Re: [PATCH] powercap: Adjust printing the constraint name with new line

[Lukasz Luba]

On 11/17/20 12:51 PM, Rafael J. Wysocki wrote:
> On Tue, Nov 17, 2020 at 10:22 AM Lukasz Luba <lukasz.luba@xxxxxxx> wrote:
> > Hi Rafael,
> >
> > On 11/10/20 7:19 PM, Rafael J. Wysocki wrote:
> > > On Mon, Nov 9, 2020 at 6:25 PM Lukasz Luba <lukasz.luba@xxxxxxx> wrote:
> > > > The constraint name has limit of size 30, which sometimes might be hit.
> > > > When this happens the new line might be lost. Prevent this and set the
> > > > new line when the name string is too long."
> > > >
> > > > Signed-off-by: Lukasz Luba <lukasz.luba@xxxxxxx>
> > > > ---
> > > >    drivers/powercap/powercap_sys.c | 4 ++++
> > > >    1 file changed, 4 insertions(+)
> > > >
> > > > diff --git a/drivers/powercap/powercap_sys.c b/drivers/powercap/powercap_sys.c
> > > > index f808c5fa9838..575f9fdb810e 100644
> > > > --- a/drivers/powercap/powercap_sys.c
> > > > +++ b/drivers/powercap/powercap_sys.c
> > > > @@ -174,6 +174,10 @@ static ssize_t show_constraint_name(struct device *dev,
> > > >                                                                   "%s\n", name);
> > > >                           buf[POWERCAP_CONSTRAINT_NAME_LEN] = '\0';
> > > >                           len = strlen(buf);
> > > > +
> > > > +                       /* When the 'name' is too long, don't lose new line */
> > > > +                       if (strlen(name) >= POWERCAP_CONSTRAINT_NAME_LEN)
> > > > +                               buf[POWERCAP_CONSTRAINT_NAME_LEN - 1] = '\n';
> > >
> > > Wouldn't it be better to pass POWERCAP_CONSTRAINT_NAME_LEN - 1 to
> > > snprintf() above?
> >
> > My apologies for delay, I was on holidays.
> >
> > The snprintf() overwrites the '\n' in that case also.
>
> The snprintf() doesn't do that AFAICS, but the next assignment
> overwrites the newline with the zero character.
>
> Anyway, it should have been POWERCAP_CONSTRAINT_NAME_LEN - 2, because
> it needs to leave space for 2 characters, the newline and the
> terminating zero.
>
> > I've experimented with it a bit more and tried to come with a bit 'cleaner' solution.
> >
> > What we are looking is probably: "%.*s\n" semantic.
> > Another solution would be:
> > ------------------------------8<---------------------------
> >                   if (name) {
> > -                       snprintf(buf, POWERCAP_CONSTRAINT_NAME_LEN,
> > -                                                               "%s\n",
> > name);
> > -                       buf[POWERCAP_CONSTRAINT_NAME_LEN] = '\0';
> > +                       sprintf(buf, "%.*s\n",
> > POWERCAP_CONSTRAINT_NAME_LEN -1,
> > +                               name);
> >                           len = strlen(buf);
> >                   }
> >
> > ----------------------------->8----------------------------
> >
> > I've check this and it behaves very well.
> >
> > It looks cleaner and it is a used pattern in the kernel.
> > What do you think? Is it good for v2?
>
> This works for me too.
>
> Thanks!

Thank you for the comments. I will send the v2 then.

Regards,
Lukasz