) {
> > +               ret = -ENOMEM;
> > +               goto free_buf;
> > +       }
> > +
> > +       gname = kzalloc(PINMUX_MAX_NAME, GFP_KERNEL);
> > +       if (!buf) {
> > +               ret = -ENOMEM;
> > +               goto free_fname;
> > +       }
> > +
> > +       ret = strncpy_from_user(buf, user_buf, PINMUX_MAX_NAME * 2);
> 
> While this guarantees buf is not overflowed...
> 
> > +       if (ret < 0) {
> > +               dev_err(pctldev->dev, "failed to copy buffer from userspace");
> > +               goto free_gname;
> > +       }
> > +       buf[len-1] = '\0';
> > +
> > +       ret = sscanf(buf, "%s %s", fname, gname);
> 
> ... one of the two strings can still be longer than PINMUX_MAX_NAME,
> thus overflowing fname or gname.
> 
> As buf is already a copy, it may be easier to just find the strings in
> buf, write the NUL terminators into buf, and set up fname and gname
> to point to the strings inside buf.

Thank you for pointing this out.  I should have considered that one name
could be much larger than the other name.  I see Andy suggested
alternative to sscanf() that gets around having to allocate seperate
buffers for each name.

> > +       if (ret != 2) {
> > +               dev_err(pctldev->dev, "expected format: <function-name> <group-name>");
> > +               goto free_gname;
> > +       }
> 
> > +static const struct file_operations pinmux_select_ops = {
> > +       .owner = THIS_MODULE,
> > +       .open = pinmux_select_open,
> > +       .read = seq_read,
> 
> I don't think you need to fill in .read for a write-only file.

Thanks, I'll remove it.
> 
> > +       .write = pinmux_select,
> > +       .llseek = no_llseek,
> > +       .release = single_release,
> > +};
> > +
> >  void pinmux_init_device_debugfs(struct dentry *devroot,
> >                          struct pinctrl_dev *pctldev)
> >  {
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> -- 
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
> 
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds